finding 9.5 : key-finding-9-5

The frequency and severity of landfalling “atmospheric rivers” on the U.S. West Coast (narrow streams of moisture that account for 30%–40% of the typical snowpack and annual precipitation in the region and are associated with severe flooding events) will increase as a result of increasing evaporation and resulting higher atmospheric water vapor that occurs with increasing temperature (medium confidence).

This finding is from chapter 9 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: 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 theses along the coast (for example, northern vs. southern California).

Description of evidence base: The Key Finding and supporting text summarizes evidence documented in the climate science literature.

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 et al. 2011;04add85e-594b-4d92-83a6-fdce66b73aa4 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 et al. 2011;04add85e-594b-4d92-83a6-fdce66b73aa4 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: 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 atmospheric rivers projected/expected is based to a very large degree on the 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 atmospheric rivers. A modest uncertainty arises in quantifying the expected change at a regional level (for example, northern Oregon vs. 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: Confidence in this finding is rated as medium based on qualitatively similar projections among different studies.

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

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