finding 12.3 : key-finding-12-3

Relative sea level (RSL) rise in this century will vary along U.S. coastlines due, in part, to changes in Earth’s gravitational field and rotation from melting of land ice, changes in ocean circulation, and vertical land motion (very high confidence). For almost all future GMSL rise scenarios, RSL rise is likely to be greater than the global average in the U.S. Northeast and the western Gulf of Mexico. In intermediate and low GMSL rise scenarios, RSL rise is likely to be less than the global average in much of the Pacific Northwest and Alaska. For high GMSL rise scenarios, RSL rise is likely to be higher than the global average along all U.S. coastlines outside Alaska. Almost all U.S. coastlines experience more than global-mean sea-level rise in response to Antarctic ice loss, and thus would be particularly affected under extreme GMSL rise scenarios involving substantial Antarctic mass loss (high confidence).

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

Process for developing key messages: The part of the key finding 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 Interagency Sea Level Rise Task Force.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c

Description of evidence base: The processes that cause geographic variability in RSL change are reviewed by Kopp et al. e8f60819-839e-4772-8a49-7c57d9c53424 Long tide gauge data sets show the RSL rise caused 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 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.7c979a1d-a012-4e44-8824-fa4a44c3736a 1823b427-f097-418f-9d4b-c2f7e9291874 GCM studies indicate the potential for a Gulf Stream contribution to sea level rise in the U.S. Northeast.9d9fd9a7-2def-4cf2-8e2e-2c23423f0a6e 0e116266-7679-409f-b1d6-99c31edfcd9e Kopp et al.38924fa0-a0dd-44c9-a2a0-366ca610b280 and Slangen et al.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 indicate 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 are developed by an Interagency Sea Level Rise Task Force.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c

New information and remaining uncertainties: Since NCA3, 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.

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.

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

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