Simple models have overestimated drying over past 60 years

By Tanya Lewis

Web edition: November 14, 2012
Print edition: December 29, 2012; Vol.182 #13 (p. 10)

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.

Sheffield and colleagues calculated global drought trends from 1950 to 2008 using both equations on multiple datasets. Notably, they found a much smaller change in drought using the Penman-Monteith equation. The estimated yearly drought increase was only half as severe as that derived from the Thornthwaite equation. The weather records invariably contain some errors, but Sheffield says those errors don’t alter the conclusion that the simpler model overestimates rises in global drying.

The finding comes in stark opposition to the results of several recent studies. “It presented a somewhat different view of the drying trend for the last 60 years,” says Aiguo Dai, an atmospheric scientist at the State University of New York at Albany, whose own research suggests that the two equations yield very little difference in drought estimates. Dai says the new study fails to consider trends in soil moisture and other variables. He also claims that the new study relies on outdated weather records and questionable radiation data. However, Sheffield and colleagues attribute the disagreement to inconsistencies in the weather data used by Dai and others.

“I think the jury’s still out on why those groups looking at similar metrics come to different conclusions,” says paleoclimatologist Kevin Anchukaitis of the Woods Hole Oceanographic Institution in Massachusetts, who was not involved in either study.

If drought increases have in fact been overestimated, scientists may need to rethink how global warming may affect extreme weather going forward.

One thing scientists seem to agree on is that the Palmer metric for drought will become increasingly inaccurate in future. “I think we’re converging on the realization that it’s not just about temperature,” Sheffield says.