Precipitation Frequency Intensity Storminess

Observed changes in precipitation are broadly consistent with theoretical expectations and reasonably simulated by global climate models (Bates and Kundzewicz, 2008; Trenberth et al., 2007; Zhang et al., 2007a). While total precipitation in the United States has increased by about 5 percent over the past 50 years, there are significant regional differences, with generally wetter conditions in the Northeast and generally drier conditions in the Southeast and particularly the Southwest (Figure 8.1) (see also Field et al., 2007b). A wide range of climate models using different emissions scenarios predict that these regional trends will continue, with generally robust model results for the north and with high uncertainty for the south (Christensen et al., 2007; USGCRP, 2009a). Other factors in addition to temperature influence precipitation. Specifically, uncertainty remains in our understanding of the effects of aerosols on cloud formation and precipitation. For example, climate models underestimate the magnitude of the observed global land precipitation response to 20th-century volcanic forcing (Hegerl and Solomon, 2009) as well as human-induced aerosol changes (Gillett et al., 2004; Lambert et al., 2005).

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FIGURE 8.1 Observed annual average precipitation changes in the United States between 1958 and 2008. Blue indicates areas that have gotten wetter, while brown indicates areas that have gotten drier. SOURCE: USGCRP (2009a); data from NOAA/NCDC (2008).

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FIGURE 8.1 Observed annual average precipitation changes in the United States between 1958 and 2008. Blue indicates areas that have gotten wetter, while brown indicates areas that have gotten drier. SOURCE: USGCRP (2009a); data from NOAA/NCDC (2008).

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 thunderstorms and tornadoes are insufficient to determine if changes have occurred (CCSP, 2008f).

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