finding 8.1 : key-finding-8-1

Recent droughts and associated heat waves have reached record intensity in some regions of the United States; however, by geographical scale and duration, the Dust Bowl era of the 1930s remains the benchmark drought and extreme heat event in the historical record (very high confidence). While by some measures drought has decreased over much of the continental United States in association with long-term increases in precipitation, neither the precipitation increases nor inferred drought decreases have been confidently attributed to anthropogenic forcing.

This finding is from chapter 8 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The key finding is a statement that recent U.S. droughts, while sometimes long and severe, are not unprecedented in the historical record.

Description of evidence base: Recent droughts are well characterized and described in the literature. The Dust Bowl is not as well documented, but available observational records support the key finding. The last sentence is an “absence of evidence” statement and does not imply “evidence of absence” of future anthropogenic changes. The inferred decreases in some measures of U.S. drought or types of drought (heat wave/flash droughts) are described in Andreadis and Lettenmaier1d369e9a-53fa-4829-9bfe-9b380c83aae9 and Mo and Lettenmaier7773d6ed-7b41-4d4f-bc64-c603cf28a660.

New information and remaining uncertainties: Record-breaking temperatures are well documented with low uncertainty.62bcd3f7-a6f1-4b0b-85a8-29cbf7412579 The magnitude of the Dust Bowl relative to present times varies with location. Uncertainty in the key finding is affected by the quality of pre-World War II observations but is relatively low.

Assessment of confidence based on evidence: Precipitation is well observed in the United States, leading to very high confidence.

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

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