finding 6.3 : key-finding-6-3

Annual average temperature over the contiguous United States is projected to rise (very high confidence). Increases of about 2.5°F (1.4°C) are projected for the period 2021–2050 relative to 1976–2005 in all RCP scenarios, implying recent record-setting years may be “common” in the next few decades (high confidence). Much larger rises are projected by late century (2071–2100): 2.8°–7.3°F (1.6°–4.1°C) in a lower scenario (RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in a higher scenario (RCP8.5) (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 average temperature 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. Similar statements about changes have also been made in other reports (e.g., NCA3;dd5b893d-4462-4bb3-9205-67b532919566 Global Climate Change Impacts in the United Statese251f590-177e-4ba6-8ed1-6f68b5e54c8a). The basic physics underlying the impact of human emissions on climate has also been documented in every IPCC assessment.

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 The range is defined as the difference between the average increase in the three coolest models and the average increase in 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 average temperature. 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 for projected change in annual average temperature; high confidence for record-setting years becoming the norm in the near future; high confidence for much larger temperature increases by late century under a higher scenario (RCP8.5).

Likelihood of Impact:

Extremely likely.

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

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