finding 2.8 : key-message-2-8

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).



This finding is from chapter 2 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

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),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,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,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 and what is new since the release of the Third NCA in 2014.dd5b893d-4462-4bb3-9205-67b532919566

Description of evidence base:

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 assessment47a5196b-4fba-4fdb-8647-8945627725bb and a large number of more recent studies that examined the magnitude of the observed tropical widening and various causes.a80ce47f-ac9a-43d2-9179-acad0e28e05a,798360ca-4177-462c-991a-c7a512d9287c,025b4915-8b20-4d53-b097-c737d39f3d62,37d85f6f-8d91-45e8-bf65-0ae8aee523a6,d377ad38-12f5-4186-998f-5643dfc6d141,a03de91a-43b9-4ca8-ba49-b9a55c28c493,d5eb689b-306c-4092-92ab-80958283a00c,c933c170-d353-4a2b-8855-d6b147d58d16,633af288-eb2c-4e4f-b5ba-9397ad897d7d,afc9d3fd-49bb-413c-bdec-75d9b75c2b8b 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.938444d5-cf1e-43b2-93d8-d8e403a23344,46b31794-a9c0-402d-a813-a65c4e87097b There is emerging evidence for an impact of anthropogenic aerosols on the tropical expansion in the Northern Hemisphere.aaf41b2e-d066-40c4-8bbf-d14ed62e13d6,6e2d8723-4a29-4bea-97d2-b300a6c18cd6 Recent studies provide new evidence on the significance of internal variability on recent changes in the tropical width.d5eb689b-306c-4092-92ab-80958283a00c,c18f45a8-0b79-49ed-b24f-98e994961b8a,c84efce6-8036-4c20-bf98-d0d5b987cd2c

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)dd5b893d-4462-4bb3-9205-67b532919566 and internationalf03117be-ccfe-4f88-b70a-ffd4351b8190 assessments. Since these assessments, more recent downscaling studies have further supported these assessments (e.g., Knutson et al. 20154f1e7aa1-0c36-4220-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).67ee7e56-b6a2-4ada-a7e8-ff836b1c58d1 Warner et al. (2015),40ffbbdf-74f1-4511-b1f1-a2b2a165185e Payne and Magnusdottir (2015),d13ddcaa-9080-4fab-9514-c45365ed3740 Gao et al. (2015),60ce531d-0064-4170-8b4d-e63bbb9f0c67 Radić et al. (2015),8927a54e-415e-4af2-aeb8-665cfe2d17ee and Hagos et al. (2016).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),59dfa0b2-2e94-4eb9-89fd-3adbbd1d61d4 Dettinger (2011),67ee7e56-b6a2-4ada-a7e8-ff836b1c58d1 Ralph et al. (2006),8caee927-3ee1-4084-a42e-e9487f4ebedf Neiman et al. (2011),a73e96c6-679f-4f76-a749-571f43601e5c Moore et al. (2012),ad8a08da-1ddc-452c-ac17-a5208fa4fe09 and Dettinger (2013).84acc46e-9dcf-43e7-8acc-07f07167ee8e

New information and remaining uncertainties:

The rate of observed expansion of the tropics depends on which metric is used.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.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.

Assessment of confidence based on evidence:

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.

References :

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