In addition to average temperature, a host of other climate variables are projected to experience significant changes over the 21st century, just as they have during the past century. For example, the frequency and intensity of heat waves is projected to continue to increase, both in the United States (Figure 6.21) and around the world. This projection is considered robust because a shift in the average value of a temperature distribution (or in another climate variable) typically entails an increase in the frequency of extreme and unprecedented events (see, e.g., Solomon et al., 2007). Similarly, there is considerable confidence that the frequency of cold extremes will decrease and that the number of frost days will decline in the middle and high latitudes, following current trends (Meehl et al., 2007a; USGCRP, 2009a). Projections of future climate also indicate that snow cover and sea ice extent will continue to decrease (Meehl et al., 2007a; USGCRP, 2009a; Zhang, 2010), while sea level will continue to rise (see Chapter 7).
Projections of precipitation change are generally more uncertain than projections of temperature and temperature-related changes. However, most models project increased precipitation in northern regions of the United States, while it is considered very likely that the southwestern United States will experience a net decrease in precipitation (USGCRP, 2009a). Another robust projection, which results from the fact