- Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II
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finding 8.1 : key-message-8-1
America’s trillion-dollar coastal property market and public infrastructure are threatened by the ongoing increase in the frequency, depth, and extent of tidal flooding due to sea level rise, with cascading impacts to the larger economy. Higher storm surges due to sea level rise and the increased probability of heavy precipitation events exacerbate the risk. Under a higher scenario (RCP8.5), many coastal communities will be transformed by the latter part of this century, and even under lower scenarios (RCP4.5 or RCP2.6), many individuals and communities will suffer financial impacts as chronic high tide flooding leads to higher costs and lower property values. Actions to plan for and adapt to more frequent, widespread, and severe coastal flooding would decrease direct losses and cascading economic impacts. (Likely, High Confidence)
This finding is from chapter 8 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.
Process for developing key messages:
The selection of the author team for the Coastal Effects chapter took into consideration the wide scope and relative sufficiency of the Third National Climate Assessment (NCA3) Coastal chapter. With input and guidance from the NCA4 Federal Steering Committee, the coordinating lead authors made the decision to convene an all-federal employee team with representation from key federal agencies with science, management, and policy expertise in climate-related coastal effects, and to focus the content of the chapter on Key Messages and themes that would both update the work conducted under NCA3 and introduce new themes. For additional information on the author team process and structure, refer to Appendix 1: Process.
A central component of the assessment process was a chapter lead authors’ meeting held in Washington, DC, in May 2017. The Key Messages were initially developed at this meeting. Key vulnerabilities were operationally defined as those challenges that can fundamentally undermine the functioning of human and natural coastal systems. They arise when these systems are highly exposed and sensitive to climate change and (given present or potential future adaptive capacities) insufficiently prepared or able to respond. The vulnerabilities that the team decided to focus on were informed by a review of the existing literature and by ongoing interactions of the author team with coastal managers, planners, and stakeholders. In addition, the author team conducted a thorough review of the technical inputs and associated literature. Chapter development was supported by numerous chapter author technical discussions via teleconference from April to September 2017.
Description of evidence base:
Significant impacts to coastal communities, properties, infrastructure, and services are already occurring in low-lying areas of the country such as Miami Beach and Fort Lauderdale in Florida; Norfolk, Virginia; and Charleston, South Carolina.66ea5840-4fdb-457c-a206-5c09d331445c,1a961dc6-30e7-44b6-80be-dc32ef761fe4,5f4de85b-be39-4ffd-ac94-1950932c0140,58862f49-f6d0-43bd-ac88-522a61696973,048006a1-a72d-44a1-bdab-fff317c842f7
Satellite and tide gauge data show that sea level rise (SLR) rates are increasing,c66bf5a9-a6d7-4043-ad99-db0ae6ae562c and research has shown that this increase is driven by emissions that are warming the planet.94a8514e-063e-45ef-b893-11c82b49a597,a0130167-b319-493d-bedc-7cab8f8fe9d9 The latest SLR scienced29e5080-da7b-41a3-a144-3a38225a0bd5,c66bf5a9-a6d7-4043-ad99-db0ae6ae562c,3bae2310-7572-47e2-99a4-9e4276764934,8e8cff98-5658-4597-8fb4-9088556acfae finds that even if RCP2.6 were achieved, it is likely that global mean sea level will rise by 1.5 feet by 2100; under RCP8.5, a rise of about 3 feet is within the likely range for 2100. Recent probabilistic studies and assessments of future SLR and rapid ice loss from Antarctica find that although a low probability, there is a possibility of upwards of 8 feet of rise by 2100 under a high-emission, extreme melt scenario.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c,38924fa0-a0dd-44c9-a2a0-366ca610b280,ae82c8a3-3033-4103-91e9-926a27d1fa18,c748bd06-bc78-4b9c-b511-7dab1974211e,825b2e35-d2cb-45d5-b52b-1beb4c9f0153,8e8cff98-5658-4597-8fb4-9088556acfae
Applying digital elevation models to determine the extent and number of communities and the amount of property and infrastructure that would be impacted by different amounts of SLR illustrates the magnitude of investments that are at risk.2db2b107-2e02-4f3a-b1e7-98301e28395d,6e83fde3-5f98-4fd1-ae2c-d11aced414ac,5f4de85b-be39-4ffd-ac94-1950932c0140,0e427bc4-00fb-4c0f-af2d-c4ae9ac7feec,a05520ac-73d9-4939-8aa0-0587ec9bae09,3a93b470-16dc-47ae-bf96-87558ce7eb35,bd3dbfa7-8dc4-4442-9cf2-14f583dc4a36 These same analyses demonstrate the savings that could be achieved by lowering emissions. Finally, implementing adaptation measures to ensure that public infrastructure is resilient to current and future flood scenarios will be tremendously expensive. To date there are few economic sectoral models that quantify damages under alternative climate scenarios,6e83fde3-5f98-4fd1-ae2c-d11aced414ac,bd3dbfa7-8dc4-4442-9cf2-14f583dc4a36 so additional modeling work would be useful.
The importance of coastal economies and infrastructure to the overall national economy is well documented (for example, the National Oceanic and Atmospheric Administration’s [NOAA] Economics: National Ocean Watch; NOAA port data), as are the economic ripple effects of impacts to property markets.6e83fde3-5f98-4fd1-ae2c-d11aced414ac,741c98c3-07ef-468e-a9c8-0126361756f0,3a93b470-16dc-47ae-bf96-87558ce7eb35,021da15c-93a1-483a-9f2f-7e3d7f104b59,d307bcf3-8688-40c9-a5a7-240a567e3dfb Similarly, much has been written about how the National Flood Insurance Program has subsidized development in risky areas and how raising flood insurance rates to be actuarially sound could make it impossible for many coastal residents to afford flood insurance.741c98c3-07ef-468e-a9c8-0126361756f0,3e49b758-3670-4137-ac4b-9093427fead9,63f095eb-2a3d-4647-a14d-84160d60dbe6,f6b2dda9-0abf-40d8-baf6-aa461e1b2b3d,32404d0b-928c-4338-9c1a-b14282cb9491 The evidence for the economic savings provided by adaptation investments is still fairly limited but growing.1ce5a44f-53b3-42b8-92f1-e2ecf686c74f,6e83fde3-5f98-4fd1-ae2c-d11aced414ac,5b27123a-8c6d-4e85-bd48-841436fdf9eb,5239d072-e6d2-4d86-baa3-f978a93c48e1
New information and remaining uncertainties:
The main source of uncertainty is in the magnitude of SLR that will occur and how it will vary across different regions, which depend in part on the amount and speed with which global society will reduce emissions. While global climate models and SLR models have improved since NCA3,dd5b893d-4462-4bb3-9205-67b532919566 uncertainty remains about exactly how much SLR will occur where and by when with different emissions levels. Even though there is uncertainty about the magnitude, the probabilistic approach to the SLR technical report to the Fourth National Climate Assessment,c66bf5a9-a6d7-4043-ad99-db0ae6ae562c together with impacts already documented around the country from high tide flooding,33a582e6-f606-4e31-bb1f-6c3da8cfd45d gives us high confidence of the threat to coastal property and infrastructure. Adaptive responses to SLR risk and impacts, including individual action and public policy development, are also significant sources of uncertainty. For example, there is uncertainty about future development patterns in coastal regions, including both new development and migration inland, which has the potential to change the magnitude of coastal property and infrastructure at risk. The U.S.-specific research on potential migration away from the coast due to SLR and other climate impacts is very limited.2ddba35f-6036-4428-b4c7-800dd57b3313
Future flood insurance policy is another specific source of uncertainty. Under the latest legislation (the Federal Emergency Management Agency’s Homeowner Flood Insurance Affordability Act32404d0b-928c-4338-9c1a-b14282cb9491), flood insurance rates are gradually rising; development of new policies related to affordability or to the requirement to carry flood insurance in order to have a federally backed mortgage could change behaviors.
While figures for the economic value of certain sectors dependent on the ocean and Great Lakes are available through NOAA’s “Economics: National Ocean Watch,”2aa0aa30-d41d-4c09-8557-2cb1e40c3f7f similar information for the economic and social value of other sectors, such as real estate and insurance/reinsurance, would be beneficial for the audience of this assessment report, especially decision-makers.
Assessment of confidence based on evidence:
There is very high confidence that the frequency and extent of tidal flooding is already increasing and will continue to increase with SLR and that this flooding threatens the trillion-dollar coastal property market and public infrastructure. There is limited research using varied methods to quantify the direct and indirect economic impacts that will be experienced under different amounts of SLR. Nevertheless, there is a high level of confidence that these losses will be dramatic under SLR associated with the higher emission scenario (RCP8.5) and significant even under lower scenarios (RCP4.5 or RCP2.6), based on property values and geographic exposure to inundation. U.S. economic history provides strong evidence that extensive property market losses have the potential to impact businesses, personal wealth, and mortgage-related securities. Similarly, historic disaster events such as hurricanes and earthquakes provide a very high level of confidence that impacts to critical transportation and energy networks will harm the economy. Considering the uncertainty inherent in future human behavior and policy responses, including flood insurance policy, it is possible that individuals and institutions will act to reduce future flooding, to lessen the exposure and sensitivity of critical assets, and to create policies that assist individuals and businesses most impacted; hence, there is medium confidence that many coastal communities will be transformed by 2100 under any scenario and that many individuals will be financially devastated under lower emission scenarios (RCP4.5 or RCP2.6). Considering current exposure of assets and the latest SLR science, large economic losses in coastal regions that will generate cascading impacts to the overall economy of the United States are considered to be likely. The overall high confidence is the net result of considering the evidence base, the well-established accumulation of economic assets and activities in coastal areas, and the directional trend of sea level rise.
- webpage Climate Change and Housing: Will a Rising Tide Sink All Homes? (021da15c)
- State of U.S. Nuisance Tidal Flooding. Supplement to State of the Climate: National Overview for May 2016 (048006a1)
- webpage Sea Level Rise Viewer [web tool] (0e427bc4)
- Natural catastrophes and man-made disasters in 2012: A year of extreme weather events in the US (1ce5a44f)
- webpage Economics: National Ocean Watch [data] (2aa0aa30)
- Water's edge: The crisis of rising sea levels (2db2b107)
- Migration induced by sea-level rise could reshape the US population landscape (2ddba35f)
- webpage Homeowner Flood Insurance Affordability Act (32404d0b)
- Cumulative hazard: The case of nuisance flooding (33a582e6)
- Probabilistic 21st and 22nd century sea-level projections at a global network of tide-gauge sites (38924fa0)
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- chapter climate-science-special-report chapter 12 : Sea Level Rise (3bae2310)
- Financing Flood Losses: A Discussion of the National Flood Insurance Program (3e49b758)
- Natural Hazard Mitigation Saves: 2017 Interim Report – An Independent Study (5239d072)
- Joint effects of storm surge and sea-level rise on US Coasts: New economic estimates of impacts, adaptation, and benefits of mitigation policy (5b27123a)
- Sea level rise drives increased tidal flooding frequency at tide gauges along the U.S. East and Gulf Coasts: Projections for 2030 and 2045 (5f4de85b)
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- Risky Business: the Economic Risks of Climate Change in The United States (6e83fde3)
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- Impacts of Antarctic fast dynamics on sea-level projections and coastal flood defense (825b2e35)
- Evolving understanding of Antarctic ice‐sheet physics and ambiguity in probabilistic sea‐level projections (8e8cff98)
- Sea-Level Rise from the Late 19th to the Early 21st Century (94a8514e)
- Temperature-driven global sea-level variability in the Common Era (a0130167)
- webpage Coastal Storm Modeling System (CoSMoS) [web tool] (a05520ac)
- Contribution of Antarctica to past and future sea-level rise (ae82c8a3)
- Climate Change in the United States: Benefits of Global Action (bd3dbfa7)
- Global and Regional Sea Level Rise Scenarios for the United States (c66bf5a9)
- A probabilistic approach to 21st century regional sea-level projections using RCP and High-end scenarios (c748bd06)
- Patterns and projections of high tide flooding along the U.S. coastline using a common impact threshold (d29e5080)
- webpage Climate Change and Homes: Who Would Lose the Most to a Rising Tide? (d307bcf3)
- Climate Change Impacts in the United States: The Third National Climate Assessment (dd5b893d)
- webpage Flood insurance reform - The law (f6b2dda9)
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