Spatial and seasonal patterns in climate change, temperatures, and precipitation across the United States
- aNational Oceanic and Atmospheric Administration Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO 80305; and
- bSchool of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom
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Edited by Isaac M. Held, National Oceanic and Atmospheric Administration, Princeton, NJ, and approved March 17, 2009 (received for review August 28, 2008)
Abstract
Changes in climate during the 20th century differ from region to region across the United States. We provide strong evidence that spatial variations in US temperature trends are linked to the hydrologic cycle, and we also present unique information on the seasonal and latitudinal structure of the linkage. We show that there is a statistically significant inverse relationship between trends in daily temperature and average daily precipitation across regions. This linkage is most pronounced in the southern United States (30–40°N) during the May-June time period and, to a lesser extent, in the northern United States (40–50°N) during the July-August time period. It is strongest in trends in maximum temperatures (Tmax) and 90th percentile exceedance trends (90PET), and less pronounced in the Tmax 10PET and the corresponding Tmin statistics, and it is robust to changes in analysis period. Although previous studies suggest that areas of increased precipitation may have reduced trends in temperature compared with drier regions, a change in sign from positive to negative trends suggests some additional cause. We show that trends in precipitation may account for some, but not likely all, of the cause point to evidence that shows that dynamical patterns (El Niño/Southern Oscillation, North Atlantic Oscillation, etc.) cannot account for the observed effects during May-June. We speculate that changing aerosols, perhaps related to vegetation changes, and increased strength of the aerosol direct and indirect effect may play a role in the observed linkages between these indices of temperature change and the hydrologic cycle.
Footnotes
- 1To whom correspondence should be addressed. E-mail: robert.w.portmann{at}noaa.gov
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Author contributions: R.W.P., S.S., and G.C.H. designed research; R.W.P. performed research; R.W.P. analyzed data; and R.W.P., S.S., and G.C.H. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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↵* Note that other articles have labeled the maximum temperature 90th percentile rate TX90p (see, for example, refs. 11 and 13). We use different nomenclature because we do not use the standard 1961–1990 time interval in computation of the percentiles because we do not use the standard set of indices from ref. 11. Thus, our “Tmax 90PET” corresponds to “trend of TX90p” discussed elsewhere (e.g., refs. 11 and 13). We also compute the trend in 10PET in the same direction (i.e., toward the warm part of the distribution) as 90PET to facilitate comparison between 10 and 90PET. This is opposite to some other articles, so our “Tmax 10PET” corresponds to their “trend of -TX10p.”
