finding 6.1 : key-finding-6-1

Annual average temperature over the contiguous United States has increased by 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960 and by 1.8°F (1.0°C) based on a linear regression for the period 1895–2016 (very high confidence). Surface and satellite data are consistent in their depiction of rapid warming since 1979 (high confidence). Paleo-temperature evidence shows that recent decades are the warmest of the past 1,500 years (medium 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 observed changes in average temperature over the United States based upon the convergence of evidence from multiple data sources, 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 exist in other reports (e.g., NCA3;dd5b893d-4462-4bb3-9205-67b532919566 Global Climate Change Impacts in the United States;e251f590-177e-4ba6-8ed1-6f68b5e54c8a SAP 1.1: Temperature trends in the lower atmospheref135add4-6d4c-4d88-a8f1-b880dbf5334f).

Evidence for changes in U.S. climate arises from multiple analyses of data from in situ, satellite, and other records undertaken by many groups over several decades. The primary dataset for surface temperatures in the United States is nClimGrid,3d9da69e-293a-4492-a418-682590c676c7 cee1b7ee-b34f-409f-9b29-ca957b272e83 though trends are similar in the U.S. Historical Climatology Network, the Global Historical Climatology Network, and other datasets. Several atmospheric reanalyses (e.g., 20th Century Reanalysis, Climate Forecast System Reanalysis, ERA-Interim, Modern Era Reanalysis for Research and Applications) confirm rapid warming at the surface since 1979, with observed trends closely tracking the ensemble mean of the reanalyses. Several recently improved satellite datasets document changes in middle tropospheric temperatures.0215f34d-335f-4105-a3eb-b660e0ff8a78 42bc6c69-ca8d-4e06-8ad0-2fbad9cfd924 272f858e-f538-4096-b4dc-563fa824a538 Longer-term changes are depicted using multiple paleo analyses (e.g., Wahl and Smerdon 2012;dc59a0d7-9c9d-45d8-966d-cf4bdedddc5a Trouet et al. 20135a3d5be0-e40f-4ea7-8f99-422db7954577).

New information and remaining uncertainties: The primary uncertainties for surface data relate to historical changes in station location, temperature instrumentation, observing practice, and spatial sampling (particularly in areas and periods with low station density, such as the intermountain West in the early 20th century). Satellite records are similarly impacted by non-climatic changes such as orbital decay, diurnal sampling, and instrument calibration to target temperatures. Several uncertainties are inherent in temperature-sensitive proxies, such as dating techniques and spatial sampling.

Assessment of confidence based on evidence: Very high (since 1895), High (for surface/satellite agreement since 1979), Medium (for paleo)

Likelihood of Impact:

Extremely Likely

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

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