finding 2.9 : key-message-2-9

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).

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 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.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).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.ab69428a-34a4-412f-8c85-b3bb8043509c,4c41b38a-7d35-470c-82cb-fec14a4307cf These observations are corroborated by glacio-isostatic adjustment models, by global positioning satellite (GPS) observations, and by geological data (e.g., Engelhart and Horton 2012427648bc-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.1823b427-f097-418f-9d4b-c2f7e9291874,7c979a1d-a012-4e44-8824-fa4a44c3736a GCM studies indicate the potential for a Gulf Stream contribution to sea level rise in the U.S. Northeast.0e116266-7679-409f-b1d6-99c31edfcd9e,9d9fd9a7-2def-4cf2-8e2e-2c23423f0a6e Kopp et al. (2014)38924fa0-a0dd-44c9-a2a0-366ca610b280 and Slangen et al. (2014)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.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.3bae2310-7572-47e2-99a4-9e4276764934,c66bf5a9-a6d7-4043-ad99-db0ae6ae562c,38924fa0-a0dd-44c9-a2a0-366ca610b280,be9c98a9-915f-4d78-80b9-239b70f93ae2,7f2a9c53-487c-4aed-84e1-43a8cd090c0b,728f4919-d8c6-4749-968e-18a16550540b,4c5c2377-b849-4a82-bb6d-26078d4d2d32,01052852-079c-4b99-b224-001c783288bd,d2dc9855-41bc-4e94-bb79-f0ba2ff2684b 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.d6bd92ad-67ef-4df7-aca9-68944523e863,49de726f-4295-42f5-9c2f-81b28882856a,94fe6e03-5a6e-4ec0-88e7-bb4f1a839af4,523a596c-a2b3-458f-b8f3-6e386f21c026,753592b0-089d-4e67-a1d6-a8e985a7e9a8 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. 2013d6bd92ad-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.134b5712-13e2-4837-b710-027fe9028e8f

New information and remaining uncertainties:

Since NCA3,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.

Assessment of confidence based on evidence:

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.

References :

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