Dry conditions in the U.S. are classified according to the Palmer Drought Severity Index, which compares the amount of moisture in the soil to that in the air closest to the surface. Using long-term temperature and precipitation records from 1870 to 2002, Aiguo Dai of the National Center for Atmospheric Research and colleagues calculated the Palmer index for locations around the world in which it is not routinely used. "Droughts and floods are extreme climate events that are likely to change more rapidly than the average climate," says Dai. "Because they are among the world's costliest natural disasters and affect a very large number of people each year, it is important to monitor them and perhaps predict their variability."
The results reveal that widespread drying has occurred in large regions of Canada, Europe and Asia, as well as western and southern Africa, and eastern Australia. The U.S. experienced the opposite trend, exhibiting increased wetness over the past 50 years. By controlling for rain and snowfall, the scientists determined the amount of drying caused by increasing global temperatures, which lead to elevated rates of evaporation. They found that about half of the change is a result of rising temperatures, particularly in areas at northern middle and high latitudes. "Global climate models predict increased drying over most land areas during their warm season, as carbon dioxide and other greenhouse gases increase," says Dai. "Our analyses suggest that this drying may have already begun."