finding 12.5 : key-finding-12-5

Assuming storm characteristics do not change, sea level rise will increase the frequency and extent of extreme flooding associated with coastal storms, such as hurricanes and nor’easters (very high confidence). A projected increase in the intensity of hurricanes in the North Atlantic (medium confidence) could increase the probability of extreme flooding along most of the U.S. Atlantic and Gulf Coast states beyond what would be projected based solely on RSL rise. However, there is low confidence in the projected increase in frequency of intense Atlantic hurricanes, and the associated flood risk amplification and flood effects could be offset or amplified by such factors as changes in overall storm frequency or tracks.

This finding is from chapter 12 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: This key finding is based upon several modeling studies of future hurricane characteristics and associated increases in major storm surge risk amplification.

Description of evidence base: The frequency, extent, and depth of extreme event-driven (for example, 5- to 100-year event probabilities) coastal flooding relative to existing infrastructure will continue to increase in the future as local RSL rises.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c 38924fa0-a0dd-44c9-a2a0-366ca610b280 d2dc9855-41bc-4e94-bb79-f0ba2ff2684b 4c5c2377-b849-4a82-bb6d-26078d4d2d32 be9c98a9-915f-4d78-80b9-239b70f93ae2 7f2a9c53-487c-4aed-84e1-43a8cd090c0b 728f4919-d8c6-4749-968e-18a16550540b 01052852-079c-4b99-b224-001c783288bd Extreme flood probabilities will increase regardless of change 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.49de726f-4295-42f5-9c2f-81b28882856a 94fe6e03-5a6e-4ec0-88e7-bb4f1a839af4 523a596c-a2b3-458f-b8f3-6e386f21c026 753592b0-089d-4e67-a1d6-a8e985a7e9a8 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, since a number of models project a substantial decrease in the overall number of tropical storms and hurricanes in the North Atlantic. Changes in the frequency of intense hurricanes depends on changes in both the overall frequency of tropical cyclones storms and their intensities. High-resolution models generally project an increase in mean hurricane intensity in the Atlantic (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: 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: There is low confidence that the flood risk at specific locations will be amplified from a major tropical storm this century.

This finding was derived from figure -.2: Confidence / Likelihood

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