finding 6.4 : key-finding-6-4

Extreme temperatures in the contiguous United States are projected to increase even more than average temperatures. The temperatures of extremely cold days and extremely warm days are both expected to increase. Cold waves are projected to become less intense while heat waves will become more intense. The number of days below freezing is projected to decline while the number above 90°F will rise. (Very high confidence)

This finding is from chapter 6 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: There is very high confidence in projected changes in temperature extremes over the United States based upon the convergence of evidence from multiple model simulations, analyses, and assessments.

Description of evidence base: The key finding and supporting text summarize extensive evidence documented in the climate science literature (e.g., Fischer et al. 2013;fd46b5d5-da35-4c38-80cc-8d9bd688f116 Sillmann et al. 2013;68c9d1ed-fd78-4967-b47d-2504ac27b649 Wuebbles et al. 2014;b91893b4-24a8-46ba-b09a-013d462caf1b Sun et al. 2015b63c9720-f770-4718-89cc-53b3616e2bec). Similar statements about changes have also been made in other national assessments (such as NCA3) and in reports by the Climate Change Science Program (such as SAP 3.3: Weather and Climate Extremes in a Changing Climate12d42a98-494b-4cec-bf08-060021c85ec2).

Projections are based on global model results and associated downscaled products from CMIP5 for RCP4.5 (lower scenario) and RCP8.5 (higher scenario). Model weighting is employed to refine projections for each RCP. Weighting parameters are based on model independence and skill over North America for seasonal temperature and annual extremes. The multimodel mean is based on 32 model projections that were statistically downscaled using the Localized Constructed Analogs technique.62c66ef3-cddb-4797-ba0e-5672fbcc27b3 Downscaling improves on the coarse model output, establishing a more geographically accurate baseline for changes in extremes and the number of days per year over key thresholds. The upper bound for projected changes is the average of the three warmest models. All increases are significant (i.e., more than 50% of the models show a statistically significant change, and more than 67% agree on the sign of the changeb63c9720-f770-4718-89cc-53b3616e2bec).

New information and remaining uncertainties: Global climate models are subject to structural and parametric uncertainty, resulting in a range of estimates of future changes in temperature extremes. This is partially mitigated through the use of model weighting and pattern scaling. Furthermore, virtually every ensemble member of every model projection contains an increase in temperature by mid- and late-century. Empirical downscaling introduces additional uncertainty (e.g., with respect to stationarity).

Assessment of confidence based on evidence: Very high

Likelihood of Impact:

Extremely likely

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

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