Historical data also show an increase in precipitation intensity. In the United States, the fraction of total precipitation falling in the heaviest 1 percent of rain events increased by about 20 percent over the past century (Gutowski et al., 2008). Most climate models project that this trend will continue (Bates and Kundzewicz, 2008) and also project a strong seasonality, with notable summer drying across much of the Midwest, the Pacific Northwest, and California (Hesselbjerg and Hewitson, 2007).
Changes in major storm events are of interest both because a significant fraction of total U.S. precipitation is associated with storm events and because storms often bring wind, storm surges, tornadoes, and other threats. Tropical storms, which become hurricanes if they grow to a certain intensity, are of particular interest because of their socioeconomic impacts (e.g., Hurricane Katrina; see Box 4.3). Changes in the intensity of hurricanes have been documented and attributed to changes in sea surface temperatures (Emanuel, 2005; Trenberth and Fasullo, 2008), but the link between these changes and climate change remains uncertain (Knutson et al., 2010). Recent model projections indicate growing certainty that climate change could lead to increases in the strength of hurricanes, but how their overall frequency of occurrence might change is still an active area of research (Bender et al., 2010; Knutson et al., 2010). Extratropical storms, including snowstorms, have moved northward in both the North Pacific and the North Atlantic (CCSP, 2008f), but the body of work analyzing current and projected future changes in the frequency and intensity of these storms is somewhat inconclusive (Albrecht et al., 2009; Hayden, 1999). Historical data for thunder-storms and tornadoes are insufficient to determine if changes have occurred (CCSP, 2008f).