finding 9.4 : key-finding-9-4

There has been a trend toward earlier snowmelt and a decrease in snowstorm frequency on the southern margins of climatologically snowy areas (medium confidence). Winter storm tracks have shifted northward since 1950 over the Northern Hemisphere (medium confidence). Projections of winter storm frequency and intensity over the United States vary from increasing to decreasing depending on region, but model agreement is poor and confidence is low. Potential linkages between the frequency and intensity of severe winter storms in the United States and accelerated warming in the Arctic have been postulated, but they are complex, and, to some extent, contested, and confidence in the connection is currently low.

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

Process for developing key messages: Decreases in snowfall on southern and low elevation margins of currently climatologically snowy areas are likely but winter storm frequency and intensity changes are uncertain.

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

Evidence for changes in winter storm track changes are documented in a small number of studies.57610605-1682-4827-878f-c12c6e9b674c d4efd07e-2886-41f4-bc6d-14cbbe00c382 Future changes are documented in one study,72c3b2b9-4739-4506-9c71-017d8b2e5181 but there are large model-to-model differences. The effects of arctic amplification on U.S. winter storms have been studied, but the results are mixed,90b56ea1-7d35-473e-8d09-fc55778055a6 29ef1bb5-092f-4a1b-ab0e-1fdc5a0e2ae0 9eef4c2e-2b3f-4386-af91-10e83338a398 38b8da08-bb41-4f40-87d1-24e10a2e2ab9 leading to considerable uncertainties.

New information and remaining uncertainties: Key remaining uncertainties relate to the sensitivity of observed snow changes to the spatial distribution of observing stations and to historical changes in station location and observing practices. There is conflicting evidence about the effects of arctic amplification on CONUS winter weather.

Assessment of confidence based on evidence: There is high confidence that warming has resulted in earlier snowmelt and decreased snowfall on the warm margins of areas with consistent snowpack based on a number of observational studies. There is medium confidence that Northern Hemisphere storm tracks have shifted north based on a small number of studies. There is low confidence in future changes in winter storm frequency and intensity based on conflicting evidence from analysis of climate model simulations.

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

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