finding 8.4 : key-finding-8-4

Substantial reductions in western U.S. winter and spring snowpack are projected as the climate warms. Earlier spring melt and reduced snow water equivalent have been formally attributed to human induced warming (high confidence) and will very likely be exacerbated as the climate continues to warm (very high confidence). Under higher scenarios, and assuming no change to current water resources management, chronic, long-duration hydrological drought is increasingly possible by the end of this century (very high confidence).

This finding is from chapter 8 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Warmer temperatures lead to less snow and more rain if total precipitation remains unchanged. Projected winter/spring precipitation changes are a mix of increases in northern states and decreases in the Southwest. In the northern Rocky Mountains, snowpack is projected to decrease even with a projected precipitation increase due to this phase change effect. This will lead to, at the very least, profound changes to the seasonal and sub-seasonal timing of the western U.S. hydrological cycle even where annual precipitation remains nearly unchanged with a strong potential for water shortages.

Description of evidence base: First principles tell us that as temperatures rise, minimum snow levels also must rise. Certain changes in western U.S. hydrology have already been attributed to human causes in several papers following Barnett et al.87875dde-385b-4f57-b0ae-aa21648b2833 and are cited in the text. The CMIP3/5 models project widespread warming with future increases in atmospheric GHG concentrations, although these are underestimated in the current generation of global climate models (GCMs) at the high altitudes of the western United States due to constraints on orographic representation at current GCM spatial resolutions.
CMIP5 models were not designed or constructed for direct projection of locally relevant snowpack amounts. However, a high-resolution climate model, selected for its ability to simulate western U.S. snowpack amounts and extent, projects devastating changes in the hydrology of this region assuming constant water resource management practices.2d28c6ea-6b35-41bb-824a-9f18afbced09 This conclusion is also supported by a statistical downscaling result shown in Figure 3.1 of Walsh et al.a6a312ba-6fd1-4006-9a60-45112db52190 and Cayan et al.59cf5a07-b64b-418c-acc6-73b7ff4d7d7e and by the more recent statistical downscaling study of Klos et al.df25e033-b388-4aab-b7a4-00d6a9ef3e7e.

New information and remaining uncertainties: The major uncertainty is not so much “if” but rather “how much” as changes to precipitation phases (rain or snow) are sensitive to temperature increases that in turn depend on greenhouse gas (GHG) forcing changes. Also, changes to the lower-elevation catchments will be realized prior to those at higher elevations that, even at 25 km, are not adequately resolved. Uncertainty in the final statement also stems from the usage of one model but is tempered by similar findings from statistical downscaling studies. However, this simulation is a so-called “prescribed temperature” experiment with the usual uncertainties about climate sensitivity wired in by the usage of one particular ocean temperature change. Uncertainty in the equator-to-pole differential ocean warming rate is also a factor.

Assessment of confidence based on evidence: All CMIP5 models project large-scale warming in the western United States as GHG forcing increases. Warming is underestimated in most of the western United States due to elevation deficiencies that are a consequence of coarse model resolution.

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

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