Climate Fact: Heaviest One Percent Now Even Heavier (Southeast)
In terms of total annual rainfall, most of the United States became “wetter” over the 20th century. Most of this increase, however, is being expressed in “extreme” rainfall events, which are now more frequent and even more extreme than they were in the 1950s. In the Southeast, for example, the amount of rain that falls during the heaviest one percent of rainy days has grown by 20 percent over the last 50 years. While more moisture is generally good for the region’s agriculture and wildlife, the general trend of more rainfall happening during extreme events has corresponded to more frequent and longer periods when no rain falls. The longer soil goes without moisture, the less absorptive it is when the rains finally return. Heavy rainfall also causes much greater amounts of soil erosion (soil loss to streams and ultimately the ocean) than moderate events do. In some locations, increases in rainfall severity can cause the amount of soil erosion to grow exponentially.
To see the region-by-region changes in the frequency of heavy rainfall events since the 1950’s, visit http://www.earthgauge.net/climate-facts-image-library#8. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.
Seasons: Spring, Summer, Fall
Sources: Groisman, PY and Knight RW. “Prolonged Dry Episodes over the Conterminous United States: New Tendencies Emerging during the Last 40 Years.” Journal of Climate 21 (2008): 1850-1862 and National Weather Service: Climate Prediction Center. U.S. Temperature and Precipitation Trends: Annual. Accessed Online 3 July 2007 and Trenberth, K et al. “The Changing Character of Precipitation.” Bulletin of the American Meteorological Society, September 2003: 1205-1217 and Easterling, D et al. “Observed climate variability and change of relevance to the biosphere.” Journal of Geophysical Research 105 (2000): 101-120 and Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009.