finding 6.2 : key-finding-6-2

There have been marked changes in temperature extremes across the contiguous United States. The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s. The Dust Bowl era of the 1930s remains the peak period for extreme heat. The number of high temperature records set in the past two decades far exceeds the number of low temperature records. (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 observed changes in temperature extremes 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 have also been made in other reports (e.g., NCA3;dd5b893d-4462-4bb3-9205-67b532919566 SAP 3.3: Weather and Climate Extremes in a Changing Climate;12d42a98-494b-4cec-bf08-060021c85ec2 IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation089d8050-f4c8-4d07-bc35-25bf61691be3).

Evidence for changes in U.S. climate arises from multiple analyses of in situ data using widely published climate extremes indices. For the analyses presented here, the source of in situ data is the Global Historical Climatology Network–Daily dataset,9b433446-b58f-4358-9737-5a6ccc2f6fcf with changes in extremes being assessed using long-term stations with minimal missing data to avoid network-induced variability on the long-term time series. Cold wave frequency was quantified using the Cold Spell Duration Index,e6ecbe14-fe1b-46f8-bad5-bde9e4cc658a heat wave frequency was quantified using the Warm Spell Duration Index,e6ecbe14-fe1b-46f8-bad5-bde9e4cc658a and heat wave intensity were quantified using the Heat Wave Magnitude Index Daily.546ef0fe-bfae-43ee-969e-5870c581e426 Station-based index values were averaged into 4° grid boxes, which were then area-averaged into a time series for the contiguous United States. Note that a variety of other threshold and percentile-based indices were also evaluated, with consistent results (e.g., the Dust Bowl was consistently the peak period for extreme heat). Changes in record-setting temperatures were quantified as in Meehl et al. (2016).72301197-e20a-4328-accb-4276341a25db

New information and remaining uncertainties: The primary uncertainties for in situ data relate to historical changes in station location, temperature instrumentation, observing practice, and spatial sampling (particularly the precision of estimates of change in areas and periods with low station density, such as the intermountain West in the early 20th century).

Assessment of confidence based on evidence:

Very high


Likelihood of Impact:

Extremely likely


References :

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