finding 7.1 : key-finding-7-1

Annual precipitation has decreased in much of the West, Southwest, and Southeast and increased in most of the Northern and Southern Plains, Midwest, and Northeast. A national average increase of 4% in annual precipitation since 1901 is mostly a result of large increases in the fall season. (Medium confidence)

This finding is from chapter 7 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Based on the patterns shown in Figure 7.1 and numerous additional studies of precipitation changes in the United States, there is medium confidence in the observed changes in annual and seasonal precipitation over the various regions and the United States as a whole.

Description of evidence base: The key finding and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Evidence of long-term changes in precipitation is based on analysis of daily precipitation observations from the U.S. Cooperative Observer Network ( and shown in Figure 7.1. Published work, such as the Third National Climate Assessment,dd5b893d-4462-4bb3-9205-67b532919566 and Figure 7.1 show important regional and seasonal differences in U.S. precipitation change since 1901.

New information and remaining uncertainties: The main source of uncertainty is the sensitivity of observed precipitation trends to the spatial distribution of observing stations and to historical changes in station location, rain gauges, the local landscape, and observing practices. These issues are mitigated somewhat by new methods to produce spatial grids through time.3d9da69e-293a-4492-a418-682590c676c7

Assessment of confidence based on evidence: Based on the evidence and understanding of the issues leading to uncertainties, confidence is medium that average annual precipitation has increased in the United States. Furthermore, confidence is also medium that the important regional and seasonal differences in changes documented in the text and in Figure 7.1 are robust.

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

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