finding 8.5 : key-finding-8-5

Detectable changes in some classes of flood frequency have occurred in parts of the United States and are a mix of increases and decreases. Extreme precipitation, one of the controlling factors in flood statistics, is observed to have generally increased and is projected to continue to do so across the United States in a warming atmosphere. However, formal attribution approaches have not established a significant connection of increased riverine flooding to human-induced climate change, and the timing of any emergence of a future detectible anthropogenic change in flooding is unclear. (Medium confidence)

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 relatively weak statement reflecting the lack of definitive detection and attribution of anthropogenic changes in U.S. flooding intensity, duration, and frequency.

Description of evidence base: Observed changes are a mix of increases and decreases and are documented by Walsh et al.a6a312ba-6fd1-4006-9a60-45112db52190 and other studies cited in the text. No attribution statements have been made.

New information and remaining uncertainties: Floods are highly variable both in space and time. The multivariate nature of floods complicates detection and attribution.

Assessment of confidence based on evidence: Confidence is limited to medium due to both the lack of an attributable change in observed flooding to date and the complicated multivariate nature of flooding. However, confidence is high in the projections of increased future extreme precipitation, the principal driver (among several) of many floods. It is unclear when an observed long-term increase in U.S. riverine flooding will be attributed to anthropogenic climate change. Hence, confidence is medium in this part of the key message at this time.

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

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