--- - chapter_identifier: north-american-and-other-international-effects confidence: '

There is high confidence in the main message. There is sufficient empirical analysis on the relationships between past climate variations along U.S. international borders. The statement about the likelihood that impacts on shared resources will affect the bilateral frameworks established to manage these resources is based on expert understanding of the integration of climate risk into existing and future frameworks.

' evidence: "

In the U.S.–Mexico drylands region, large areas are projected to become drier,{{< tbib '102' '37d85f6f-8d91-45e8-bf65-0ae8aee523a6' >}} which will present increasing demands for water resources on top of existing stresses related to population growth.{{< tbib '103' 'c9075dbc-f7c8-4d85-b534-e97282562b3e' >}}^,{{}} There is high confidence that resources critical to livelihoods at borders between the United States and neighboring nations are becoming increasingly vulnerable to impacts of climate change and that the multinational frameworks that manage these resources are increasingly incorporating research-based understanding of the climate risks that these resources face. The literature supporting the Key Message is substantial, increasing in quantity and robustness.{{< tbib '96' '5be5a1f9-7144-41ba-a80d-e3c77a49986f' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} The current impacts are well documented, and the projections of future impacts are aligned with the robust projections of future climate variability.{{< tbib '94' '69fe7cf7-52d4-4bc1-ba55-f6d994a47687' >}}^,{{}} The literature also provides examples of bilateral agreements and management frameworks in place to manage these resources. Examples of the impacts include the migration northward into Canadian waters of Pacific hake, a migratory species sensitive to water temperature, during periods of warmer water temperature.{{< tbib '100' '49117baa-d1a6-475b-b5d5-7391a7c272b1' >}} One example of a bilateral management framework is the inclusion in 2012 of a climate change impacts annex to the U.S.–Canada Great Lakes Water Quality Agreement to identify, quantify, understand, and predict climate change impacts on the water quality of the Great Lakes.{{< tbib '109' 'c3722455-6da6-40ca-aca3-6568594c80fe' >}}

" href: https://data.globalchange.gov/report/nca4/chapter/north-american-and-other-international-effects/finding/key-message-16-4.yaml identifier: key-message-16-4 ordinal: 4 process: "

The Fourth National Climate Assessment (NCA4) is the first U.S. National Climate Assessment (NCA) to include a chapter that addresses the impacts of climate change beyond the borders of the United States. This chapter was included in NCA4 in response to comments received during public review of the Third National Climate Assessment (NCA3) that proposed that future NCAs include an analysis of international impacts of climate change as they relate to U.S. interests.

This chapter focuses on the implications of international impacts of climate change on U.S. interests. It does not address or summarize all international impacts of climate change; that very broad topic is covered by Working Group II of the Intergovernmental Panel on Climate Change (IPCC; e.g., IPCC 2014{{< tbib '1' 'c390e13f-8517-40a9-a236-ac4dede3a7a0' >}}). The U.S. government supports and participates in the IPCC process. The more focused topic of how U.S. interests can be affected by climate impacts outside of the United States is not specifically addressed by the IPCC.

The topics in the chapter—economics and trade, international development and humanitarian assistance, national security, and transboundary resources—were selected because they illustrate ways in which U.S. interests can be affected by international climate impacts. These topics cut across the world, so the chapter does not focus on impacts in specific regions.

The transboundary section was added to address climate-related impacts across U.S. borders. While the regional chapters address local and regional transboundary impacts, they do not address impacts that exist in multiple regions or agreements between the United States and its neighbors that create mechanisms for addressing such impacts.

The science section is part of the chapter because of the importance of international scientific cooperation to our understanding of climate science. That topic is not treated as a separate section because it is not a risk-based issue and therefore not an appropriate candidate to have as a Key Message.

The U.S. Global Change Research Program (USGCRP) put out a call for authors for the International chapter both inside and outside the Federal Government. The USGCRP asked for nominations of and by individuals with experience and knowledge on international climate change impacts and implications for the United States as well as experience in assessments such as the NCA.

All of the authors selected for the chapter have extensive experience in international climate change, and several had been authors on past NCAs. Section lead assignments were made based on the expertise of the individuals and, for those authors who are current federal employees, based on the expertise of the agencies. The author team of ten individuals is evenly divided between federal and non-federal personnel.

The coordinating lead author (CLA) and USGCRP organized two public outreach meetings. The first meeting was held at the Wilson Center in Washington, DC, on September 15, 2016, as part of the U.S. Agency for International Development’s (USAID) Adaptation Community Meetings and solicited input on the outline of the chapter and asked for volunteers to become chapter authors or otherwise contribute to the chapter. A public review meeting regarding the International chapter was held on April 6, 2017, at Chemonics in Washington, DC, also as part of USAID’s Adaptation Community Meetings series. The USGCRP and chapter authors shared information about the progress to date of the International chapter and sought input from stakeholders to help inform further development of the chapter, as well as to raise general awareness of the process and timeline for NCA4.

The chapter was revised in response to comments from the public and from the National Academy of Sciences. The comments were reviewed and discussed by the entire author team and the review editor, Dr. Diana Liverman of the University of Arizona. Individual authors drafted responses to comments on their sections, while the CLA and the chapter lead (CL) drafted responses to comments that pertained to the entire chapter. All comments were reviewed by the CLA and CL. The review editor reviewed responses to comments and revisions to the chapter to ensure that all comments had been considered by the authors.

" report_identifier: nca4 statement: '

Shared resources along U.S. land and maritime borders provide direct benefits to Americans and are vulnerable to impacts from a changing climate, variability, and extremes (very likely, high confidence). Multinational frameworks that manage shared resources are increasingly incorporating climate risk in their transboundary decision-making processes.

' uncertainties: "

Impacts on shared resources along U.S. international borders are already being experienced. Uncertainties about the impacts are aligned with the uncertainties associated with projections of future climate variability. As elaborated upon in multiple regional chapters of this report (Ch. 18: Northeast; Ch. 20: U.S. Caribbean; Ch. 21: Midwest; Ch. 24: Northwest; Ch. 25: Southwest; Ch. 26: Alaska; Ch. 27: Hawai‘i & Pacific Islands), weather patterns in these border regions are projected to continue to change with varying degrees of likelihood and confidence.

" uri: /report/nca4/chapter/north-american-and-other-international-effects/finding/key-message-16-4 url: ~ - chapter_identifier: sectoral-interdependencies-and-compounding-stressors confidence: '

We have high confidence in this message, because there is high agreement and extensive evidence that a range of critical intersectoral interdependencies and compounding stressors are present and relevant to climate risk assessment. At the same time, the precise impact of these on system dynamics is not well understood.

' evidence: "

A suite of examples across this assessment and within this chapter demonstrate the interactions between systems and the potentially important implications of these linkages. Examples in this chapter include Hurricane Harvey; the 2003 Northeast blackout; energy–water–land systems in California and throughout the nation; forest systems facing influences from wildfires, drought, and pine bark beetles; and the implications of the reintroduction of wolves in Yellowstone. Each of these examples is supported by its own evidence base; the linkages between systems and the importance of non-climate influences is self-evident from these examples. Beyond these examples, a small set of recent literature has begun to explore ways to more systematically quantify the implications of including sectoral interdependencies in climate risk assessment (e.g., Harrison et al. 2016{{< tbib '8' '2a131189-94cc-4c86-bb51-2fc0bf6a4504' >}}).

In addition to literature specific to risk assessment in the context of climate change, there is a long history of research on complex systems{{< tbib '11' '87e9e534-034f-450c-b205-f268be5c2152' >}} that raises the potential for a range of dynamics that might emerge from sectoral interdependencies and compounding stressors. This includes research spanning disciplines from meteorology{{< tbib '12' 'ff6f1e9a-1875-438b-b628-c107c5de2396' >}} to ecology{{< tbib '13' 'ceb49ae3-99c4-4009-a382-c3f26891e687' >}} to paleontology{{< tbib '14' '82abbb5d-1c8e-4178-82c3-249fb0fdf168' >}} to computer science.{{< tbib '15' 'a4feb2d0-0a82-4f20-98af-89c295b177c0' >}} This literature supports the conclusion that more complex dynamics may occur when multiple systems interact with one another.

" href: https://data.globalchange.gov/report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-1.yaml identifier: key-message-17-1 ordinal: 1 process: '

The scope of this chapter was developed to fill a gap in previous National Climate Assessments (NCAs), notably the risks that emerge from interactions among sectors. Previous NCAs have touched on this subject, for example the energy, water, and land use chapter in the Third National Climate Assessment (NCA3). However, these assessments never included a chapter specifically focused on a general treatment of this topic. Emerging scientific research is highlighting the links between sectors and the potential complexity and implications of these interactions, from complex system dynamics such as cascading failures to management approaches and approaches to risk. These concepts were then incorporated into a detailed terms of reference for the chapter, outlining the scope and the general content to be included in the document.

The author team for this chapter was constructed to bring together the necessary diverse experience, expertise, and perspectives. Our authors brought expertise and experience in multiscale, multisector research and modeling, with a focus in specific sectors or sectoral combinations including critical infrastructure, energy–water–land interactions, and ecosystems. The authors also had expertise in complex systems science and previous experience in assessment processes.

The chapter was developed through technical discussions, a literature review, and expert deliberation by chapter authors through email and phone discussions. The team evaluated the state of the science on the analysis of sectoral interdependencies, compounding stressors, and complex system science. Case studies were drawn from a range of sources intended to represent the key themes in the chapter.

' report_identifier: nca4 statement: '

The sectors and systems exposed to climate (for example, energy, water, and agriculture) interact with and depend on one another and other systems less directly exposed to climate (such as the financial sector). In addition, these interacting systems are not only exposed to climate-related stressors such as floods, droughts, and heat waves, they are also subject to a range of non-climate factors, from population movements to economic fluctuations to urban expansion. These interactions can lead to complex behaviors and outcomes that are difficult to predict. It is not possible to fully understand the implications of climate change on the United States without considering the interactions among sectors and their consequences. (High Confidence)

' uncertainties: '

The interactions between sectors and systems relevant to climate risk assessment are self-evident, and there are clear examples of unanticipated dynamics emerging from these interactions in the past. Yet our understanding is limited regarding the precise nature of complex system behavior in the context of climate risk assessment and its ultimate influence on the outcomes of such assessments. As noted in Key Message 4, the available tools and frameworks are simply not sufficient at this point to identify key risks emerging from intersectoral interdependencies and compounding stressors.

' uri: /report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-1 url: ~ - chapter_identifier: sectoral-interdependencies-and-compounding-stressors confidence: '

We have high confidence in this Key Message because there is high agreement that a multisector perspective alters risk assessment, as is reflected in recent climate change assessments. However, the evidence basis for multisector evaluations is emerging.

' evidence: "

Recent climate change assessments (e.g., Oppenheimer et al. 2014, Houser et al. 2015{{< tbib '45' '0ea6d723-5df9-4b45-8d5f-be269119ccf8' >}}^,{{}}) emphasize that a multisector perspective expands the scope of relevant risks and uncertainties associated with climate change impacts. Assessing these risks requires attention to multiple interacting sectors, geographic regions, and stressors, such as 1) interactions in the management of water, land, and energy (see Box 17.3), or 2) spatial compounding of impacts if, for example, multiple infrastructure systems fail within a city (see Box 17.1). Risk assessment also requires attention to indirect and long-distance climate change impacts, for instance resulting from human migration or conflict.{{< tbib '45' '0ea6d723-5df9-4b45-8d5f-be269119ccf8' >}}^,{{}} Analyses of historical events (see Box 17.5), evaluations of statistical risk (e.g., Carleton and Hsiang 2016{{< tbib '101' 'a73835ed-0558-4fe3-bccf-e61b4014ed63' >}}), and process-based modeling projections are some of the methods demonstrating these complex interactions across sectors, scales, and stressors.

Different tools and approaches are required to assess multisector risks. Approaches can be applied to integrate diverse evidence, combining quantitative and qualitative results and drawing from the natural and social sciences and other forms of analysis.{{< tbib '47' '0bf999f3-8291-493a-bf19-525a26af5125' >}}^,{{}}^,{{}} For instance, models and expert judgment have been used together to inform our understanding of future sea level rise,{{< tbib '52' '843b8feb-de6f-42be-88f9-657915e75601' >}} and scenarios can also be used to explore preparedness across possible futures.{{< tbib '53' 'ae138b1a-a619-4312-a671-0f671a85662b' >}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-2.yaml identifier: key-message-17-2 ordinal: 2 process: '

' report_identifier: nca4 statement: '

Climate change risk assessment benefits from a multisector perspective, encompassing interactions among sectors and both climate and non-climate stressors. Because such interactions and their consequences can be challenging to identify in advance, effectively assessing multisector risks requires tools and approaches that integrate diverse evidence and that consider a wide range of possible outcomes. (High Confidence)

' uncertainties: '

For interdependent systems affected by multiple stressors, the number and complexity of possible interactions are greater, presenting deeper uncertainties. It is often difficult or impossible to represent all relevant processes and interactions in analyses of risks, especially quantitatively. For example, quantitative projections can evaluate probabilities of well-understood sectoral interactions but will be limited by processes or parameters that are poorly known or unknowable. This is why the integration of diverse evidence and attention to deeper uncertainties are important in multisector risk assessment.

' uri: /report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-2 url: ~ - chapter_identifier: sectoral-interdependencies-and-compounding-stressors confidence: '

There is high agreement and extensive evidence that institutional arrangements and governance are critical to the management of systems and their interdependencies. This finding is reflected in scientific assessments, modeling studies, and observations of system responses and performance, as well as in theories emerging from complex systems science. Furthermore, a history of management practice associated with water, energy, transportation, telecommunications, food, and health systems that spans decades to centuries provides evidence for the importance of system interdependencies. Thus, there is high Confidence in this message.

' evidence: "

Recent literature has documented that the management of interacting infrastructure systems is a key factor influencing their resilience to climate and other stressors. A range of studies have argued that the complexity of institutional arrangements in mature, democratic economies like the United States poses challenges to the pursuit of climate adaptation objectives and sustainability more broadly.{{< tbib '72' '747e6b30-6afc-4520-af4b-660389e167ba' >}}^,{{}}^,{{}}^,{{}}^,{{}} The complexity associated with interacting systems of systems poses significant challenges to integrated management.{{< tbib '105' '9f316b11-0ea5-4aff-b638-9bb9737cd7b6' >}} The allocation of authority and responsibility for system management across multiple levels of government as well as between public and private sectors often contributes to decision-making by one actor being enabled or constrained by other actors.{{< tbib '72' '747e6b30-6afc-4520-af4b-660389e167ba' >}}^,{{}}

The interdependencies among systems reflect the potential value in the development of more integrated management strategies.{{< tbib '72' '747e6b30-6afc-4520-af4b-660389e167ba' >}} This concept of integrated management is reflected in existing literatures, particularly those associated with integrated water resources management {{< tbib '106' 'f43680e8-feb9-4e43-aaa3-26b843935b35' >}}^,{{}}^,{{}}^,{{}} and integrated infrastructure planning.{{< tbib '110' 'e065f634-1a56-417f-b541-f90862b11623' >}}^,{{}}^,{{}} Such studies often address integration within sectors or systems, with less consideration for integration between or among systems. This has the potential to lead to missed opportunities for improving management practice.{{< tbib '72' '747e6b30-6afc-4520-af4b-660389e167ba' >}} However, assessments of energy,{{< tbib '113' '66fa5de6-5f51-4d35-a6dc-ecc243575ac6' >}} urban infrastructure,{{< tbib '75' 'd2f3853a-5f20-4132-92c8-57da1b4d95fc' >}} and coupled energy–water–land{{< tbib '114' '552cc5f5-a7b3-4a64-8bee-98ae0cced150' >}} systems conducted as part of NCA3{{< tbib '44' 'aa1fec1f-b5c3-48b8-b17e-ca88da35eb4c' >}} identified a range of interdependencies across multiple sectors (see Dawson 2015{{< tbib '115' '38a397d4-812d-4af6-98fb-8f74dd8632ac' >}}).

A range of strategies have been proposed for enhancing the capacity to manage system interdependencies and climate change risk. Significant effort has been invested in understanding and modeling system dynamics to enhance capabilities for risk and vulnerability assessment. These efforts have largely focused on physical infrastructure systems, infrastructure networks, and the potential for cascading failures.{{< tbib '116' 'd5343adc-cad7-4ec5-89db-02b4e7432c1a' >}}^,{{}}^,{{}}^,{{}} Such capabilities help to identify what can be monitored in complex systems to enhance situational awareness, anticipate disruptions, and increase resilience.{{< tbib '71' '9278107f-e3d4-4d4b-92db-a72057d3a5fa' >}}^,{{}}^,{{}}

There is ample evidence of comanagement of interdependent systems, often as a function of resource assurance and/or contingency planning. For example, the use of water for electricity generation (hydropower or cooling in thermal generation) involves regulatory constraints around water use as well as operational decision-making regarding water management.{{< tbib '72' '747e6b30-6afc-4520-af4b-660389e167ba' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} These interactions have been a major focus of studies addressing the climate–water–energy nexus. Meanwhile, emergency managers as well as agricultural, commercial, and industrial supply chains often develop contingency plans in the event of disruptions of transportation, telecommunications, water, and/or electricity.{{< tbib '81' '25c22917-41da-4f27-82db-1d40c3b4f677' >}}^,{{}}^,{{}}^,{{}}^,{{}}

A key element of such planning is to build redundancy and flexibility into system operations.{{< tbib '73' 'e70ad283-4e8b-4a9e-8279-6f7f830f98f5' >}} Evidence suggests that adding flexibility or robustness to systems or transforming systems such that they interact or behave in fundamentally different ways can increase construction, maintenance, or procurement costs.{{< tbib '82' '9a6c7a87-5c0f-4d64-904c-c707f68f2115' >}}^,{{}}^,{{}} However, a number of studies exploring the valuation of resilience actions and investments have concluded that the benefits of resilience interventions can be significantly greater than the costs, provided the long-term mitigating effects of the intervention are factored in.{{< tbib '132' '97189668-36ce-4b55-9550-f10a6ebdea24' >}}^,{{}}^,{{}}

Given the complexity of governance systems, the responsibility for the design and implementation of such strategies for integrated management rests on a broad range of actors. Over the latter part of the 20th century, the privatization of infrastructure, including energy, telecommunications, and water, transferred infrastructure management, responsibility, and risk to the private sector.{{< tbib '135' '18325d52-df0d-4729-9e02-c0b0e8945fef' >}} Nevertheless, local, state, and federal governments continue to have critical roles in regulation, risk assessment, and research and development. In addition, many institutions, organizations, and individuals either have infrastructure dependencies or influence the dynamics, operations, investment, and performance of infrastructure.{{< tbib '136' '57da6191-41b4-48a5-8fe6-0d55fd26a01b' >}} The increasing interconnectedness of both infrastructure and the people who use and manage that infrastructure is leading to both new challenges and opportunities for comanaging these systems, particularly in urban areas.{{< tbib '137' 'c75e24cb-498e-400b-8f25-a47526666cf5' >}}^,{{}}^,{{}} A growing literature is identifying opportunities to enhance consideration of human health and other benefits in the design of urban landscapes and infrastructure.{{< tbib '67' 'f1e633d5-070a-4a7d-935b-a2281a0c9cb6' >}}^,{{}}^,{{}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-3.yaml identifier: key-message-17-3 ordinal: 3 process: '

' report_identifier: nca4 statement: '

The joint management of interacting systems can enhance the resilience of communities, industries, and ecosystems to climate-related stressors. For example, during drought events, river operations can be managed to balance water demand for drinking water, navigation, and electricity production. Such integrated approaches can help avoid missed opportunities or unanticipated tradeoffs associated with the implementation of management responses to climate-related stressors. (High Confidence)

' uncertainties: '

The dominant uncertainties associated with the management of climate risks and system interdependencies include understanding indirect effects and feedbacks between systems, particularly with respect to predicting system responses. Technological change could have significant implications for the resilience, interconnectedness, and responses of systems to climate-related stressors and other disturbances. Such change could increase the complexity of integrated management with implications that could be positive or negative with respect to vulnerability. In addition, the future evolution of governance and regulatory dimensions of infrastructures systems, as well as consumer choices and behavior, are associated with irreducible uncertainty, largely because they involve choices yet to be made.

' uri: /report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-3 url: ~ - chapter_identifier: sectoral-interdependencies-and-compounding-stressors confidence: '

See above. No likelihood statement is appropriate, and the high confidence is based on the authors’ assessment of the underlying literature and development of methods and modeling tools.

' evidence: '

This Key Message is based on an understanding of a range of analyses and modeling tools described throughout the chapter.

' href: https://data.globalchange.gov/report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-4.yaml identifier: key-message-17-4 ordinal: 4 process: '

' report_identifier: nca4 statement: '

Predicting the responses of complex, interdependent systems will depend on developing meaningful models of multiple, diverse systems, including human systems, and methods for characterizing uncertainty. (High Confidence)

' uncertainties: '

Because the Key Message is the authors’ assessment of the overall state of development of research tools and models, and the subsequent importance of developing research tools, the concept of major uncertainties is not entirely appropriate. This is a matter of the authors’ judgment, not calculation or assessment of underlying probabilities.

' uri: /report/nca4/chapter/sectoral-interdependencies-and-compounding-stressors/finding/key-message-17-4 url: ~ - chapter_identifier: northeast confidence: '

There is high confidence that the combined effects of increasing winter and early-spring temperatures and increasing winter precipitation (very high confidence) are changing aquatic and terrestrial habitats and affecting the species adapted to them. The impact of changing seasonal temperature, moisture conditions, and habitats will vary geographically and impact interactions among species. It is likely that some will not adapt. There is high confidence that over the next century, some species will decline while other species introduced to the region thrive as conditions change. There is high confidence that increased precipitation in early spring will negatively impact farming, but the response of vegetation to future changes in seasonal temperature and moisture conditions depends on plant hardiness for medium confidence in the level of risk to specialty crops and forestry. A reduction in the length of the snow season by mid-century is highly likely under lower and higher scenarios, with very high confidence that the winter recreation industry will be negatively impacted by the end of the century under lower and higher scenarios (RCP4.5 and RCP8.5).

' evidence: "

Multiple lines of evidence show that changes in seasonal temperature and precipitation cycles have been observed in the Northeast.{{< tbib '3' '56148bf0-62f5-4ec7-8dbc-1e356e40bd42' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Projected increases in winter air temperatures under lower and higher scenarios (RCP4.5 and RCP8.5){{< tbib '3' '56148bf0-62f5-4ec7-8dbc-1e356e40bd42' >}}^,{{}} will result in shorter and milder cold seasons, a longer frost-free season,{{< tbib '3' '56148bf0-62f5-4ec7-8dbc-1e356e40bd42' >}} and decreased regional snow cover and earlier snowmelt.{{< tbib '108' '25c6b777-dbb6-49a7-8b04-4b509b58966b' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Observed seasonal changes to streamflows in response to increased winter precipitation, changes in snow hydrology,{{< tbib '112' 'f9dc4907-65ae-4582-a285-29b5d4732a9f' >}}^,{{}}^,{{}}^,{{}} and an earlier but prolonged transition into spring{{< tbib '68' '74f1e179-6237-49cd-ae8c-acf2980a5803' >}} are projected to continue.{{< tbib '105' 'dc7250b8-b20f-4b9f-818e-1729746d08c2' >}}

These changes are affecting a number of plant and animal species throughout the region, including earlier bloom times and leaf-out,{{< tbib '71' '3307a62c-ed45-4399-bcb9-f77e71b1e626' >}}^,{{}}^,{{}} spawning,{{< tbib '164' '0ce13198-a924-4762-bd5c-00519d8ae3fc' >}} migration,{{< tbib '84' '39d1cdc8-6302-4642-a4e3-457ee307946f' >}}^,{{}}^,{{}} and insect emergence,{{< tbib '74' '5d339b6f-883e-4cec-afe2-e75a0f638730' >}} as well as longer growing seasons,{{< tbib '72' '2cf345a8-4496-4f48-899f-4cbc020b8039' >}} delayed senescence, and enhanced leaf color change.{{< tbib '103' 'f947ac52-b0ce-4279-9925-63584393c70b' >}} Milder winters will likely contribute to the range expansion of wildlife and insect species,{{< tbib '399' '1985bce4-5738-4ba6-ac9a-0d676d2ce4a3' >}} increase the size of certain herbivore populations{{< tbib '78' 'c94e7da1-3648-49d7-8dc2-6ca97ec26738' >}} and their exposure to parasitism,{{< tbib '81' '440bd774-59c5-420f-9fa3-460ece82c2b4' >}}^,{{}} and increase the vulnerability of an array of plant and animal species to change.{{< tbib '66' '0095368f-c79d-47b5-89bd-856b38752d03' >}}^,{{}}^,{{}}

Warmer winters will likely contribute to declining yields for specialty crops{{< tbib '35' 'b6e8b67c-7042-4b85-b432-033983875e14' >}} and fewer operational days for logging{{< tbib '88' '8a427d3d-8b74-4ed8-8ec0-530b4a2fcdc1' >}} and snow-dependent recreation.{{< tbib '115' 'bff3f502-9bc7-4d5f-859f-636a30c71624' >}}^,{{}}^,{{}} Excess moisture is the leading cause of crop loss in the Northeast,{{< tbib '35' 'b6e8b67c-7042-4b85-b432-033983875e14' >}} and the observed increase in precipitation amount, intensity, and persistence is projected to continue under both lower and higher scenarios.{{< tbib '3' '56148bf0-62f5-4ec7-8dbc-1e356e40bd42' >}}^,{{}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/northeast/finding/key-message-18-1.yaml identifier: key-message-18-1 ordinal: 1 process: "

It is understood that authors for a regional assessment must have scientific and regional credibility in the topical areas. Each author must also be willing and interested in serving in this capacity. Author selection for the Northeast chapter proceeded as follows:

First, the U.S. Global Change Research Program (USGCRP) released a Call for Public Nominations. Interested scientists were either nominated or self-nominated and their names placed into a database. The concurrent USGCRP Call for Public Nominations also solicited scientists to serve as chapter leads. Both lists were reviewed by the USGCRP with input from the coordinating lead author (CLA) and from the National Climate Assessment (NCA) Steering Committee. All regional chapter lead (CL) authors were selected by the USGCRP at the same time. The CLA and CL then convened to review the author nominations list as a “first cut” in identifying potential chapter authors for this chapter. Using their knowledge of the Northeast’s landscape and challenges, the CLA and CL used the list of national chapter topics that would be most relevant for the region. That topical list was associated with scientific expertise and a subset of the author list.

In the second phase, the CLA and CL used both the list of nominees as well as other scientists from around the region to build an author team that was representative of the Northeast’s geography, institutional affiliation (federal agencies and academic and research institutions), depth of subject matter expertise, and knowledge of selected regional topics. Eleven authors were thus identified by December 2016, and the twelfth author was invited in April 2017 to better represent tribal knowledge in the chapter.

Lastly, the authors were contacted by the CL to determine their level of interest and willingness to serve as experts on the region's topics of water resources, agriculture and natural resources, oceans and marine ecosystems, coastal issues, health, and the built environment and urban issues.

On the due diligence of determining the region’s topical areas of focus

The first two drafts of the Northeast chapter were structured around the themes of water resources, agriculture and natural resources, oceans and marine ecosystems, coastal issues, health, and the built environment and urban issues. During the USGCRP-sponsored Regional Engagement Workshop held in Boston on February 10, 2017, feedback was solicited from approximately 150 online participants (comprising transportation officials, coastal managers, urban planners, city managers, fisheries managers, forest managers, state officials, and others) around the Northeast and other parts of the United States, on both the content of these topical areas and important focal areas for the region. Additional inputs were solicited from other in-person meetings such as the ICNet workshop and American Association of Geographers meetings, both held in April 2017. All feedback was then compiled with the lessons learned from the USGCRP CLA-CL meeting in Washington, DC, also held in April 2017. On April 28, 2017, the author team met in Burlington, Vermont, and reworked the chapter’s structure around the risk-based framing of interest to 1) changing seasonality, 2) coastal/ocean resources, 3) rural communities and livelihoods, 4) urban interconnectedness, and 5) adaptation.

" report_identifier: nca4 statement: '

The seasonality of the Northeast is central to the region’s sense of place and is an important driver of rural economies. Less distinct seasons with milder winter and earlier spring conditions (very high confidence) are already altering ecosystems and environments (high confidence) in ways that adversely impact tourism (very high confidence), farming (high confidence), and forestry (medium confidence). The region’s rural industries and livelihoods are at risk from further changes to forests, wildlife, snowpack, and streamflow (likely).

' uncertainties: "

Warmer fall temperatures affect senescence, fruit ripening, migration, and hibernation, but are less well studied in the region{{< tbib '98' 'f773b2e9-428c-455b-82f9-a4dbf065d44b' >}} and must be considered alongside other climatic factors such as drought. Projections for summer rainfall in the Northeast are uncertain,{{< tbib '4' '4de020df-232e-45f8-8d44-f864565f0b84' >}} but evaporative demand for surface moisture is expected to increase with projected increases in summer temperatures.{{< tbib '3' '56148bf0-62f5-4ec7-8dbc-1e356e40bd42' >}}^,{{}} Water use is highest during the warm season;{{< tbib '141' 'd4e6a0d4-8428-40e4-8dd9-34b0276d0d40' >}}^,{{}} how much this will affect water availability for agricultural use depends on the frequency and intensity of drought during the growing season.{{< tbib '302' '5e012394-93f7-431c-8e19-1022945a6506' >}}

" uri: /report/nca4/chapter/northeast/finding/key-message-18-1 url: ~ - chapter_identifier: northeast confidence: '

Warming ocean temperatures (high confidence), acidification (high confidence), and sea level rise (very high confidence) will alter coastal and ocean ecosystems (likely) and threaten the ecosystems services provided by the coasts and oceans (likely) in the Northeast. There is high confidence that ocean temperatures have caused shifts in the distribution, productivity, and phenology of marine species and very high confidence that high tide flooding and storm surge impacts are being amplified by sea level rise. Because much will depend on how humans choose to address or adapt to these problems, and as there is considerable uncertainty over the extent to which many of these coastal systems will be able to adapt, there is medium confidence in the level of risk to traditions and livelihoods. It is likely that under higher scenarios, sea level rise will significantly alter the coastal landscape, and rising temperatures and acidification will affect marine populations and fisheries.

' evidence: "

Warming rates on the Northeast Shelf have been higher than experienced in other ocean regions,{{< tbib '39' 'fb1f46cd-8b70-4a44-923a-66df61ffa0be' >}} and climate projections indicate that warming in this region will continue to exceed rates expected in other ocean regions.{{< tbib '48' 'f44f9474-6d98-43a9-8d7f-ee808ecaf41e' >}}^,{{}} Multiple lines of research have shown that changes in ocean temperatures and acidification have resulted in distribution,{{< tbib '7' '8b09bbe8-9f42-412e-a4d6-ef4889f56556' >}}^,{{}}^,{{}} productivity,{{< tbib '39' 'fb1f46cd-8b70-4a44-923a-66df61ffa0be' >}}^,{{}}^,{{}}^,{{}} and phenology shifts{{< tbib '155' '1dfd2171-2be3-40b2-a8e2-c0df84ec462a' >}}^,{{}}^,{{}}^,{{}}^,{{}} in marine populations. These shifts have impacted marine fisheries and prompted industry adaptations to changes.{{< tbib '155' '1dfd2171-2be3-40b2-a8e2-c0df84ec462a' >}}^,{{}}^,{{}}

Research also shows that sea level rise has been{{< tbib '12' 'b58704d1-b4ec-46d0-9dd5-e7573523951e' >}}^,{{}}^,{{}}^,{{}} and will be higher in the Northeast with respect to the rest of the United States{{< tbib '12' 'b58704d1-b4ec-46d0-9dd5-e7573523951e' >}}^,{{}}^,{{}}^,{{}} due largely to vertical land movement,{{< tbib '207' 'a26008f1-a98f-45f7-b964-9dda3dee8a0c' >}}^,{{}}^,{{}} varying atmospheric shifts and ocean dynamics,{{< tbib '210' '199b0e91-24ab-429c-ab3f-1930b96c62a0' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} and ice mass loss from the polar regions.{{< tbib '214' '4531ff9e-d432-4564-9b72-2d334532ceb8' >}} High tide flooding has increased{{< tbib '216' '91aeffdb-e82f-4645-abe9-f6ea6909e979' >}}^,{{}} and will continue to increase,{{< tbib '403' 'bbf3043e-9999-4f0e-8d0c-6012450d9d84' >}} and storm surges due to stronger and more frequent hurricanes{{< tbib '50' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}}^,{{}}^,{{}} have been and will be amplified by sea level rise.{{< tbib '217' 'b072d10e-db78-421e-a708-e2bdcb25de6e' >}}^,{{}}^,{{}}^,{{}} Climate-related coastal impacts on the landscape include greater potential for coastal flooding, erosion, overwash, barrier island breaching and disaggregation, and marsh conversion to open water,{{< tbib '12' 'b58704d1-b4ec-46d0-9dd5-e7573523951e' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} which will directly affect the ability of ecosystems to sustain many of the services they provide. Changes to salt marshes in response to sea level rise have already been observed in some coastal settings in the region, although their impacts are site specific and variable.{{< tbib '265' 'feb05a46-1a26-4007-85d7-d455699e651d' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Studies quantifying sea level rise impacts on other types of coastal settings (such as beaches) in the region are more limited; however, there is consensus on what impacts under higher rates of relative sea level rise might look like due to geologic history and modern analogs elsewhere (such as the Louisiana coast).{{< tbib '12' 'b58704d1-b4ec-46d0-9dd5-e7573523951e' >}}^,{{}}^,{{}} Although probabilistically low, worst-case sea level rise projections that account for ice sheet collapse{{< tbib '47' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}}^,{{}} would result in sea level rise rates far beyond the rates at which natural systems are likely able to adapt,{{< tbib '274' 'f4de2290-29cf-421e-b520-55e3e2fbde02' >}}^,{{}}^,{{}} affecting not only ecosystems function and services but also likely substantially changing the coastal landscape largely through inundation.{{< tbib '223' '97387e44-8bfc-413a-948c-e6dc67f5e7cd' >}}

" href: https://data.globalchange.gov/report/nca4/chapter/northeast/finding/key-message-18-2.yaml identifier: key-message-18-2 ordinal: 2 process: "

On the due diligence of determining the region’s topical areas of focus

" report_identifier: nca4 statement: '

The Northeast’s coast and ocean support commerce, tourism, and recreation that are important to the region’s economy and way of life. Warmer ocean temperatures, sea level rise, and ocean acidification (high confidence) threaten these services (likely). The adaptive capacity of marine ecosystems and coastal communities will influence ecological and socioeconomic outcomes as climate risks increase (high confidence).

' uncertainties: "

Although work to value coastal and marine ecosystems services is still evolving,{{< tbib '6' '874f9406-dd99-4e92-b64a-4542c23d0d16' >}}^,{{}}^,{{}} changes to coastal ecosystem services will depend largely on the adaptability of the coastal landscape, direct hits from storms, and rate of sea level rise, which have identified uncertainties. Lower sea level rise rates are more probable, though the timing of ice sheet collapse{{< tbib '407' 'ae82c8a3-3033-4103-91e9-926a27d1fa18' >}} and the variability of ocean dynamics are still not well understood{{< tbib '210' '199b0e91-24ab-429c-ab3f-1930b96c62a0' >}}^,{{}}^,{{}} and will dramatically affect the rate of rise.{{< tbib '47' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}}^,{{}} It is also difficult to anticipate how humans will contend with changes along the coast{{< tbib '389' '7e02aaef-bbc5-4d00-a53e-31ee9e8095f6' >}} and how adjacent natural settings will respond. Furthermore, specific tipping points for many coastal ecosystems are still not well resolved{{< tbib '275' '4071d07b-0079-4055-abaa-71db3428a613' >}}^,{{}}^,{{}} and vary due to site-specific conditions{{< tbib '224' '5fde38f4-be63-40ce-88fa-392a869e7c88' >}}^,{{}}

The Northeast Shelf is sensitive to ocean acidification, and many fisheries in the region are dependent on shell-forming organisms.{{< tbib '181' '07043123-9da3-43da-a9fa-36885cd77331' >}}^,{{}}^,{{}} However, few studies that have investigated the impacts of ocean acidification on species biology and ecology used native populations from the region{{< tbib '182' '713bb290-1c8c-4d9a-8b65-3941399b48b9' >}} or tested the effects at acidification levels expected over the next 20–40 years.{{< tbib '143' '7da52043-249d-4413-9e1a-40ed0659b2f8' >}} Moreover, there are limited studies that consider the effects of climate change in conjunction with multiple other stressors that affect marine populations.{{< tbib '39' 'fb1f46cd-8b70-4a44-923a-66df61ffa0be' >}}^,{{}}^,{{}}^,{{}} Limited understanding of the adaptive capacity of species to environmental changes presents major uncertainties in ecosystem responses to climate change.{{< tbib '143' '7da52043-249d-4413-9e1a-40ed0659b2f8' >}}^,{{}} How humans will respond to changes in ecosystems is also not well known, yet these decisions will shape how marine industries and coastal communities are affected by climate change.{{< tbib '45' '548bc141-7fd7-45bb-9c1b-e2a25bcb4e24' >}}

" uri: /report/nca4/chapter/northeast/finding/key-message-18-2 url: ~ - chapter_identifier: northeast confidence: "

There is high confidence that weather-related impacts on urban centers already experienced today will become more common under a changing climate. For the Northeast, sea level rise is projected to occur at a faster rate than the global average, potentially increasing the impact of moderate and severe coastal flooding.{{< tbib '47' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}}

By the end of the century and under a higher scenario (RCP8.5), Coupled Model Intercomparison Project Phase 5 (CMIP5) models suggest that annual average temperatures will increase by more than 9°F (16°C) for much of the region (2071–2100 compared to 1976–2005), while precipitation is projected to increase, particularly during winter and spring.{{< tbib '50' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}}

Extreme events that impact urban environments have been observed to increase over much of the United States and are projected to continue to intensify. There is high confidence that heavy precipitation events have increased in intensity and frequency since 1901, with the largest increase in the Northeast, a trend projected to continue.{{< tbib '50' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}} There is very high confidence that extreme heat events are increasing across most regions worldwide, a trend very likely to continue.{{< tbib '50' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}} Extreme precipitation from tropical cyclones has not demonstrated a clear observed trend but is expected to increase in the future.{{< tbib '50' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}}^,{{}} Research has suggested that the number of tropical cyclones will overall increase with future warming.{{< tbib '416' '4230c77b-1f8c-4452-aa52-5690aac2a72e' >}} However, this finding is contradicted by results using a high-resolution dynamical downscaling study under a lower scenario (RCP4.5), which suggests overall reduction in frequency of tropical cyclones but an increase in the occurrence of storms of Saffir–Simpson categories 4 and 5.{{< tbib '50' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}}

" evidence: "

The urban built environment and related supply and management systems are at increased risk of disruption from a variety of increasing climate risks. These risks emerge from accelerated sea level rise as well as increased frequency of coastal and estuarine flooding, intense precipitation events, urban heating and heat waves, and drought.

Coastal flooding can lead to adverse health consequences, loss of life, and damaged property and infrastructure.{{< tbib '368' '641ac0a3-aad2-4422-a632-f07117fe694a' >}} Much of the region’s major industries and cities are located along the coast, with 88% of the region’s population and 68% of the regional gross domestic product.{{< tbib '260' '9f559c9b-c78e-4593-bcbe-f07661d29e16' >}} High tide flooding is also increasingly problematic and costly.{{< tbib '47' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}} Rising sea level and amplified storm events can increase the magnitude and geographic size of a coastal flood event. The frequency of dangerous coastal flooding in the Northeast would more than triple with 2 feet of sea level rise.{{< tbib '93' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} In Boston, the areal extent of a 1% (1 in 100 chance of occurring in any given year) flood is expected to increase multifold in many coastal neighborhoods.{{< tbib '295' '4d61fbc8-2282-49e8-bb8c-e7d87075f424' >}} However, there will likely be notable variability across coastal locations. Using the 2014 U.S. National Climate Assessment’s Intermediate-High scenario for sea level rise (a global rise of 1.2 meters by 2100), the median number of flood events per year for the Northeast is projected to increase from 1 event per year experienced today to 5 events by 2030 and 25 events by 2045, with significant variation within the region.{{< tbib '410' '5f4de85b-be39-4ffd-ac94-1950932c0140' >}}

Intense precipitation events can lead to riverine and street-level flooding affecting urban environments. Over recent decades, the Northeast has experienced an increase of intense precipitation events, particularly in the spring and fall.{{< tbib '411' '9131626f-95e5-4b4c-8a4e-08183ff2fe12' >}} From 1958 to 2016, the number of heaviest 1% precipitation events (that is, an event that has a 1% chance of occurring in any given year) in the Northeast has increased by 55%.{{< tbib '58' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} A recent study suggests that this trend began rather abruptly after 1996, though uniformly across the region.{{< tbib '411' '9131626f-95e5-4b4c-8a4e-08183ff2fe12' >}}

Urban heating and heat waves threaten the health of the urban population and the integrity of the urban landscape. Due to the urban heat island effect, summer surface temperatures across Northeast cities were an average of 13°F to 16°F (7°C to 9°C) warmer than surrounding rural areas over a three-year period, 2003 to 2005.{{< tbib '412' '6b78125d-611b-402d-ab56-c409b15d52aa' >}} This is of concern, as rising temperatures increase heat- and pollution-related mortality while also stressing energy demands across the urban environment.{{< tbib '413' '5d45fe96-3a2f-4f4c-b989-406af17fc9af' >}} However, the degree of urban heat island intensity varies across cities depending on local factors such as whether the city is coastal or inland.{{< tbib '414' '8d5cd278-b5eb-4a23-aba3-79fc4b0f5544' >}} Recent analysis of mortality in major cities of the Northeast suggests that the region could experience an additional 2,300 deaths per year by 2090 from extreme heat under RCP8.5 (compared to an estimated 970 deaths per year under the lower scenario, RCP4.5) compared to 1989–2000.{{< tbib '29' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}} Another study that considered 1,692 cities around the world suggested that without mitigation, total economic costs associated with climate change could be 2.6 times higher due to the warmer temperatures in urban versus extra-urban environments.{{< tbib '415' 'e904b5f2-2c5e-4e55-8365-2ba748291939' >}}

Changes in temperature and precipitation can have dramatic impacts on urban water supply available for municipal and industrial uses. Under a higher scenario (RCP8.5), the Northeast is projected to experience cumulative losses of $730 million (discounted at 3% in 2015 dollars) due to water supply shortfalls for the period 2015 to 2099.{{< tbib '29' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}} Under a lower scenario (RCP4.5), the Northeast is projected to sustain losses of $510 million (discounted at 3% in 2015 dollars).{{< tbib '29' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}} The losses are largely projected for the more southern and coastal areas in the region.

" href: https://data.globalchange.gov/report/nca4/chapter/northeast/finding/key-message-18-3.yaml identifier: key-message-18-3 ordinal: 3 process: "

On the due diligence of determining the region’s topical areas of focus

" report_identifier: nca4 statement: '

The Northeast’s urban centers and their interconnections are regional and national hubs for cultural and economic activity. Major negative impacts on critical infrastructure, urban economies, and nationally significant historic sites are already occurring and will become more common with a changing climate. (High Confidence)

' uncertainties: "

Projecting changes in urban pollution and air quality under a changing climate is challenging given the associated complex chemistry and underlying factors that influence it. For example, fine particulates (PM_2.5; that is, particles with a diameter of or less than 2.5 micrometers) are affected by cloud processes and precipitation, amongst other meteorological processes, leading to considerable uncertainty in the geographic distribution and overall trend in both modeling analysis and the literature.{{< tbib '29' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}} Land use can also play an unexpected role, such as planting trees as a mitigation option that may lead to increases in volatile organic compounds (VOCs), which, in a VOC-limited environment that can exist in some urban areas such as New York City, may increase ozone concentrations (however, it is noted that most of the Northeast region is limited by the availability of nitrogen oxides).{{< tbib '327' '5b52af56-61c6-4663-9d7d-302e8570800f' >}}

Interdependencies among infrastructure sectors can lead to unexpected and amplified consequences in response to extreme weather events. However, it is unclear how society may choose to invest in the built environment, possibly strengthening urban infrastructure to plausible future conditions.

" uri: /report/nca4/chapter/northeast/finding/key-message-18-3 url: ~ - chapter_identifier: northeast confidence: '

There is very high confidence that extreme weather, warmer temperatures, degradation of air and water quality, and sea level rise threaten the health and well-being of people in the Northeast. There is very high confidence that these climate-related environmental changes will lead to additional adverse health-related impacts and costs, including premature deaths, more emergency department visits and hospitalizations, and lower quality of life. There is very high confidence that climate-related health impacts will vary by location, age, current health, and other characteristics of individuals and communities.

' evidence: "

Extreme storms and temperatures, overall warmer temperatures, degradation of air and water quality, and sea level rise are all associated with adverse health outcomes from heat,{{< tbib '20' '6b3cd0ec-1e3e-42e8-ad82-5c12ed7ab0e8' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} poor air quality,{{< tbib '324' '54a66159-1675-43bb-b5d3-a9b7f283e4de' >}}^,{{}}^,{{}} disease-transmitting vectors,{{< tbib '67' '953d1436-e0d0-426c-8dcc-68e5c02eef30' >}}^,{{}}^,{{}} contaminated food and water,{{< tbib '322' 'd746e578-c6fd-4b73-8a3c-d91365668348' >}}^,{{}}^,{{}}^,{{}} harmful algal blooms,{{< tbib '335' '59d0bcfb-805b-472d-b6fe-3b70bacc3d25' >}} and traumatic stress or health service disruption.{{< tbib '17' '15df801e-f052-4327-9b08-47c13d894ea7' >}}^,{{}} The underlying susceptibility of populations determines whether or not there are health impacts from an exposure and the severity of such impacts.{{< tbib '307' 'd7ef401f-6a34-4410-ab1e-4690f3c4d161' >}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/northeast/finding/key-message-18-4.yaml identifier: key-message-18-4 ordinal: 4 process: "

On the due diligence of determining the region’s topical areas of focus

" report_identifier: nca4 statement: '

Changing climate threatens the health and well-being of people in the Northeast through more extreme weather, warmer temperatures, degradation of air and water quality, and sea level rise (very high confidence). These environmental changes are expected to lead to health-related impacts and costs, including additional deaths, emergency room visits and hospitalizations, and a lower quality of life (very high confidence). Health impacts are expected to vary by location, age, current health, and other characteristics of individuals and communities (very high confidence).

' uncertainties: "

Uncertainty remains in projections of the magnitude of future changes in particulate matter, humidity, and wildfires and how these changes may influence health risks. For example, health effects of future extreme heat may be exacerbated by future changes in absolute or relative humidity.

Health impacts are ultimately determined by not just the environmental hazard but also the amount of exposure, size and underlying susceptibility of the exposed population, and other factors such as health insurance coverage and access to timely healthcare services. In projecting future health risks, researchers acknowledge these challenges and use different analytic approaches to address this uncertainty or note it as a limitation.{{< tbib '23' '028a4c4b-3a7f-47b3-8a78-432fd7840f21' >}}^,{{}}^,{{}}

In addition, there is a paucity of literature that considers the joint or cumulative impacts on health of multiple climatic hazards. Additional areas where the literature base is limited include specific health impacts related to different types of climate-related migration, the impact of climatic factors on mental health, and the specific timing and geographic range of shifting disease-carrying vectors.

" uri: /report/nca4/chapter/northeast/finding/key-message-18-4 url: ~ - chapter_identifier: northeast confidence: '

There is high confidence that there are communities in the Northeast undertaking planning efforts to reduce risks posed from climate change and medium confidence that they are implementing climate adaptation. There is high confidence that decision support tools are informative and medium confidence that these communities are using decision support tools to find solutions for adaptation that are workable. There is high confidence that early adoption is occurring in some communities and that this provides a foundation for future efforts. This Key Message does not address trends into the future, and therefore likelihood is not applicable.

' evidence: "

Reports on climate adaptation and resilience planning have been published by city, state, and tribal governments and by regional and federal agencies in the Northeast. Examples include the Interstate Commission on the Potomac River Basin (for the Washington, DC, metropolitan area),{{< tbib '304' '3258fcdc-5c7f-46a1-be18-29e440a0489a' >}} Boston,{{< tbib '295' '4d61fbc8-2282-49e8-bb8c-e7d87075f424' >}} the Port Authority of New York and New Jersey,{{< tbib '357' '62f465d8-b42c-42f7-81ec-0f5378ba9f19' >}} the St. Regis Mohawk Tribe,{{< tbib '360' '479edcdc-3e35-4859-86aa-5733316e0aa1' >}} the U.S. Army Corps of Engineers,{{< tbib '368' '641ac0a3-aad2-4422-a632-f07117fe694a' >}} the State of Maine,{{< tbib '381' 'a3a5fe5c-f49b-4c6b-a008-5647194a88a7' >}} and southeastern Connecticut.{{< tbib '417' '4afcd82e-a3d5-4d98-80e9-33ce546fabd9' >}} Structured decision-making is being applied to design management plans, determine research needs, and allocate resources{{< tbib '365' 'eddcff40-a0a0-426d-880b-a73730e9497f' >}} to preserve habitat and resources throughout the region.{{< tbib '151' '68fcc8c6-b20a-4739-aae6-e98b893d5163' >}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/northeast/finding/key-message-18-5.yaml identifier: key-message-18-5 ordinal: 5 process: "

On the due diligence of determining the region’s topical areas of focus

" report_identifier: nca4 statement: '

Communities in the Northeast are proactively planning (high confidence) and implementing (medium confidence) actions to reduce risks posed by climate change. Using decision support tools to develop and apply adaptation strategies informs both the value of adopting solutions and the remaining challenges (high confidence). Experience since the last assessment provides a foundation to advance future adaptation efforts (high confidence).

' uncertainties: '

The percentage of communities in the Northeast that are planning for climate adaptation and resilience and the percentage of those using decision support tools are not known. More case studies would be needed to evaluate the effectiveness of adaptation actions.

' uri: /report/nca4/chapter/northeast/finding/key-message-18-5 url: ~ - chapter_identifier: southeast confidence: '

There is very high confidence that southeastern cities will likely be impacted by climate change, especially in the areas of infrastructure and human health.

' evidence: "

Multiple studies have projected that urban areas, including those in the Southeast, will be adversely affected by climate change in a variety of ways. This includes impacts on infrastructure{{< tbib '41' '00e98394-26f1-45da-a5a3-e79b2b1a356f' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} and human health.{{< tbib '30' '9cef8d69-7596-480a-81b6-abd09ff1c1e3' >}}^,{{}}^,{{}}^,{{}} Increases in climate-related impacts have already been observed in some Southeast metropolitan areas (e.g., Habeeb et al. 2015, Tzung-May Fu et al. 2015{{< tbib '12' '4b55e347-52cb-4301-9eea-ad3858c6fc1d' >}}^,{{}}).

Southeastern cities may be more vulnerable than cities in other regions of the United States due to the climate being more conducive to some vector-borne diseases, the presence of multiple large coastal cities at low elevation that are vulnerable to flooding and storms, and a rapidly growing urban and coastal population.{{< tbib '22' '446a98e1-77e4-4654-9125-277eab402a9f' >}}^,{{}}^,{{}}

Many city and county governments, utilities, and other government and service organizations have already begun to plan and prepare for the impacts of climate change (e.g., Gregg et al. 2017.; FTA 2013; City of Fayetteville 2017; City of Charleston 2015; City of New Orleans 2015; Tampa Bay Water 2014; EPA 2015; City of Atlanta 2015, 2017; Southeast Florida Regional Climate Change Compact 2017{{< tbib '44' '29100037-c24e-4309-b6a2-e4397db7ed01' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}). A wide variety of adaptation options are available, offering opportunities to improve the climate resilience, quality of life, and economy of urban areas.{{< tbib '77' '7bdd9d20-6e83-40ab-8d50-68272c2b3dc9' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/southeast/finding/key-message-19-1.yaml identifier: key-message-19-1 ordinal: 1 process: "

Prior to identifying critical issues for the Southeast assessment focuses for the Fourth National Climate Assessment (NCA4), the Chapter Lead (CL) contacted numerous professional colleagues representing various geographic areas (e.g., Florida, Louisiana, and South Carolina) for expert opinions on critical climate change related issues impacting the region, with a particular emphasis on emerging issues since the Third National Climate Assessment (NCA3) effort.{{< tbib '77' '7bdd9d20-6e83-40ab-8d50-68272c2b3dc9' >}} Following those interviews, the CL concluded that the most pressing climate change issues to focus on for the NCA4 effort were extreme events, flooding (both from rainfall and sea level rise), wildfire, health issues, ecosystems, and adaptation actions. Authors with specific expertise in each of these areas were sought, and a draft outline built around these issues was developed. Further refinement of these focal areas occurred in conjunction with the public Regional Engagement Workshop, held on the campus of North Carolina State University in March 2017 and in six satellite locations across the Southeast region. The participants agreed that the identified issues were important and suggested the inclusion of several other topics, including impacts on coastal and rural areas and people, forests, and agriculture. Based on the subsequent authors’ meeting and input from NCA staff, the chapter outline and Key Messages were updated to reflect a risk-based framing in the context of a new set of Key Messages. The depth of discussion for any particular topic and Key Message is dependent on the availability of supporting literature and chapter length limitations.

" report_identifier: nca4 statement: '

Many southeastern cities are particularly vulnerable to climate change compared to cities in other regions, with expected impacts to infrastructure and human health (very likely, very high confidence). The vibrancy and viability of these metropolitan areas, including the people and critical regional resources located in them, are increasingly at risk due to heat, flooding, and vector-borne disease brought about by a changing climate (likely, high confidence). Many of these urban areas are rapidly growing and offer opportunities to adopt effective adaptation efforts to prevent future negative impacts of climate change (very likely, high confidence).

' uncertainties: '

Population projections are inherently uncertain over long time periods, and shifts in immigration or migration rates and shifting demographics will influence urban vulnerabilities to climate change. The precise impacts on cities are difficult to project. The scope and scale of adaptation efforts, which are already underway, will affect future vulnerability and risk. Technological developments (such as a potential shift in transportation modes) will also affect the scope and location of risk within cities. Newly emerging pathogens could increase risk of disease in the future, while successful adaptations could reduce public health risk.

' uri: /report/nca4/chapter/southeast/finding/key-message-19-1 url: ~ - chapter_identifier: southeast confidence: '

There is high confidence that flood risks will very likely increase in coastal and low-lying regions of the Southeast due to rising sea level and an increase in extreme rainfall events. There is high confidence that Southeast coastal cities are already experiencing record numbers of high tide flooding events, and without significant adaptation measures, it is likely they will be impacted by daily high tide flooding.

' evidence: "

Multiple lines of research have shown that global sea levels have increased in the past and are projected to continue to accelerate in the future due to increased global temperature and that higher local sea level rise rates in the Mid-Atlantic and Gulf Coasts have occurred.{{< tbib '51' '3bae2310-7572-47e2-99a4-9e4276764934' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}

Annual occurrences of high tide flooding have increased, causing several Southeast coastal cities to experience all-time records of occurrences that are posing daily risks.{{< tbib '1' 'df029572-7e7a-4f65-91c2-da86756620c4' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}

There is scientific consensus that sea level rise will continue to cause increases in high tide flooding in the Southeast as well as impact the frequency and duration of extreme water level events, causing an increase in the vulnerability of coastal populations and property.{{< tbib '1' 'df029572-7e7a-4f65-91c2-da86756620c4' >}}^,{{}}^,{{}}^,{{}}^,{{}}

In the future, coastal flooding is projected to become more serious, disruptive, and costly as the frequency, depth, and inland extent grow with time.{{< tbib '1' 'df029572-7e7a-4f65-91c2-da86756620c4' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}

Many analyses have determined that extreme rainfall events have increased in the Southeast, and under higher scenarios, the frequency and intensity of these events are projected to increase.{{< tbib '19' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}}^,{{}}^,{{}}

Rainfall records have shown that since NCA3, many intense rainfall events (approaching 500-year events) have occurred in the Southeast, with some causing billions of dollars in damage and many deaths.{{< tbib '68' '03e51664-273d-40e5-8af0-ab885436ac8e' >}}^,{{}}^,{{}}

The flood events in Baton Rouge, Louisiana, in 2016 and in South Carolina in 2015 provide real examples of how vulnerable inland and coastal communities are to extreme rainfall events.{{< tbib '81' '6acb342f-f144-4fad-ae46-a6ff80f812cf' >}}^,{{}}^,{{}}

The socioeconomic impacts of climate change on the Southeast is a developing research field.{{< tbib '65' 'e1f4f1b2-6b77-465a-bddb-ed992079deea' >}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/southeast/finding/key-message-19-2.yaml identifier: key-message-19-2 ordinal: 2 process: "

" report_identifier: nca4 statement: '

The Southeast’s coastal plain and inland low-lying regions support a rapidly growing population, a tourism economy, critical industries, and important cultural resources that are highly vulnerable to climate change impacts (very likely, very high confidence). The combined effects of changing extreme rainfall events and sea level rise are already increasing flood frequencies, which impacts property values and infrastructure viability, particularly in coastal cities. Without significant adaptation measures, these regions are projected to experience daily high tide flooding by the end of the century (likely, high confidence).

' uncertainties: '

The amount of confidence associated with the historical rate of global sea level rise is impacted by the sparsity of tide gauge records and historical proxies as well as different statistical approaches for estimating sea level change. The amount of unpredictability in future projected rates of sea level rise is likely caused by a range of future climate scenarios projections and rate of ice sheet mass changes. Flooding events are highly variable in both space and time. Detection and attribution of flood events are difficult due to multiple variables that cause flooding.

' uri: /report/nca4/chapter/southeast/finding/key-message-19-2 url: ~ - chapter_identifier: southeast confidence: '

There is high confidence that climate change (e.g., changing winter temperatures extremes, changing fire regimes, rising sea levels and hurricanes, warming ocean temperatures, and more extreme rainfall and drought) will very likely affect natural systems in the Southeast region. These climatic drivers play critical roles and greatly influence the distribution, structure, and functioning of ecosystems; hence, changes in these climatic drivers will transform ecosystems in the region and greatly alter the distribution and abundance of species.

' evidence: "

Winter temperature extremes, fire regimes, sea levels, hurricanes, rainfall extremes, drought extremes, and warming ocean temperatures greatly influence the distribution, abundance, and performance of species and ecosystems.

Winter air temperature extremes (for example, freezing and chilling events) constrain the northern limit of many tropical and subtropical species.{{< tbib '30' '9cef8d69-7596-480a-81b6-abd09ff1c1e3' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} In the future, warmer winter temperatures are expected to facilitate the northward movement of cold-sensitive species, often at the expense of cold-tolerant species.{{< tbib '132' '30e64f09-40ad-4aa8-8a20-ecc203f91914' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Certain ecosystems are located near thresholds where small changes in winter air temperature regimes can trigger comparatively large and abrupt landscape-scale ecological changes (i.e., ecological regime shifts).{{< tbib '135' '31446ba7-4409-483b-b467-ae773a9ba950' >}}^,{{}}^,{{}}

Changing fire regimes are expected to have a large impact on natural systems. Fire has historically played an important role in the region, and ecological diversity in many southeastern natural systems is dependent upon fire.{{< tbib '115' '56b229a1-fc34-4010-9b6e-3ab94c77b49c' >}}^,{{}}^,{{}}^,{{}} In the future, rising temperatures and increases in the duration and intensity of drought are expected to increase wildfire occurrence and also reduce the effectiveness of prescribed fire.{{< tbib '3' 'de4a77df-03ba-4319-a13f-7fdefbb353a5' >}}^,{{}}^,{{}}^,{{}}

Hurricanes and rising sea levels are aspects of climate change that will have a tremendous effect on coastal ecosystems in the Southeast. Historically, coastal ecosystems in the region have adjusted to sea level rise via vertical and/or horizontal movement across the landscape.{{< tbib '125' '6c5f197a-cfe5-4433-9bce-2c53a1939f2d' >}}^,{{}}^,{{}}^,{{}} As sea levels rise in the future, some coastal ecosystems will be submerged and converted to open water, and some coastal ecosystems will move inland at the expense of upslope and upriver ecosystems.{{< tbib '203' '79a38ee0-fa95-411e-bb5d-48d0e34554cb' >}}^,{{}} Since coastal terrestrial and freshwater ecosystems are highly sensitive to increases in inundation and/or salinity, sea level rise will result in the comparatively rapid conversion of these systems to tidal saline habitats. In addition to sea level rise, climate change is expected to increase the impacts of hurricanes; the high winds, storm surges, inundation, and salts that accompany hurricanes will have large ecological impacts to terrestrial and freshwater ecosystems.{{< tbib '209' 'babe9483-5a5a-4167-b004-9e80ab8f0db1' >}}^,{{}}

Climate change is expected to intensify the hydrologic cycle and increase the frequency and severity of extreme events. Extreme drought events are expected to become more frequent and severe. Drought and extreme heat can result in tree mortality and transform southeastern forested ecosystems.{{< tbib '217' 'a073cf8e-8d74-4f11-bfe2-d3494b9bcc7a' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Drought can also affect aquatic and wetland ecosystems.{{< tbib '224' '989a57fc-3c12-4ed1-a80d-0c765a119a3f' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Extreme rainfall events are also expected to become more frequent and severe in the future. The prolonged inundation and lack of oxygen that result from extreme rainfall events can also result in mortality and large impacts to natural systems.{{< tbib '233' 'f3efb037-04cf-442a-8d41-812d21f7a6c8' >}} In combination, future increases in both extreme drought and extreme rainfall are expected to transform many southeastern ecosystems.

Warming ocean temperatures due to climate change are expected to have a large effect on marine and coastal ecosystems.{{< tbib '234' 'cfdaea11-95e2-4789-914b-74901b2f26b0' >}}^,{{}}^,{{}} Many species are sensitive to small changes in ocean temperature; hence, the distribution and abundance of marine organisms are expected to be greatly altered by increasing ocean temperatures. For example, the distribution of tropical herbivorous fish has been expanding in response to warmer waters, which has resulted in the tropicalization of some temperate marine ecosystems and decreases in the cover of valuable macroalgal plant communities.{{< tbib '179' 'e4313895-fb80-4d31-906c-2fadb9da71de' >}} A decrease in the growth of sea turtles in the West Atlantic has been linked to higher ocean temperatures.{{< tbib '237' '6a37fde2-5a4b-470e-b14b-304ca956b4b6' >}} The impacts to coral reef ecosystems have been and are expected to be particularly dire. Coral reef mortality in the Florida Keys and across the globe has been very high in recent decades, due in part to warming ocean temperatures, nutrient enrichment, overfishing, and coastal development.{{< tbib '240' 'b09adbe5-6a17-4d3c-ab96-b3d9e306af67' >}}^,{{}}^,{{}}^,{{}}^,{{}} Coral elevation and volume in the Florida Keys have been declining in recent decades,{{< tbib '245' '5c1b02bb-0002-4f57-9a64-68a8d0539cfa' >}} and present-day temperatures in the region are already close to bleaching thresholds; hence, it is likely that many of the remaining coral reefs in the Southeast will be lost in the coming decades.{{< tbib '246' 'bd3dbfa7-8dc4-4442-9cf2-14f583dc4a36' >}}^,{{}} In addition to warming temperatures, accelerated ocean acidification is also expected to contribute to coral reef mortality and decline.{{< tbib '248' 'e684169c-60a2-4c78-a724-36fb93fb385a' >}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/southeast/finding/key-message-19-3.yaml identifier: key-message-19-3 ordinal: 3 process: "

" report_identifier: nca4 statement: '

The Southeast’s diverse natural systems, which provide many benefits to society, will be transformed by climate change (very likely, high confidence). Changing winter temperature extremes, wildfire patterns, sea levels, hurricanes, floods, droughts, and warming ocean temperatures are expected to redistribute species and greatly modify ecosystems (very likely, high confidence). As a result, the ecological resources that people depend on for livelihood, protection, and well-being are increasingly at risk, and future generations can expect to experience and interact with natural systems that are much different than those that we see today (very likely, high confidence).

' uncertainties: '

In the Southeast, winter temperature extremes, fire regimes, sea level fluctuations, hurricanes, extreme rainfall, and extreme drought all play critical roles and greatly influence the distribution, structure, and function of species and ecosystems. Changing climatic conditions (particularly, changes in the frequency and severity of climate extremes) are, however, difficult to replicate via experimental manipulations; hence, ecological responses to future climate regimes have not been fully quantified for all species and ecosystems. Natural ecosystems are complex and governed by many interacting biotic and abiotic processes. Although it is possible to make general predictions of climate change effects, specific future ecological transformations can be difficult to predict, especially given the number of interacting and changing biotic and abiotic factors in any specific location. Uncertainties in the range of potential future changes in multiple and concurrent facets of climate and land-use change also affect our ability to predict changes to natural systems.

' uri: /report/nca4/chapter/southeast/finding/key-message-19-3 url: ~ - chapter_identifier: southeast confidence: '

There is high confidence that climate change (e.g., rising temperatures, changing fire regimes, rising sea levels, and more extreme rainfall and drought) will very likely affect agricultural and forest products industries, potentially resulting in economic impacts. There is high confidence that increases in temperature are very likely to increase heat-related illness, deaths, and loss of labor productivity without greater adaptation efforts.

' evidence: "

Analysis of the sensitivity of some manufacturing sectors to climate changes anticipates secondary risks associated with crop and livestock productivity.{{< tbib '64' '9f559c9b-c78e-4593-bcbe-f07661d29e16' >}}^,{{}}

Multiple analyses anticipate that energy- or water-intensive industries could face water stress and increased energy costs.{{< tbib '8' '75a38932-a8a4-4eeb-b94c-bbb65b580efe' >}}^,{{}}^,{{}}^,{{}}

A large body of evidence addresses the sensitivity of many crops grown in the Southeast to changing climate conditions including increased temperatures, decreased summer rainfall, drought, and change in the timing and duration of chill periods.{{< tbib '7' 'cc31a438-8e10-4957-88f9-cb6e763e2b5e' >}}^,{{}} Extensive research documents livestock sensitivity to heat stress.{{< tbib '7' 'cc31a438-8e10-4957-88f9-cb6e763e2b5e' >}}

Multiple lines of evidence indicate that forests are likely to be impacted by changing climate, particularly moisture regimes and potential changes in wildfire activity.{{< tbib '191' 'a182cf3b-2113-4680-99e8-4e17abed758a' >}}^,{{}}^,{{}}^,{{}} There is extensive research on heat-related illness and mortality among those living and working in the Southeast. While there is more evidence focused on urban areas, limited research has identified higher levels of heat-related illness in rural areas.{{< tbib '280' 'b5eb05f2-a3f3-4265-b1e9-10a9c382101c' >}}^,{{}} Research on occupational heat-related mortality identifies some of the Nation’s highest levels in southeastern states.{{< tbib '282' '7a2f62de-a38c-46dc-94ed-656cbf3e625d' >}} Computer model simulations of heat-related reductions in labor productivity anticipate the greatest losses will occur in the Southeast. However, these models do not account for adaptations that may reduce estimated losses.{{< tbib '35' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}^,{{}} By the end of the century, mean annual electricity costs are estimated at $3.3 billion each year under RCP8.5 (model range: $2.4 to $4.2 billion; in 2015 dollars, undiscounted) and mean $1.2 billion each year under RCP4.5 (model range $0.9 to $1.9 billion; in 2015 dollars, undiscounted).{{< tbib '35' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}

Rural communities tend to be vulnerable due to factors such as demography, occupations, earnings, literacy, and poverty incidence.{{< tbib '8' '75a38932-a8a4-4eeb-b94c-bbb65b580efe' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Reducing the stress created by such factors can improve resilience.{{< tbib '9' '6f8b234a-206a-498c-af9d-fb4b9b355d0a' >}}^,{{}} The availability and accessibility of planning and health services to support coping with climate-related stresses are limited in the rural Southeast.{{< tbib '288' 'bc63cd69-0f13-4d07-8854-1e0e759a31b2' >}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/southeast/finding/key-message-19-4.yaml identifier: key-message-19-4 ordinal: 4 process: "

" report_identifier: nca4 statement: '

Rural communities are integral to the Southeast’s cultural heritage and to the strong agricultural and forest products industries across the region. More frequent extreme heat episodes and changing seasonal climates are projected to increase exposure-linked health impacts and economic vulnerabilities in the agricultural, timber, and manufacturing sectors (very likely, high confidence). By the end of the century, over one-half billion labor hours could be lost from extreme heat-related impacts (likely, medium confidence). Such changes would negatively impact the region’s labor-intensive agricultural industry and compound existing social stresses in rural areas related to limited local community capabilities and associated with rural demography, occupations, earnings, literacy, and poverty incidence (very likely, high confidence). Reduction of existing stresses can increase resilience (very likely, high confidence).

' uncertainties: "

There are limited studies documenting direct connections between climate changes and economic impacts. Models are limited in their ability to incorporate adaptation that may reduce losses. These factors restrict the potential to strongly associate declines in agricultural and forest productivity with the level of potential economic impact.

Projections of potential change in the frequency and extent of wildfires depend in part on models of future population growth and human behavior, which are limited, adding to the uncertainty associated with climate and forest modeling.

Many indicators of vulnerability are dynamic, so that adaptation and other changes can affect the patterns of vulnerability to heat and other climate stressors over time. Limited studies indicate concerns over the planning and preparedness of capacity at local levels; however, information is limited.

Projected labor hours lost vary by global climate model, time frame, and scenario, with a mean of 0.57 and a model range of 0.34–0.82 billion labor hours lost each year for RCP8.5 by 2090. The annual mean projected losses are roughly halved (0.28 billion labor hours) and with a model range from 0.19 to 0.43 billion labor hours lost under RCP4.5 by 2090.{{< tbib '35' '0b30f1ab-e4c4-4837-aa8b-0e19faccdb94' >}}

" uri: /report/nca4/chapter/southeast/finding/key-message-19-4 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is very high confidence for a major human influence on climate.

Assessments of the natural forcings of solar irradiance changes and volcanic activity show with very high confidence that both forcings are small over the industrial era relative to total anthropogenic forcing. Total anthropogenic forcing is assessed to have become larger and more positive during the industrial era, while natural forcings show no similar trend.

' evidence: "

The Key Message and supporting text summarize extensive evidence documented in the climate science literature and are similar to statements made in previous national (NCA3){{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} and international{{< tbib '249' 'f03117be-ccfe-4f88-b70a-ffd4351b8190' >}} assessments. The human effects on climate have been well documented through many papers in the peer-reviewed scientific literature (e.g., see Fahey et al. 2017{{< tbib '18' '0615b4ff-d185-4e14-9d4d-5bea1ce6ca51' >}} and Knutson et al. 2017{{< tbib '16' '0725eae6-7458-4ec2-8f66-880d88118148' >}} for more discussion of supporting evidence).

The finding of an increasingly strong positive forcing over the industrial era is supported by observed increases in atmospheric temperatures (see Wuebbles et al. 2017{{< tbib '10' '666daffe-2c3b-4e2d-9157-16b989860618' >}}) and by observed increases in ocean temperatures.{{< tbib '10' '666daffe-2c3b-4e2d-9157-16b989860618' >}}^,{{}}^,{{}} The attribution of climate change to human activities is supported by climate models, which are able to reproduce observed temperature trends when radiative forcing from human activities is included and considerably deviate from observed trends when only natural forcings are included (Wuebbles et al. 2017; Knutson et al. 2017, Figure 3.1^{{{< tbib '10' '666daffe-2c3b-4e2d-9157-16b989860618' >}}^,{{< tbib '16' '0725eae6-7458-4ec2-8f66-880d88118148' >}}}).

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-1.yaml identifier: key-message-2-1 ordinal: 1 process: "

This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),{{< tbib '208' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}} a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.

The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,{{< tbib '208' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}} e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,{{< tbib '208' '75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1' >}} and what is new since the release of the Third NCA in 2014.{{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}}

" report_identifier: nca4 statement: '

Global climate is changing rapidly compared to the pace of natural variations in climate that have occurred throughout Earth’s history. Global average temperature has increased by about 1.8°F from 1901 to 2016, and observational evidence does not support any credible natural explanations for this amount of warming; instead, the evidence consistently points to human activities, especially emissions of greenhouse or heat-trapping gases, as the dominant cause. (Very High Confidence)

' uncertainties: "

Key remaining uncertainties relate to the precise magnitude and nature of changes at global, and particularly regional, scales and especially for extreme events and our ability to simulate and attribute such changes using climate models. The exact effects from land-use changes relative to the effects from greenhouse gas emissions need to be better understood.

The largest source of uncertainty in radiative forcing (both natural and anthropogenic) over the industrial era is quantifying forcing by aerosols. This finding is consistent across previous assessments (e.g., IPCC 2007, IPCC 2013{{< tbib '249' 'f03117be-ccfe-4f88-b70a-ffd4351b8190' >}}^,{{}}).

Recent work has highlighted the potentially larger role of variations in ultraviolet solar irradiance, versus total solar irradiance, in solar forcing. However, this increase in solar forcing uncertainty is not sufficiently large to reduce confidence that anthropogenic activities dominate industrial-era forcing.

" uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-1 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is very high confidence in the likelihood of the existence of positive feedbacks and tipping elements based on a large body of literature published over the last 25 years that draws from basic physics, observations, paleoclimate data, and modeling.

There is very high confidence that some feedbacks can be quantified, others are known but cannot be quantified, and others may yet exist that are currently unknown.

There is very high confidence that the models are incomplete representations of the real world; and there is medium confidence that their tendency is to under- rather than overestimate the amount of long-term future change.

' evidence: "

This Key Message is based on a large body of scientific literature recently summarized by Lenton et al. (2008),{{< tbib '197' 'd64a3dbf-d45e-49de-98b9-b4ea32da888f' >}} NRC (2013),{{< tbib '339' '3dcd5a73-de83-4b37-884a-5236407c170e' >}} and Kopp et al. (2016).{{< tbib '198' '08bc6610-586b-421c-a788-f5e18781ac52' >}} As NRC (2013){{< tbib '339' '3dcd5a73-de83-4b37-884a-5236407c170e' >}} states, “A study of Earth’s climate history suggests the inevitability of ‘tipping points’—thresholds beyond which major and rapid changes occur when crossed—that lead to abrupt changes in the climate system” and “Can all tipping points be foreseen? Probably not. Some will have no precursors, or may be triggered by naturally occurring variability in the climate system. Some will be difficult to detect, clearly visible only after they have been crossed and an abrupt change becomes inevitable.” As IPCC AR5 WG1 Chapter 12, Section 12.5.5{{< tbib '26' 'b3bbc7b5-067e-4c23-8d9b-59faee21e58e' >}} further states, “A number of components or phenomena within the Earth system have been proposed as potentially possessing critical thresholds (sometimes referred to as tipping points) beyond which abrupt or nonlinear transitions to a different state ensues.” Collins et al. (2013){{< tbib '26' 'b3bbc7b5-067e-4c23-8d9b-59faee21e58e' >}} further summarize critical thresholds that can be modeled and others that can only be identified.

This Key Message is also based on the conclusions of IPCC AR5 WG1,{{< tbib '249' 'f03117be-ccfe-4f88-b70a-ffd4351b8190' >}} specifically Chapter 7;{{< tbib '196' 'a46eaad1-5c17-46f7-bba6-d3fee718a092' >}} the state of the art of global models is briefly summarized in Hayhoe et al. (2017).{{< tbib '24' '9c909a77-a1d9-477d-82fc-468a6b1af771' >}} This Key Message is also based upon the tendency of global climate models to underestimate, relative to geological reconstructions, the magnitude of both long-term global mean warming and the amplification of warming at high latitudes in past warm climates (e.g., Salzmann et al. 2013, Goldner et al. 2014, Caballeo and Huber 2013, Lunt et al. 2012{{< tbib '199' '9f061a0a-e32d-417f-8404-c5ad0d4b01f4' >}}^,{{}}^,{{}}^,{{}}).

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-10.yaml identifier: key-message-2-10 ordinal: 10 process: "

" report_identifier: nca4 statement: '

The climate change resulting from human-caused emissions of carbon dioxide will persist for decades to millennia. Self-reinforcing cycles within the climate system have the potential to accelerate human-induced change and even shift Earth’s climate system into new states that are very different from those experienced in the recent past. Future changes outside the range projected by climate models cannot be ruled out (very high confidence), and due to their systematic tendency to underestimate temperature change during past warm periods, models may be more likely to underestimate than to overestimate long-term future change (medium confidence).

' uncertainties: '

The largest uncertainties are 1) whether proposed tipping elements actually undergo critical transitions, 2) the magnitude and timing of forcing that will be required to initiate critical transitions in tipping elements, 3) the speed of the transition once it has been triggered, 4) the characteristics of the new state that results from such transition, and 5) the potential for new positive feedbacks and tipping elements to exist that are yet unknown.

The largest uncertainties in models are structural: are the models including all the important components and relationships necessary to model the feedbacks and, if so, are these correctly represented in the models?

' uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-10 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is very high confidence for continued changes in climate and high confidence for the levels shown in the Key Message.

' evidence: "

The Key Message and supporting text summarize extensive evidence documented in the climate science literature and are similar to statements made in previous national (NCA3){{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} and international{{< tbib '249' 'f03117be-ccfe-4f88-b70a-ffd4351b8190' >}} assessments. The projections for future climate have been well documented through many papers in the peer reviewed scientific literature (e.g., see Hayhoe et al. 2017{{< tbib '24' '9c909a77-a1d9-477d-82fc-468a6b1af771' >}} for descriptions of the scenarios and the models used).

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-2.yaml identifier: key-message-2-2 ordinal: 2 process: "

" report_identifier: nca4 statement: '

Earth’s climate will continue to change over this century and beyond (very high confidence). Past mid-century, how much the climate changes will depend primarily on global emissions of greenhouse gases and on the response of Earth’s climate system to human-induced warming (very high confidence). With significant reductions in emissions, global temperature increase could be limited to 3.6°F (2°C) or less compared to preindustrial temperatures (high confidence). Without significant reductions, annual average global temperatures could increase by 9°F (5°C) or more by the end of this century compared to preindustrial temperatures (high confidence).

' uncertainties: '

Key remaining uncertainties relate to the precise magnitude and nature of changes at global, and particularly regional scales, and especially for extreme events and our ability to simulate and attribute such changes using climate models. Of particular importance are remaining uncertainties in the understanding of feedbacks in the climate system, especially in ice–albedo and cloud cover feedbacks. Continued improvements in climate modeling to represent the physical processes affecting the Earth’s climate system are aimed at reducing uncertainties. Enhanced monitoring and observation programs also can help improve the understanding needed to reduce uncertainties.

' uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-2 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is very high confidence in measurements that show increases in the ocean heat content and warming of the ocean, based on the agreement of different methods. However, long-term data in total ocean heat uptake in the deep ocean are sparse, leading to limited knowledge of the transport of heat between and within ocean basins.

Major ocean deoxygenation is taking place in bodies of water inland, at estuaries, and in the coastal and the open ocean (high confidence). Regionally, the phenomenon is exacerbated by local changes in weather, ocean circulation, and continental inputs to the oceans.

' evidence: "

The Key Message and supporting text summarize the evidence documented in climate science literature as summarized in Rhein et al. (2013).{{< tbib '31' 'bc140b4c-c2d9-4d99-a684-5c054dc5134f' >}} Oceanic warming has been documented in a variety of data sources, most notably by the World Ocean Circulation Experiment (WOCE),{{< tbib '251' '4ef3eb98-3ce7-4c94-8b1b-9a09ee951bfd' >}} Argo,{{< tbib '252' '295cc0c4-536f-49c5-abdc-3a3b4916fdba' >}} and the Extended Reconstructed Sea Surface Temperature v4 (ERSSTv4).{{< tbib '253' '865e132e-dd4a-4195-9ea0-c3c7d32d447e' >}} There is particular confidence in calculated warming for the time period since 1971 due to increased spatial and depth coverage and the level of agreement among independent sea surface temperature (SST) observations from satellites, surface drifters and ships, and independent studies using differing analyses, bias corrections, and data sources.{{< tbib '20' 'db777261-ee2e-4bf6-944e-a8831c595300' >}}^,{{}}^,{{}} Other observations such as the increase in mean sea level rise (see Sweet et al. 2017{{< tbib '76' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}}) and reduced Arctic/Antarctic ice sheets (see Taylor et al. 2017{{< tbib '122' '61d6757d-3f7a-4e90-add7-b03de796c6c4' >}}) further confirm the increase in thermal expansion. For the purpose of extending the selected time periods back from 1900 to 2016 and analyzing U.S. regional SSTs, the ERSSTv4{{< tbib '253' '865e132e-dd4a-4195-9ea0-c3c7d32d447e' >}} is used. For the centennial time scale changes over 1900–2016, warming trends in all regions are statistically significant with the 95% confidence level. U.S. regional SST warming is similar between calculations using ERSSTv4 in this report and those published by Belkin (2016),{{< tbib '254' '594bee23-c085-4cdd-8480-9d6fd1658c4e' >}} suggesting confidence in these findings.

Evidence for oxygen trends arises from extensive global measurements of WOCE after 1989 and individual profiles before that.{{< tbib '43' '2dbd3f8b-a4f8-421f-b75f-8cb165b1a867' >}} The first basin-wide dissolved oxygen surveys were performed in the 1920s.{{< tbib '255' 'b2a0160d-032f-4a96-8cb1-321e09950172' >}} The confidence level is based on globally integrated O₂ distributions in a variety of ocean models. Although the global mean exhibits low interannual variability, regional contrasts are large.

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-3.yaml identifier: key-message-2-3 ordinal: 3 process: "

" report_identifier: nca4 statement: '

The world’s oceans have absorbed 93% of the excess heat from human-induced warming since the mid-20th century and are currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually from human activities, making the oceans warmer and more acidic (very high confidence). Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations in many locations (high confidence).

' uncertainties: '

Uncertainties in the magnitude of ocean warming stem from the disparate measurements of ocean temperature over the last century. There is high confidence in warming trends of the upper ocean temperature from 0–700 m depth, whereas there is more uncertainty for deeper ocean depths of 700–2,000 m due to the short record of measurements from those areas. Data on warming trends at depths greater than 2,000 m are even more sparse. There are also uncertainties in the timing and reasons for particular decadal and interannual variations in ocean heat content and the contributions that different ocean basins play in the overall ocean heat uptake.

Uncertainties in ocean oxygen content (as estimated from the intermodel spread) in the global mean are moderate mainly because ocean oxygen content exhibits low interannual variability when globally averaged. Uncertainties in long-term decreases of the global averaged oxygen concentration amount to 25% in the upper 1,000 m for the 1970–1992 period and 28% for the 1993–2003 period. Remaining uncertainties relate to regional variability driven by mesoscale eddies and intrinsic climate variability such as ENSO.

' uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-3 url: ~ - chapter_identifier: our-changing-climate confidence: '

This Key Message is based upon multiple analyses of tide gauge and satellite altimetry records, on a meta-analysis of multiple geological proxies for pre-instrumental sea level change, and on both statistical and physical analyses of the human contribution to GMSL rise since 1900.

It is also based upon multiple methods for estimating the probability of future sea level change and on new modeling results regarding the stability of marine-based ice in Antarctica.

Confidence is very high in the rate of GMSL rise since 1900, based on multiple different approaches to estimating GMSL rise from tide gauges and satellite altimetry. Confidence is high in the substantial human contribution to GMSL rise since 1900, based on both statistical and physical modeling evidence. There is medium confidence that the magnitude of the observed rise since 1900 is unprecedented in the context of the previous 2,700 years, based on meta-analysis of geological proxy records.

There is very high confidence that GMSL rise over the next several decades will be at least as fast as a continuation of the historical trend over the last quarter century would indicate. There is medium confidence in the upper end of very likely ranges for 2030 and 2050. Due to possibly large ice sheet contributions, there is low confidence in the upper end of very likely ranges for 2100. Based on multiple projection methods, there is high confidence that differences between scenarios are small before 2050 but significant beyond 2050.

' evidence: "

Multiple researchers, using different statistical approaches, have integrated tide gauge records to estimate global mean sea level (GMSL) rise since the late 19th century (e.g., Church and White 2006, 2011; Hay et al. 2015; Jevrejeva et al. 2009{{< tbib '61' '1295b731-1d4c-44e2-b877-74df46d8e58d' >}}^,{{}}^,{{}}^,{{}}). The most recent published rate estimates are 1.2 ± 0.2 mm/year{{< tbib '73' '7c318710-b8fb-4e09-9982-546f2b60be67' >}} or 1.5 ± 0.2 mm/year{{< tbib '74' '94a8514e-063e-45ef-b893-11c82b49a597' >}} over 1901–1990. Thus, these results indicate about 4–5 inches (11–14 cm) of GMSL rise from 1901 to 1990. Tide gauge analyses indicate that GMSL rose at a considerably faster rate of about 0.12 inches/year (3 mm/year) since 1993,{{< tbib '73' '7c318710-b8fb-4e09-9982-546f2b60be67' >}}^,{{}} a result supported by satellite data indicating a trend of 0.13 inches/year (3.4 ± 0.4 mm/year) over 1993–2015 (update to Nerem et al. 2010;{{< tbib '75' '7b7ffcb0-766c-43b3-ac22-db29fbffef71' >}} see also Sweet et al. 2017,{{< tbib '57' '3bae2310-7572-47e2-99a4-9e4276764934' >}} Figure 12.3a). These results indicate an additional GMSL rise of about 3 inches (7 cm) since 1990. Thus, total GMSL rise since 1900 is about 7–8 inches (18–21 cm).

The finding regarding the historical context of the 20th-century change is based upon Kopp et al. (2016),{{< tbib '58' 'a0130167-b319-493d-bedc-7cab8f8fe9d9' >}} who conducted a meta-analysis of geological regional sea level (RSL) reconstructions, spanning the last 3,000 years, from 24 locations around the world, as well as tide gauge data from 66 sites and the tide-gauge-based GMSL reconstruction of Hay et al. (2015).{{< tbib '73' '7c318710-b8fb-4e09-9982-546f2b60be67' >}} By constructing a spatiotemporal statistical model of these datasets, they identified the common global sea level signal over the last three millennia, and its uncertainties. They found a 95% probability that the average rate of GMSL change over 1900–2000 was greater than during any preceding century in at least 2,800 years.

The lower bound of the very likely range is based on a continuation of the observed, approximately 3 mm/year rate of GMSL rise. The upper end of the very likely range is based on estimates for a higher scenario (RCP8.5) from three studies producing fully probabilistic projections across multiple RCPs. Kopp et al.(2014){{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}} fused multiple sources of information accounting for the different individual process contributing to GMSL rise. Kopp et al. (2016){{< tbib '58' 'a0130167-b319-493d-bedc-7cab8f8fe9d9' >}} constructed a semi-empirical sea level model calibrated to the Common Era sea level reconstruction. Mengel et al. (2016){{< tbib '257' '94117a50-acc5-4dbf-8029-368aa3fc9680' >}} constructed a set of semi-empirical models of the different contributing processes. All three studies show negligible scenario dependence in the first half of this century but increasing in prominence in the second half of the century. A sensitivity study by Kopp et al. (2014),{{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}} as well as studies by Jevrejeva et al. (2014){{< tbib '78' 'be9f25a7-6fb1-4599-b971-47aeb2abf967' >}} and by Jackson and Jevrejeva (2016),{{< tbib '258' 'c748bd06-bc78-4b9c-b511-7dab1974211e' >}} used frameworks similar to Kopp et al. (2016){{< tbib '58' 'a0130167-b319-493d-bedc-7cab8f8fe9d9' >}} but incorporated an expert elicitation study on ice sheet stability.{{< tbib '259' '86851f34-1534-4feb-aa11-8e0d7eeb0b11' >}} (This study was incorporated in the main results of Kopp et al. 2014{{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}} with adjustments for consistency with Church et al. 2013.{{< tbib '56' 'da0fddf2-c9c9-40d0-8e33-a86342d8b864' >}}) These studies extend the very likely range for RCP8.5 as high as 5–6 feet (160–180 cm; see Kopp et al. 2014, sensitivity study; Jevrejeva et al. 2014; Jackson and Jevrejeva 2016).{{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}}^,{{< tbib '78' 'be9f25a7-6fb1-4599-b971-47aeb2abf967' >}}^,{{}}

As described in Sweet et al. (2017),{{< tbib '57' '3bae2310-7572-47e2-99a4-9e4276764934' >}} Miller et al. (2013),{{< tbib '260' 'b58704d1-b4ec-46d0-9dd5-e7573523951e' >}} and Kopp et al. (2017),{{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}} several lines of arguments exist that support a plausible worst-case GMSL rise scenario in the range of 2.0 m to 2.7 m by 2100. Pfeffer et al. (2008){{< tbib '261' 'bfa425f2-e044-44c2-8bdb-5f8491c577de' >}} constructed a “worst-case” 2.0 m scenario, based on acceleration of mass loss from Greenland, that assumed a 30 cm GMSL contribution from thermal expansion. However, Sriver et al. (2012){{< tbib '262' 'b15cbb81-a2ac-4201-a184-a361bbd238d6' >}} find a physically plausible upper bound from thermal expansion exceeding 50 cm (an additional ~20-cm increase). The ~60 cm maximum contribution by 2100 from Antarctica in Pfeffer et al. (2008){{< tbib '261' 'bfa425f2-e044-44c2-8bdb-5f8491c577de' >}} could be exceeded by ~30 cm, assuming the 95th percentile for Antarctic melt rate (~22 mm/year) of the Bamber and Aspinall (2013){{< tbib '259' '86851f34-1534-4feb-aa11-8e0d7eeb0b11' >}} expert elicitation study is achieved by 2100 through a linear growth in melt rate. The Pfeffer et al. (2008){{< tbib '261' 'bfa425f2-e044-44c2-8bdb-5f8491c577de' >}} study did not include the possibility of a net decrease in land-water storage due to groundwater withdrawal; Church et al. (2013){{< tbib '56' 'da0fddf2-c9c9-40d0-8e33-a86342d8b864' >}} find a likely land-water storage contribution to 21st century GMSL rise of −1 cm to +11 cm. These arguments all point to the physical plausibility of GMSL rise in excess of 8 feet (240 cm).

Additional arguments come from model results examining the effects of marine ice-cliff collapse and ice-shelf hydro-fracturing on Antarctic loss rates.{{< tbib '80' 'ae82c8a3-3033-4103-91e9-926a27d1fa18' >}} To estimate the effect of incorporating the DeConto and Pollard (2016){{< tbib '80' 'ae82c8a3-3033-4103-91e9-926a27d1fa18' >}} projections of Antarctic ice sheet melt, Kopp et al. (2017){{< tbib '81' '387b7906-07c3-431f-a441-5a103220a974' >}} substituted the bias-corrected ensemble of DeConto and Pollard{{< tbib '80' 'ae82c8a3-3033-4103-91e9-926a27d1fa18' >}} into the Kopp et al. (2014){{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}} framework. This elevates the projections for 2100 to 3.1–8.9 feet (93–243 cm) for RCP8.5, 1.6–5.2 feet (50–158 cm) for RCP4.5, and 0.9–3.2 feet (26–98 cm) for RCP2.6. DeConto and Pollard{{< tbib '80' 'ae82c8a3-3033-4103-91e9-926a27d1fa18' >}} is just one study, not designed in a manner intended to produce probabilistic projections, and so these results cannot be used to ascribe probability; they do, however, support the physical plausibility of GMSL rise in excess of 8 feet.

Very likely ranges, 2030 relative to 2000 in cm (feet)

	Kopp et al. (2014){{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}}	Kopp et al. (2016){{< tbib '58' 'a0130167-b319-493d-bedc-7cab8f8fe9d9' >}}	Kopp et al. (2017){{< tbib '81' '387b7906-07c3-431f-a441-5a103220a974' >}} DP16	Mengel et al. (2016){{< tbib '257' '94117a50-acc5-4dbf-8029-368aa3fc9680' >}}
RCP8.5 (higher)	11–18 (0.4–0.6)	8–15 (0.3–0.5)	6–22 (0.2–0.7)	7–12 (0.2–0.4)
RCP4.5 (lower)	10–18 (0.3–0.6)	8–15 (0.3–0.5)	6–23 (0.2–0.8)	7–12 (0.2–0.4)
RCP2.6 (very low)	10–18 (0.3–0.6)	8–15 (0.3–0.5)	6–23 (0.2–0.8)	7–12 (0.2–0.4)

Very likely ranges, 2050 relative to 2000 in cm (feet)

	Kopp et al. (2014){{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}}	Kopp et al. (2016){{< tbib '58' 'a0130167-b319-493d-bedc-7cab8f8fe9d9' >}}	Kopp et al. (2017){{< tbib '81' '387b7906-07c3-431f-a441-5a103220a974' >}} DP16	Mengel et al. (2016){{< tbib '257' '94117a50-acc5-4dbf-8029-368aa3fc9680' >}}
RCP8.5 (higher)	21–38 (0.7–1.2)	16–34 (0.5–1.1)	17–48 (0.6–1.6)	15–28 (0.5–0.9)
RCP4.5 (lower)	18–35 (0.6–1.1)	15–31 (0.5–1.0)	14–43 (0.5–1.4)	14–25 (0.5–0.8)
RCP2.6 (very low)	18–33 (0.6–1.1)	14–29 (0.5–1.0)	12–41 (0.4–1.3)	13–23 (0.4–0.8)

Very likely ranges, 2100 relative to 2000 in cm (feet)

	Kopp et al. (2014){{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}}	Kopp et al. (2016){{< tbib '58' 'a0130167-b319-493d-bedc-7cab8f8fe9d9' >}}	Kopp et al. (2017){{< tbib '81' '387b7906-07c3-431f-a441-5a103220a974' >}} DP16	Mengel et al. (2016){{< tbib '257' '94117a50-acc5-4dbf-8029-368aa3fc9680' >}}
RCP8.5 (higher)	55–121 (1.8–4.0)	52–131 (1.7–4.3)	93–243 (3.1–8.0)	57–131 (1.9–4.3)
RCP4.5 (lower)	36–93 (1.2–3.1)	33–85 (1.1–2.8)	50–158 (1.6–5.2)	37–77 (1.2–2.5)
RCP2.6 (very low)	29–82 (1.0–2.7)	24–61 (0.8–2.0)	26–98 (0.9–3.2)	28–56 (0.9–1.8)

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-4.yaml identifier: key-message-2-4 ordinal: 4 process: "

" report_identifier: nca4 statement: '

Global average sea level has risen by about 7–8 inches (16–21 cm) since 1900, with almost half this rise occurring since 1993 as oceans have warmed and land-based ice has melted (very high confidence). Relative to the year 2000, sea level is very likely to rise 1 to 4 feet (0.3 to 1.3 m) by the end of the century (medium confidence). Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a rise exceeding 8 feet (2.4 m) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.

' uncertainties: '

Uncertainties in reconstructed GMSL change relate to the sparsity of tide gauge records, particularly before the middle of the 20th century, and to different statistical approaches for estimating GMSL change from these sparse records. Uncertainties in reconstructed GMSL change before the twentieth century also relate to the sparsity of geological proxies for sea level change, the interpretation of these proxies, and the dating of these proxies. Uncertainty in attribution relates to the reconstruction of past changes and the magnitude of unforced variability.

Since NCA3, multiple different approaches have been used to generate probabilistic projections of GMSL rise, conditional upon the RCPs. These approaches are in general agreement. However, emerging results indicate that marine-based sectors of the Antarctic ice sheet are more unstable than previous modeling indicated. The rate of ice sheet mass changes remains challenging to project.

' uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-4 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is very high confidence in trends since 1895, based on the instrumental record, since this is a long-term record with measurements made with relatively high precision. There is high confidence for trends that are based on surface/satellite agreement since 1979, since this is a shorter record. There is medium confidence for trends based on paleoclimate data, as this is a long record but with relatively low precision.

There is very high confidence in observed changes in average annual and seasonal temperature and observed changes in temperature extremes over the United States, as these are based upon the convergence of evidence from multiple data sources, analyses, and assessments including the instrumental record.

There is high confidence that the range of projected changes in average temperature and temperature extremes over the United States encompasses the range of likely change, based upon the convergence of evidence from basic physics, multiple model simulations, analyses, and assessments.

' evidence: "

The Key Message and supporting text summarize extensive evidence documented in the climate science literature. Similar statements about changes exist in other reports (e.g., NCA3,{{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} Climate Change Impacts in the United States,{{< tbib '263' 'e251f590-177e-4ba6-8ed1-6f68b5e54c8a' >}} SAP 1.1: Temperature trends in the lower atmosphere).{{< tbib '264' 'f135add4-6d4c-4d88-a8f1-b880dbf5334f' >}}

Evidence for changes in U.S. climate arises from multiple analyses of data from in situ, satellite, and other records undertaken by many groups over several decades. The primary dataset for surface temperatures in the United States is nClimGrid,{{< tbib '85' '29960c69-6168-4fb0-9af0-d50bdd91acd3' >}}^,{{}} though trends are similar in the U.S. Historical Climatology Network, the Global Historical Climatology Network, and other datasets. Several atmospheric reanalyses (e.g., 20th Century Reanalysis, Climate Forecast System Reanalysis, ERA-Interim, and Modern Era Reanalysis for Research and Applications) confirm rapid warming at the surface since 1979, and observed trends closely track the ensemble mean of the reanalyses.{{< tbib '265' '8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c' >}} Several recently improved satellite datasets document changes in middle tropospheric temperatures.{{< tbib '7' '0215f34d-335f-4105-a3eb-b660e0ff8a78' >}}^,{{}} Longer-term changes are depicted using multiple paleo analyses (e.g., Trouet et al. 2013, Wahl and Smerdon 2012).{{< tbib '86' '5a3d5be0-e40f-4ea7-8f99-422db7954577' >}}^,{{}}

Evidence for changes in U.S. climate arises from multiple analyses of in situ data using widely published climate extremes indices. For the analyses presented here, the source of in situ data is the Global Historical Climatology Network–Daily dataset.{{< tbib '268' '9b433446-b58f-4358-9737-5a6ccc2f6fcf' >}} Changes in extremes were assessed using long-term stations with minimal missing data to avoid network-induced variability on the long-term time series. Cold wave frequency was quantified using the Cold Spell Duration Index,{{< tbib '269' 'e6ecbe14-fe1b-46f8-bad5-bde9e4cc658a' >}} heat wave frequency was quantified using the Warm Spell Duration Index,{{< tbib '269' 'e6ecbe14-fe1b-46f8-bad5-bde9e4cc658a' >}} and heat wave intensity was quantified using the Heat Wave Magnitude Index Daily.{{< tbib '270' '546ef0fe-bfae-43ee-969e-5870c581e426' >}} Station-based index values were averaged into 4° grid boxes, which were then area-averaged into a time series for the contiguous United States. Note that a variety of other threshold and percentile-based indices were also evaluated, with consistent results (e.g., the Dust Bowl was consistently the peak period for extreme heat). Changes in record-setting temperatures were quantified, as in Meehl et al. (2016).{{< tbib '13' '72301197-e20a-4328-accb-4276341a25db' >}}

Projections are based on global model results and associated downscaled products from CMIP5 for a lower scenario (RCP4.5) and a higher scenario (RCP8.5). Model weighting is employed to refine projections for each RCP. Weighting parameters are based on model independence and skill over North America for seasonal temperature and annual extremes. The multimodel mean is based on 32 model projections that were statistically downscaled using the LOcalized Constructed Analogs technique.{{< tbib '247' '62c66ef3-cddb-4797-ba0e-5672fbcc27b3' >}} The range is defined as the difference between the average increase in the three coolest models and the average increase in the three warmest models. All increases are significant (i.e., more than 50% of the models show a statistically significant change, and more than 67% agree on the sign of the change).{{< tbib '271' 'b63c9720-f770-4718-89cc-53b3616e2bec' >}}

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-5.yaml identifier: key-message-2-5 ordinal: 5 process: "

" report_identifier: nca4 statement: '

Annual average temperature over the contiguous United States has increased by 1.2ºF (0.7°C) over the last few decades and by 1.8°F (1°C) relative to the beginning of the last century (very high confidence). Additional increases in annual average temperature of about 2.5°F (1.4°C) are expected over the next few decades regardless of future emissions, and increases ranging from 3°F to 12°F (1.6°–6.6°C) are expected by the end of century, depending on whether the world follows a higher or lower future scenario, with proportionally greater changes in high temperature extremes (high confidence).

' uncertainties: "

The primary uncertainties for surface data relate to historical changes in station location, temperature instrumentation, observing practice, and spatial sampling (particularly in areas and periods with low station density, such as the intermountain West in the early 20th century). Much research has been done to account for these issues, resulting in techniques that make adjustments at the station level to improve the homogeneity of the time series (e.g., Easterling and Peterson 1995, Menne and Williams 2009{{< tbib '272' 'a7bd80fe-7df0-456b-9978-8f7e222bfafa' >}}^,{{}}). Further, Easterling et al. (1996){{< tbib '274' '2b8701b9-46d8-4a52-a7a8-c074fe313126' >}} examined differences in area-averaged time series at various scales for homogeneity-adjusted temperature data versus non-adjusted data and found that when the area reached the scale of the NCA regions, little differences were found. Satellite records are similarly impacted by non-climatic changes such as orbital decay, diurnal sampling, and instrument calibration to target temperatures. Several uncertainties are inherent in temperature-sensitive proxies, such as dating techniques and spatial sampling.

Global climate models are subject to structural and parametric uncertainty, resulting in a range of estimates of future changes in average temperature. This is partially mitigated through the use of model weighting and pattern scaling. Furthermore, virtually every ensemble member of every model projection contains an increase in temperature by mid- and late-century. Empirical downscaling introduces additional uncertainty (e.g., with respect to stationarity).

" uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-5 url: ~ - chapter_identifier: our-changing-climate confidence: "

Confidence is medium that precipitation has increased and high that heavy precipitation events have increased in the United States. Furthermore, confidence is also high that the important regional and seasonal differences in changes documented here are robust.

Based on evidence from climate model simulations and our fundamental understanding of the relationship of water vapor to temperature, confidence is high that extreme precipitation will increase in all regions of the United States. However, based on the evidence and understanding of the issues leading to uncertainties, confidence is medium that more total precipitation is projected for the northern United States and less for the Southwest.

Based on the evidence and understanding of the issues leading to uncertainties, confidence is medium that average annual precipitation has increased in the United States. Furthermore, confidence is also medium that the important regional and seasonal differences in changes documented in the text and in Figure 7.1 in Easterling et al. (2017){{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} are robust.

Given the evidence base and uncertainties, confidence is medium that snow cover extent has declined in the United States and medium that extreme snowfall years have declined in recent years. Confidence is high that western U.S. snowpack will decline in the future, and confidence is medium that a shift from snow domination to rain domination will occur in the parts of the central and eastern United States cited in the text, as well as that soil moisture in the surface (top 10cm) will decrease.

" evidence: "

The Key Message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature and previous National Climate Assessments (e.g., Karl et al. 2009, Walsh et al. 2014{{< tbib '88' 'a6a312ba-6fd1-4006-9a60-45112db52190' >}}^,{{}}). Evidence of long-term changes in precipitation is based on analysis of daily precipitation observations from the U.S. Cooperative Observer Network (http://www.nws.noaa.gov/om/coop/) and shown in Easterling et al. (2017),{{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} Figure 7.1. Published work, such as the Third National Climate Assessment and Figure 7.1{{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}}, show important regional and seasonal differences in U.S. precipitation change since 1901.

Numerous papers have been written documenting observed changes in heavy precipitation events in the United States (e.g., Kunkel et al. 2003, Groisman et al. 2004{{< tbib '275' '642a65e4-fe5d-4655-97e1-49a8a9bfc297' >}}^,{{}}), which were cited in the Third National Climate Assessment, as well as those cited in this assessment. Although station-based analyses (e.g., Westra et al. 2013{{< tbib '277' 'e941a5b9-10b7-462b-9042-5760a82fc415' >}}) do not show large numbers of statistically significant station-based trends, area averaging reduces the noise inherent in station-based data and produces robust increasing signals (see Easterling et al. 2017,{{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} Figures 7.2 and 7.3). Evidence of long-term changes in precipitation is based on analysis of daily precipitation observations from the U.S. Cooperative Observer Network (http://www.nws.noaa.gov/om/coop/) and shown in Easterling et al. (2017),{{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} Figures 7.2, 7.3, and 7.4.

Evidence of historical changes in snow cover extent and reduction in extreme snowfall years is consistent with our understanding of the climate system’s response to increasing greenhouse gases. Furthermore, climate models continue to consistently show future declines in snowpack in the western United States. Recent model projections for the eastern United States also confirm a future shift from snowfall to rainfall during the cold season in colder portions of the central and eastern United States. Each of these changes is documented in the peer-reviewed literature and cited in the main text of this chapter.

Evidence of future change in precipitation is based on climate model projections and our understanding of the climate system’s response to increasing greenhouse gases, and on regional mechanisms behind the projected changes. In particular, Figure 7.7 in Easterling et al. (2017){{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} documents projected changes in the 20-year return period amount using the LOCA data, and Figure 7.6{{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} shows changes in 2-day totals for the 5-year return period using the CMIP5 suite of models. Each figure shows robust changes in extreme precipitation events as they are defined in the figure. However, Figure 7.5{{< tbib '94' 'e8089a19-413e-4bc5-8c4a-7610399e268c' >}} shows changes in seasonal and annual precipitation and shows where confidence in the changes is higher based on consistency between the models, and there are large areas where the projected change is uncertain.

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-6.yaml identifier: key-message-2-6 ordinal: 6 process: "

" report_identifier: nca4 statement: '

Annual precipitation since the beginning of the last century has increased across most of the northern and eastern United States and decreased across much of the southern and western United States. Over the coming century, significant increases are projected in winter and spring over the Northern Great Plains, the Upper Midwest, and the Northeast (medium confidence). Observed increases in the frequency and intensity of heavy precipitation events in most parts of the United States are projected to continue (high confidence). Surface soil moisture over most of the United States is likely to decrease (medium confidence), accompanied by large declines in snowpack in the western United States (high confidence) and shifts to more winter precipitation falling as rain rather than snow (medium confidence).

' uncertainties: "

The main issue that relates to uncertainty in historical trends is the sensitivity of observed precipitation trends to the spatial distribution of observing stations and to historical changes in station location, rain gauges, the local landscape, and observing practices. These issues are mitigated somewhat by new methods to produce spatial grids{{< tbib '152' '596a7f1e-6ce5-4bdf-b144-d0715a7567bd' >}} through time.

This includes the sensitivity of observed snow changes to the spatial distribution of observing stations and to historical changes in station location, rain gauges, and observing practices, particularly for snow. Future changes in the frequency and intensity of meteorological systems causing heavy snow are less certain than temperature changes.

A key issue is how well climate models simulate precipitation, which is one of the more challenging aspects of weather and climate simulation. In particular, comparisons of model projections for total precipitation (from both CMIP3 and CMIP5; see Sun et al. 2015{{< tbib '271' 'b63c9720-f770-4718-89cc-53b3616e2bec' >}}) by NCA3 region show a spread of responses in some regions (e.g., Southwest) such that they are opposite from the ensemble average response. The continental United States is positioned in the transition zone between expected drying in the subtropics and projected wetting in the mid- and higherlatitudes. There are some differences in the location of this transition between CMIP3 and CMIP5 models, and thus there remains uncertainty in the exact location of the transition zone.

" uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-6 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is very high confidence that the arctic surface and air temperatures have warmed across Alaska and the Arctic at a much faster rate than the global average is provided by the multiple datasets analyzed by multiple independent groups indicating the same conclusion. Additionally, climate models capture the enhanced warming in the Arctic, indicating a solid understanding of the underlying physical mechanisms.

There is high confidence that permafrost is thawing, becoming discontinuous, and releasing CO₂ and CH₄. Physically based arguments and observed increases in CO₂ and CH₄ emissions as permafrost thaws indicate that the feedback is positive. This confidence level is justified based on observations of rapidly changing permafrost characteristics.

There is very high confidence that arctic sea and land ice melt is accelerating and mountain glacier ice mass is declining, given the multiple observational sources and analysis techniques documented in the peer-reviewed climate science literature.

' evidence: "

Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice the global average. Observational studies using ground-based observing stations and satellites analyzed by multiple independent groups support this finding. The enhanced sensitivity of the arctic climate system to anthropogenic forcing is also supported by climate modeling evidence, indicating a solid grasp of the underlying physics. These multiple lines of evidence provide very high confidence of enhanced arctic warming with potentially significant impacts on coastal communities and marine ecosystems.

This aspect of the Key Message is supported by observational evidence from ground-based observing stations, satellites, and data model temperature analyses from multiple sources and independent analysis techniques.{{< tbib '117' 'e2086a52-de43-4628-97f8-05fb1c8e1e45' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} For more than 40 years, climate models have predicted enhanced arctic warming, indicating a solid grasp of the underlying physics and positive feedbacks driving the accelerated arctic warming.{{< tbib '26' 'b3bbc7b5-067e-4c23-8d9b-59faee21e58e' >}}^,{{}}^,{{}} Lastly, similar statements have been made in NCA3,{{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} IPCC AR5,{{< tbib '120' '47a5196b-4fba-4fdb-8647-8945627725bb' >}} and in other arctic-specific assessments such as the Arctic Climate Impacts Assessment{{< tbib '281' '56f7d484-3935-4b75-b3c0-2265edae42c2' >}} and the Snow, Water, Ice and Permafrost in the Arctic assessment report.{{< tbib '129' 'e4385436-dcc5-45ca-89a2-e281d025545d' >}}

Permafrost is thawing, becoming more discontinuous, and releasing carbon dioxide (CO₂) and methane (CH₄). Observational and modeling evidence indicates that permafrost has thawed and released additional CO₂ and CH₄, indicating that the permafrost–carbon feedback is positive, accounting for additional warming of approximately 0.08ºC to 0.50ºC on top of climate model projections. Although the magnitude and timing of the permafrost–carbon feedback are uncertain due to a range of poorly understood processes (deep soil and ice wedge processes, plant carbon uptake, dependence of uptake and emissions on vegetation and soil type, and the role of rapid permafrost thaw processes such as thermokarst), emerging science and the newest estimates continue to indicate that this feedback is more likely on the larger side of the range. Impacts of permafrost thaw and the permafrost–carbon feedback complicate our ability to limit future temperature changes by adding a currently unconstrained radiative forcing to the climate system.

This part of the Key Message is supported by observational evidence of warming permafrost temperatures and a deepening active layer, in situ gas measurements, laboratory incubation experiments of CO₂ and CH₄ release, and model studies.{{< tbib '126' 'e787a738-62a2-4c16-984c-b37f225a7510' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Alaska and arctic permafrost characteristics have responded to increased temperatures and reduced snow cover in most regions since the 1980s, with colder permafrost warming faster than warmer permafrost.{{< tbib '127' '3d339c60-bdf6-44f9-900d-249676925b4f' >}}^,{{}}^,{{}} Large carbon soil pools (approximately half of the global below-ground organic carbon pool) are stored in permafrost soil,{{< tbib '287' '05903e43-63b7-4a76-8ddf-625849add0f6' >}}^,{{}} with the potential to be released. Thawing permafrost makes previously frozen organic matter available for microbial decomposition. In situ gas flux measurements have directly measured the release of CO₂ and CH₄ from arctic permafrost.{{< tbib '289' '3a1ac4af-4295-4dff-a77f-d4d58d618d62' >}}^,{{}} The specific conditions of microbial decomposition, aerobic or anaerobic, determine the relative production of CO₂ and CH₄. This distinction is significant as CH₄ is a much more powerful greenhouse gas than CO₂.{{< tbib '17' '6c7c285c-8606-41fe-bf93-100d80f1d17a' >}} However, incubation studies indicate that 3.4 times more carbon is released under aerobic conditions than anaerobic conditions, leading to a 2.3 times stronger radiative forcing under aerobic conditions.{{< tbib '284' 'e08db6e2-291f-465b-a693-a90f6110f5af' >}} Combined data and modeling studies suggest that the impact of the permafrost–carbon feedback on global temperatures could amount to +0.52° ± 0.38°F (+0.29° ± 0.21°C) by 2100.{{< tbib '124' '5b7d739a-50de-4006-811f-5a9bd469c977' >}} Chadburn et al. (2017){{< tbib '291' '29b5eac3-49d9-47aa-9f54-fa5c2501c39b' >}} infer the sensitivity of permafrost area to globally averaged warming to be 1.5 million square miles (4 million square km), constraining a group of climate models with the observed spatial distribution of permafrost; this sensitivity is 20% higher than previous studies. Permafrost thaw is occurring faster than models predict due to poorly understood deep soil, ice wedge, and thermokarst processes.{{< tbib '125' '0ee6881f-0ceb-4192-bf18-9fe5f8e4d01c' >}}^,{{}}^,{{}}^,{{}} Additional uncertainty stems from the surprising uptake of methane from mineral soils{{< tbib '293' '12c3ea10-a785-4e52-b2cf-ecad1c207714' >}} and dependence of emissions on vegetation and soil properties.{{< tbib '294' '0992f3f4-2780-45e8-bd5c-3a1ec35a6ceb' >}} The observational and modeling evidence supports the Key Message that the permafrost–carbon feedback is positive (i.e., amplifies warming).

Arctic land and sea ice loss observed in the last three decades continues, in some cases accelerating. A diverse range of observational evidence from multiple data sources and independent analysis techniques provides consistent evidence of substantial declines in arctic sea ice extent, thickness, and volume since at least 1979, mountain glacier melt over the last 50 years, and accelerating mass loss from Greenland. An array of different models and independent analyses indicate that future declines in ice across the Arctic are expected, resulting in late summers in the Arctic very likely becoming ice free by mid-century.

This final aspect of the Key Message is supported by observational evidence from multiple ground-based and satellite-based observational techniques (including passive microwave, laser and radar altimetry, and gravimetry) analyzed by independent groups using different techniques reaching similar conclusions.{{< tbib '127' '3d339c60-bdf6-44f9-900d-249676925b4f' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}Additionally, the U.S. Geological Survey repeat photography database shows the glacier retreat for many Alaska glaciers (Taylor et al. 2017,{{< tbib '122' '61d6757d-3f7a-4e90-add7-b03de796c6c4' >}} Figure 11.4). Several independent model analysis studies using a wide array of climate models and different analysis techniques indicate that sea ice loss will continue across the Arctic, very likely resulting in late summers becoming nearly ice-free by mid-century.{{< tbib '26' 'b3bbc7b5-067e-4c23-8d9b-59faee21e58e' >}}^,{{}}^,{{}}

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-7.yaml identifier: key-message-2-7 ordinal: 7 process: "

" report_identifier: nca4 statement: '

In the Arctic, annual average temperatures have increased more than twice as fast as the global average, accompanied by thawing permafrost and loss of sea ice and glacier mass (very high confidence). Arctic-wide glacial and sea ice loss is expected to continue; by mid-century, it is very likely that the Arctic will be nearly free of sea ice in late summer (very high confidence). Permafrost is expected to continue to thaw over the coming century as well, and the carbon dioxide and methane released from thawing permafrost has the potential to amplify human-induced warming, possibly significantly (high confidence).

' uncertainties: '

The lack of high-quality data and the restricted spatial resolution of surface and ground temperature data over many arctic land regions, coupled with the fact that there are essentially no measurements over the Central Arctic Ocean, hampers the ability to better refine the rate of arctic warming and completely restricts our ability to quantify and detect regional trends, especially over the sea ice. Climate models generally produce an arctic warming between two to three times the global mean warming. A key uncertainty is our quantitative knowledge of the contributions from individual feedback processes in driving the accelerated arctic warming. Reducing this uncertainty will help constrain projections of future arctic warming.

A lack of observations affects not only the ability to detect trends but also to quantify a potentially significant positive feedback to climate warming: the permafrost–carbon feedback. Major uncertainties are related to deep soil and thermokarst processes, as well as the persistence or degradation of massive ice (e.g., ice wedges) and the dependence of CO₂ and CH₄ uptake and production on vegetation and soil properties. Uncertainties also exist in relevant soil processes during and after permafrost thaw, especially those that control unfrozen soil carbon storage and plant carbon uptake and net ecosystem exchange. Many processes with the potential to drive rapid permafrost thaw (such as thermokarst) are not included in current Earth System Models.

Key uncertainties remain in the quantification and modeling of key physical processes that contribute to the acceleration of land and sea ice melting. Climate models are unable to capture the rapid pace of observed sea and land ice melt over the last 15 years; a major factor is our inability to quantify and accurately model the physical processes driving the accelerated melting. The interactions between atmospheric circulation, ice dynamics and thermodynamics, clouds, and specifically the influence on the surface energy budget are key uncertainties. Mechanisms controlling marine-terminating glacier dynamics, specifically the roles of atmospheric warming, seawater intrusions under floating ice shelves, and the penetration of surface meltwater to the glacier bed, are key uncertainties in projecting Greenland ice sheet melt.

' uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-7 url: ~ - chapter_identifier: our-changing-climate confidence: '

There is medium to high confidence that the tropics and related features of the global circulation have expanded poleward is based upon the results of a large number of observational studies, using a wide variety of metrics and datasets, which reach similar conclusions. A large number of studies utilizing modeling of different complexity and theoretical considerations provide compounding evidence that human activities like increases in greenhouse gases, ozone depletion, and anthropogenic aerosols contributed to the observed poleward expansion of the tropics. Climate models forced with these anthropogenic drivers cannot explain the observed magnitude of tropical expansion, and some studies suggest a possibly large contribution of internal variability. These multiple lines of evidence lead to the conclusion of medium confidence that human activities contributed to observed expansion of the tropics.

Confidence is rated as high in tropical cyclone rainfall projections and medium in intensity projections since there are a number of publications supporting these overall conclusions, fairly well-established theory, general consistency among different studies, varying methods used in studies, and still a fairly strong consensus among studies. However, a limiting factor for confidence in the results is the lack of a supporting detectable anthropogenic contribution in observed tropical cyclone data.

There is low to medium confidence for increased occurrence of the most intense tropical cyclones for most basins, as there are relatively few formal studies focused on these changes, and the change in occurrence of such storms would be enhanced by increased intensities but reduced by decreased overall frequency of tropical cyclones.

Confidence in this finding on atmospheric rivers is rated as medium based on qualitatively similar projections among different studies.

' evidence: "

The tropics have expanded poleward in each hemisphere over the period 1979–2009 (medium to high confidence) as shown by a large number of studies using a variety of metrics, observations, and reanalysis. Modeling studies and theoretical considerations illustrate that human activities like increases in greenhouse gases, ozone depletion, and anthropogenic aerosols cause a widening of the tropics. There is medium confidence that human activities have contributed to the observed poleward expansion, taking into account uncertainties in the magnitude of observed trends and a possible large contribution of natural climate variability.

The first part of the Key Message is supported by statements of the previous international IPCC AR5 assessment{{< tbib '120' '47a5196b-4fba-4fdb-8647-8945627725bb' >}} and a large number of more recent studies that examined the magnitude of the observed tropical widening and various causes.{{< tbib '95' 'a80ce47f-ac9a-43d2-9179-acad0e28e05a' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} Additional evidence for an impact of greenhouse gas increases on the widening of the tropical belt and poleward shifts of the midlatitude jets is provided by the diagnosis of CMIP5 simulations.{{< tbib '306' '938444d5-cf1e-43b2-93d8-d8e403a23344' >}}^,{{}} There is emerging evidence for an impact of anthropogenic aerosols on the tropical expansion in the Northern Hemisphere.{{< tbib '308' 'aaf41b2e-d066-40c4-8bbf-d14ed62e13d6' >}}^,{{}} Recent studies provide new evidence on the significance of internal variability on recent changes in the tropical width.{{< tbib '302' 'd5eb689b-306c-4092-92ab-80958283a00c' >}}^,{{}}^,{{}}

Models are generally in agreement that tropical cyclones will be more intense and have higher precipitation rates, at least in most basins. Given the agreement among models and support of theory and mechanistic understanding, there is medium to high confidence in the overall projection, although there is some limitation on confidence levels due to the lack of a supporting detectable anthropogenic contribution to tropical cyclone intensities or precipitation rates.

The second part of the Key Message is also based on extensive evidence documented in the climate science literature and is similar to statements made in previous national (NCA3){{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} and international{{< tbib '249' 'f03117be-ccfe-4f88-b70a-ffd4351b8190' >}} assessments. Since these assessments, more recent downscaling studies have further supported these assessments (e.g., Knutson et al. 2015{{< tbib '170' '4f1e7aa1-0c36-4220-ac77-7d55bcb33061' >}}), though pointing out that the changes (future increased intensity and tropical cyclone precipitation rates) may not occur in all basins.

Increases in atmospheric river frequency and intensity are expected along the U.S. West Coast, leading to the likelihood of more frequent flooding conditions, with uncertainties remaining in the details of the spatial structure of these systems along the coast (for example, northern vs. southern California). Evidence for the expectation of an increase in the frequency and severity of landfalling atmospheric rivers on the U.S. West Coast comes from the CMIP-based climate change projection studies of Dettinger (2011).{{< tbib '163' '67ee7e56-b6a2-4ada-a7e8-ff836b1c58d1' >}} Warner et al. (2015),{{< tbib '164' '40ffbbdf-74f1-4511-b1f1-a2b2a165185e' >}} Payne and Magnusdottir (2015),{{< tbib '312' 'd13ddcaa-9080-4fab-9514-c45365ed3740' >}} Gao et al. (2015),{{< tbib '165' '60ce531d-0064-4170-8b4d-e63bbb9f0c67' >}} Radić et al. (2015),{{< tbib '313' '8927a54e-415e-4af2-aeb8-665cfe2d17ee' >}} and Hagos et al. (2016).{{< tbib '314' 'a2470cdb-4b8f-4ed6-8c5f-38cd301053a2' >}} The close connection between atmospheric rivers and water availability and flooding is based on the present-day observation studies of Guan et al. (2010),{{< tbib '315' '59dfa0b2-2e94-4eb9-89fd-3adbbd1d61d4' >}} Dettinger (2011),{{< tbib '163' '67ee7e56-b6a2-4ada-a7e8-ff836b1c58d1' >}} Ralph et al. (2006),{{< tbib '316' '8caee927-3ee1-4084-a42e-e9487f4ebedf' >}} Neiman et al. (2011),{{< tbib '317' 'a73e96c6-679f-4f76-a749-571f43601e5c' >}} Moore et al. (2012),{{< tbib '318' 'ad8a08da-1ddc-452c-ac17-a5208fa4fe09' >}} and Dettinger (2013).{{< tbib '319' '84acc46e-9dcf-43e7-8acc-07f07167ee8e' >}}

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-8.yaml identifier: key-message-2-8 ordinal: 8 process: "

" report_identifier: nca4 statement: '

Human-induced change is affecting atmospheric dynamics and contributing to the poleward expansion of the tropics and the northward shift in Northern Hemisphere winter storm tracks since the 1950s (medium to high confidence). Increases in greenhouse gases and decreases in air pollution have contributed to increases in Atlantic hurricane activity since 1970 (medium confidence). In the future, Atlantic and eastern North Pacific hurricane rainfall (high confidence) and intensity (medium confidence) are projected to increase, as are the frequency and severity of landfalling “atmospheric rivers” on the West Coast (medium confidence).

' uncertainties: "

The rate of observed expansion of the tropics depends on which metric is used.{{< tbib '161' '798360ca-4177-462c-991a-c7a512d9287c' >}} The linkages between different metrics are not fully explored. Uncertainties also result from the utilization of reanalysis to determine trends and from limited observational records of free atmosphere circulation, precipitation, and evaporation. The dynamical mechanisms behind changes in the width of the tropical belt (e.g., tropical–extratropical interactions, baroclinic eddies) are not fully understood. There is also a limited understanding of how various climate forcings, such as anthropogenic aerosols, affect the width of the tropics. The coarse horizontal and vertical resolution of global climate models may limit the ability of these models to properly resolve latitudinal changes in the atmospheric circulation. Limited observational records affect the ability to accurately estimate the contribution of natural decadal to multi-decadal variability on observed expansion of the tropics.

A key uncertainty in tropical cyclones (TCs) is the lack of a supporting detectable anthropogenic signal in the historical data to add further confidence to these projections. As such, confidence in the projections is based on agreement among different modeling studies and physical understanding (for example, potential intensity theory for TC intensities and the expectation of stronger moisture convergence, and thus higher precipitation rates, in TCs in a warmer environment containing greater amounts of environmental atmospheric moisture). Additional uncertainty stems from uncertainty in both the projected pattern and magnitude of future SST.{{< tbib '170' '4f1e7aa1-0c36-4220-ac77-7d55bcb33061' >}}

In terms of atmospheric rivers (ARs), a modest uncertainty remains in the lack of a supporting detectable anthropogenic signal in the historical data to add further confidence to these projections. However, the overall increase in ARs projected/expected is based to a very large degree on very high confidence that the atmospheric water vapor will increase. Thus, increasing water vapor coupled with little projected change in wind structure/intensity still indicates increases in the frequency/intensity of ARs. A modest uncertainty arises in quantifying the expected change at a regional level (for example, northern Oregon versus southern Oregon), given that there are some changes expected in the position of the jet stream that might influence the degree of increase for different locations along the west coast. Uncertainty in the projections of the number and intensity of ARs is introduced by uncertainties in the models’ ability to represent ARs and their interactions with climate.

" uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-8 url: ~ - chapter_identifier: our-changing-climate confidence: '

Because of the enumerated physical processes, there is very high confidence that RSL change will vary across U.S. coastlines. There is high confidence in the likely differences of RSL change from GMSL change under different levels of GMSL change, based on projections incorporating the different relevant processes. There is low confidence that the flood risk at specific locations will be amplified from a major tropical storm this century.

' evidence: "

The part of the Key Message regarding the existence of geographic variability is based upon a broader observational, modeling, and theoretical literature. The specific differences are based upon the scenarios described by the Federal Interagency Sea Level Rise Task Force.{{< tbib '76' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}} The processes that cause geographic variability in regional sea level (RSL) change are also reviewed by Kopp et al. (2015).{{< tbib '320' 'e8f60819-839e-4772-8a49-7c57d9c53424' >}} Long tide gauge datasets reveal where RSL rise is largely driven by vertical land motion due to glacio-isostatic adjustment and fluid withdrawal along many U.S. coastlines.{{< tbib '321' 'ab69428a-34a4-412f-8c85-b3bb8043509c' >}}^,{{}} These observations are corroborated by glacio-isostatic adjustment models, by global positioning satellite (GPS) observations, and by geological data (e.g., Engelhart and Horton 2012{{< tbib '323' '427648bc-547c-4161-8d97-14ec813adcc8' >}}). The physics of the gravitational, rotational, and flexural “static-equilibrium fingerprint” response of sea level to redistribution of mass from land ice to the oceans is well-established.{{< tbib '324' '1823b427-f097-418f-9d4b-c2f7e9291874' >}}^,{{}} GCM studies indicate the potential for a Gulf Stream contribution to sea level rise in the U.S. Northeast.{{< tbib '326' '0e116266-7679-409f-b1d6-99c31edfcd9e' >}}^,{{}} Kopp et al. (2014){{< tbib '77' '38924fa0-a0dd-44c9-a2a0-366ca610b280' >}} and Slangen et al. (2014){{< tbib '59' '9a5f3738-4283-4df2-adb6-8a0cac785d22' >}} accounted for land motion (only glacial isostatic adjustment for Slangen et al.), fingerprint, and ocean dynamic responses. Comparing projections of local RSL change and GMSL change in these studies indicates that local rise is likely to be greater than the global average along the U.S. Atlantic and Gulf Coasts and less than the global average in most of the Pacific Northwest. Sea level rise projections in this report were developed by a Federal Interagency Sea Level Rise Task Force.{{< tbib '76' 'c66bf5a9-a6d7-4043-ad99-db0ae6ae562c' >}}

The frequency, extent, and depth of extreme event-driven (e.g., 5- to 100-year event probabilities) coastal flooding relative to existing infrastructure will continue to increase in the future as local RSL rises.{{< tbib '57' '3bae2310-7572-47e2-99a4-9e4276764934' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} These projections are based on modeling studies of future hurricane characteristics and associated increases in major storm surge risk amplification. Extreme flood probabilities will increase regardless of changes in storm characteristics, which may exacerbate such changes. Model-based projections of tropical storms and related major storm surges within the North Atlantic mostly agree that intensities and frequencies of the most intense storms will increase this century.{{< tbib '190' 'd6bd92ad-67ef-4df7-aca9-68944523e863' >}}^,{{}}^,{{}}^,{{}}^,{{}} However, the projection of increased hurricane intensity is more robust across models than the projection of increased frequency of the most intense storms. A number of models project a decrease in the overall number of tropical storms and hurricanes in the North Atlantic, although high-resolution models generally project increased mean hurricane intensity (e.g., Knutson et al. 2013{{< tbib '190' 'd6bd92ad-67ef-4df7-aca9-68944523e863' >}}). In addition, there is model evidence for a change in tropical cyclone tracks in warm years that minimizes the increase in landfalling hurricanes in the U.S. mid-Atlantic or Northeast.{{< tbib '338' '134b5712-13e2-4837-b710-027fe9028e8f' >}}

" href: https://data.globalchange.gov/report/nca4/chapter/our-changing-climate/finding/key-message-2-9.yaml identifier: key-message-2-9 ordinal: 9 process: "

" report_identifier: nca4 statement: '

Regional changes in sea level rise and coastal flooding are not evenly distributed across the United States; ocean circulation changes, sinking land, and Antarctic ice melt will result in greater-than-average sea level rise for the Northeast and western Gulf of Mexico under lower scenarios and most of the U.S. coastline other than Alaska under higher scenarios (very high confidence). Since the 1960s, sea level rise has already increased the frequency of high tide flooding by a factor of 5 to 10 for several U.S. coastal communities. The frequency, depth, and extent of tidal flooding is expected to continue to increase in the future (high confidence), as is the more severe flooding associated with coastal storms, such as hurricanes and nor’easters (low confidence).

' uncertainties: "

Since NCA3,{{< tbib '1' 'dd5b893d-4462-4bb3-9205-67b532919566' >}} multiple authors have produced global or regional studies synthesizing the major process that causes global and local sea level change to diverge. The largest sources of uncertainty in the geographic variability of sea level change are ocean dynamic sea level change and, for those regions where sea level fingerprints for Greenland and Antarctica differ from the global mean in different directions, the relative contributions of these two sources to projected sea level change.

Uncertainties remain large with respect to the precise change in future risk of a major coastal impact at a specific location from changes in the most intense tropical cyclone characteristics and tracks beyond changes imposed from local sea level rise.

" uri: /report/nca4/chapter/our-changing-climate/finding/key-message-2-9 url: ~ - chapter_identifier: us-caribbean confidence: '

There is high confidence that freshwater availability will likely be constrained by the end of the century and medium confidence that extreme rainfall events will likely increase in intensity. There is high confidence that sea level rise will very likely cause saltwater intrusion impacts on coastal freshwater aquifers. There is medium confidence about likely changes to ecological life zones but low confidence about the distributional effects on the existing terrestrial ecosystems in the region.

' evidence: "

The average global atmospheric carbon dioxide (CO₂) concentration has increased from 378 parts per million (ppm) in 2005 to over 406 ppm during April of 2017. The rate of increase over this period appears to be constant, and there is no indication that the rate will decrease in the future.{{< tbib '146' '0b94246c-91be-4f95-ae61-d36fdf775ff3' >}} Several climate change studies have concluded that owing to increased atmospheric CO₂ and the consequent global climate change, rainfall will likely decrease in the region between now and the end of the century (e.g., Meehl et al. 2007, Biasutti et al. 2012, Campbell et al. 2011, Cashman et al. 2010{{< tbib '2' '03abb6ea-0525-4fac-a321-121ca0727673' >}}^,{{}}^,{{}}^,{{}}). Neelin et al. (2006){{< tbib '147' '680629ff-ef00-462b-9c16-d98dc1d3c163' >}} and Scatena (1998){{< tbib '148' '17117749-f561-47d3-96fe-a62683b61369' >}} have predicted increasingly severe droughts in the region in the future. Several downscaling studies, which specifically considered Puerto Rico, predict a reduction in rainfall by the end of the century{{< tbib '6' '72d1011e-bdff-49c0-b00f-8222c2a350ea' >}}^,{{}}^,{{}} and constraints on freshwater availability. Furthermore, Taylor et al. (2018){{< tbib '149' '9de0b9c6-ce59-4940-8a6c-a244f1fa7a9c' >}} used the most recent generation of global climate models and demonstrated that when global warming increases from 1.5°C to 2°C above the preindustrial values (1861–1900), the Caribbean experiences a shift to predominantly drier conditions. Small watersheds that feed reservoirs are typical of the Caribbean region, and they are less able to serve as a buffer for rainfall variability. Small watersheds exhibit variable drainage patterns, which in turn affect evapotranspiration, groundwater infiltration, and surface water runoff. Drainage patterns in watersheds are also affected by the specific geometry, configuration, and orientation in relation to the average direction of wind over the region, as well as the morphology of rivers. With a projected reduction in rainfall up to 30% on average for the island by the end of the century,{{< tbib '7' '650b2907-85b1-4b76-a339-a9ec1703c5bd' >}} certain watersheds will likely be less able to buffer rainfall variability and will likely see water deficits in the near future. Increasing variability in rainfall events and increasing temperatures will likely exacerbate existing problems in water management, planning, and infrastructure capacity.

Streamflow is estimated using hydrologic models that are calibrated to networks of stream gauges and precipitation measurements. Reservoirs are considered in a permanent supply deficit if the annual streamflow leaving these reservoirs falls below zero after estimating withdrawals for human consumption, evapotranspiration, and rainfall. Projections of when deficit conditions could occur (circa 2025) are estimated using climate models.{{< tbib '46' 'a045f06c-0964-4286-9b5a-9b625da4eb2d' >}}

Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater in coastal aquifers. In Puerto Rico, groundwater quality can change when the water table is below sea level in coastal areas or when the intensity of pumping induces local upconing of deeper, poor-quality water.{{< tbib '43' '553e2d0a-c0ad-4540-9c5f-1f47374129ec' >}} Upconing is the process by which saline water underlying freshwater in an aquifer rises upward into the freshwater zone due to pumping.{{< tbib '150' '4375edd4-4f85-4a9f-bf62-37c2985ade2b' >}} When the water table is below sea level, the natural discharge of groundwater along the coast is reversed and can result in the inland movement of seawater or the upconing of low-quality water.{{< tbib '151' '1b555f67-0af6-4f16-882b-0c253117b9c8' >}}^,{{}} Diminished aquifer recharge and, to a lesser extent, increased groundwater withdrawals during 2012–2015 resulted in a reduction in the freshwater saturated thickness of the South Coast Aquifer. With sea level rise, groundwater quality will likely deteriorate even further in coastal aquifers in Puerto Rico.

" href: https://data.globalchange.gov/report/nca4/chapter/us-caribbean/finding/key-message-20-1.yaml identifier: key-message-20-1 ordinal: 1 process: '

The majority of our Key Messages were developed over the course of two separate author meetings. The first occurred March 9–10, 2017, and the second on May 3, 2017. Both meetings were held in San Juan, Puerto Rico; however, people were also able to join remotely from Washington, DC, Raleigh, North Carolina, and the U.S. Virgin Islands (USVI). In addition, the author team held weekly conference calls and organized separate Key Message calls and meetings to review and draft information that was integral to our chapter. To develop the Key Messages, the team also deliberated with outside experts who are acknowledged as our technical contributors.

' report_identifier: nca4 statement: '

Freshwater is critical to life throughout the Caribbean. Increasing global carbon emissions are projected to reduce average rainfall in this region by the end of the century (likely, high confidence), constraining freshwater availability, while extreme rainfall events, which can increase freshwater flooding impacts, are expected to increase in intensity (likely, medium confidence). Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater in coastal aquifers (very likely, high confidence). Increasing variability in rainfall events and increasing temperatures will likely alter the distribution of ecological life zones and exacerbate existing problems in water management, planning, and infrastructure capacity (likely, medium confidence).

' uncertainties: "

As global changes continue to alter the hydrological cycle across the region, water resources are expected to be affected in both quantity and quality. There is still uncertainty as to the extent and severity of these global changes on small island nations such as Puerto Rico and the USVI, despite notable advancements in downscaled modeling exercises. Current climatological observations have presented an overall increase in mean annual precipitation across Puerto Rico.{{< tbib '153' '0049e302-7751-4977-91ff-0df54d0ab326' >}} However, climate model projections point toward an overall decrease in annual mean precipitation toward 2050 and an increase in rainfall intensity for extreme rainfall,{{< tbib '6' '72d1011e-bdff-49c0-b00f-8222c2a350ea' >}}^,{{}}^,{{}}^,{{}}^,{{}}^,{{}} including rainfall associated with hurricanes. There is more uncertainty regarding the frequency and duration to changes in extreme rainfall within the region.{{< tbib '7' '650b2907-85b1-4b76-a339-a9ec1703c5bd' >}}^,{{}}^,{{}}

Selected CMIP3 (Coupled Model Intercomparison Project, phase 3) and CMIP5 global climate models (GCMs) capture the general large-scale atmospheric circulation that controls seasonal rainfall patterns within the Caribbean{{< tbib '155' '5d493a0a-db95-418d-ad99-148d753db96a' >}} and provide justification that these GCM projections can be further downscaled to capture important rainfall characteristics associated with the islands.{{< tbib '156' 'e16c77ed-0eaf-4fa6-8c98-256a28794b3b' >}} Systemic dry biases exist, however, in the GCMs.{{< tbib '155' '5d493a0a-db95-418d-ad99-148d753db96a' >}} And many GCMs fail to capture the bimodal precipitation pattern in the region.{{< tbib '28' '56d77153-c8fc-4fcf-a7f0-fa0e843936f1' >}} The CMIP3 generation of GCMs that do capture the bimodal rainfall pattern predict extreme drying at the middle and end of this century.{{< tbib '7' '650b2907-85b1-4b76-a339-a9ec1703c5bd' >}}^,{{}} The CMIP5 generation of GCMs also projects drying by the middle and end of the century, but the magnitude of drying is not as large. Local and island-scale processes could affect these projected changes, since the land surface interacts with and affects both precipitation and evaporation rates.{{< tbib '157' 'f0dee221-fc70-498e-a618-4b272642bab2' >}}

" uri: /report/nca4/chapter/us-caribbean/finding/key-message-20-1 url: ~