By Tanya Lewis
Droughts shrivel crops, threaten communities, and wither ecosystems. Studies claim global warming is increasing drought worldwide, and may already have done so. But the standard method of assessing drought has exaggerated drying trends over the past 60 years, scientists report in the Nov. 14 Nature.
The 2007 report of the Intergovernmental Panel on Climate Change concluded that “more intense and longer droughts have been observed over wider areas since the 1970s.” Its findings were largely based on a widely used model known as the Palmer Drought Severity Index, which uses temperature and rainfall to determine dryness. Originally developed in the 1960s to help apportion aid to drought-stricken farmers, the index may skew drought trends in the presence of climate change.
“It’s quite obvious that the Palmer model has been overestimating changes in drought,” says study coauthor Justin Sheffield, a hydroclimatologist at Princeton University. Other scientists have reported this effect for regional areas, but the new study is the first to show it globally, he says.
The problem has to do with the way of calculating a quantity called potential evaporation, the amount of evaporation that would occur given an unlimited water supply. Historically, scientists calculated potential evaporation using the Thornthwaite equation, which is based entirely on temperature. The more complete Penman-Monteith equation, by contrast, incorporates the influences of solar radiation, humidity and wind speed. The latter gives a much more accurate measure of potential evaporation, Sheffield says.