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In addition to the differences in the amount of precipitation, the occurrence of heavy downpours has increased in many parts of the region. For example, analyses of temporal trends over the past century have documented an increase in heavy rain- fall events across the area, extending from central Texas to the Appalachian Mountains in Tennes- see and North Carolina (Keim 1997). Increased frequency of extreme rainfall events will likely affect processes such as soil erosion, sedimentation, and stream dynamics. At the same time, many parts of the region are experiencing an increasing number of droughts. The areas of moderate to severe spring and summer drought have increased by 12 and 14 percent, respectively, since the mid -1970s (Karl et al. 2009). Continued rising temperatures will likely lead to further droughts in affected areas, as high temperatures increase evaporation of moisture from soils and plants. The projected increased variabil- ity in precipitation may have greater impacts than increases or decreases in magnitude. Table 2 -3. Observed precipitation changes in the Southeast summarized for two different periods (Source: Karl et al. 2009) 2,27 " k,'w'I Its arid Severe re V /{fe' 7 Pe'r Everits During hurricane season, tropical cyclones account for as much as 15% of the rainfall along portions of the Carolinas (Knight and Davis 2007). Changes in hurricane frequency and intensity would have impacts on precipitation patterns across the state. There has been much research into whether the significant increase in numbers of tropical storms and hurricanes in the Atlantic over the last three decades is due to increases in sea surface tempera- tures or to other factors related to multidecadal variability (Webster et al. 2005, Pielke et al. 2005). More support exists for a link between warmer sea surface temperatures and the observed increases in hurricane intensity (Emanuel 2005, Elsner et al. 2008). Globally, the number of category four and five hurricanes has almost doubled since 1975, and similar trends have been observed in the Atlan- tic basin (Webster et al. 2005). It remains unclear whether these observed patterns have exceeded the variability expected through non- anthropogenic causes. However, advances in modeling techniques have increased confidence concerning several aspects of cyclone- activity projections (reviewed in Knutson et al. 2010). A general convergence of frequency projections, in combination with fairly accurate hindcasting predictions, have begun to provide some confidence that globally the number of tropical cyclones is likely to decrease or remain unchanged under warming conditions. There is less certainty in projections for individual basins. Some increase in mean maximum wind speed of tropical cyclones is likely, although increases may not occur in all tropi- cal regions, and rainfall rates are likely to increase. High resolution models for the western Atlantic suggest there will be fewer tropical cyclones in the basin overall, but significantly more intense hurri- canes by the end of the twenty -first century (Bender et al. 2010). When storms do occur, rising sea levels will amplify the impacts of storm -surge incidence, particularly in sensitive coastal regions. Average Change in Precipitation in the Southeast Precipitation Change in % 1901 -2008 1970 -2008 Annual 6.0 -7.7 Winter 1.2 -9.6 Spring 1.7 -29.2 Summer -4.0 3.6 Fall 27.4 0.1 2,27 " k,'w'I Its arid Severe re V /{fe' 7 Pe'r Everits During hurricane season, tropical cyclones account for as much as 15% of the rainfall along portions of the Carolinas (Knight and Davis 2007). Changes in hurricane frequency and intensity would have impacts on precipitation patterns across the state. There has been much research into whether the significant increase in numbers of tropical storms and hurricanes in the Atlantic over the last three decades is due to increases in sea surface tempera- tures or to other factors related to multidecadal variability (Webster et al. 2005, Pielke et al. 2005). More support exists for a link between warmer sea surface temperatures and the observed increases in hurricane intensity (Emanuel 2005, Elsner et al. 2008). Globally, the number of category four and five hurricanes has almost doubled since 1975, and similar trends have been observed in the Atlan- tic basin (Webster et al. 2005). It remains unclear whether these observed patterns have exceeded the variability expected through non- anthropogenic causes. However, advances in modeling techniques have increased confidence concerning several aspects of cyclone- activity projections (reviewed in Knutson et al. 2010). A general convergence of frequency projections, in combination with fairly accurate hindcasting predictions, have begun to provide some confidence that globally the number of tropical cyclones is likely to decrease or remain unchanged under warming conditions. There is less certainty in projections for individual basins. Some increase in mean maximum wind speed of tropical cyclones is likely, although increases may not occur in all tropi- cal regions, and rainfall rates are likely to increase. High resolution models for the western Atlantic suggest there will be fewer tropical cyclones in the basin overall, but significantly more intense hurri- canes by the end of the twenty -first century (Bender et al. 2010). When storms do occur, rising sea levels will amplify the impacts of storm -surge incidence, particularly in sensitive coastal regions.