Many studies conclude that global warming is increasing drought worldwide, however; new research indicates that a widely used model known as the Palmer Drought Severity Index has been overestimating changes in regional, and now global drought, according to ScienceNews..
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, says study coauthor Justin Sheffield, a hydroclimatologist at Princeton University. (via ScienceNews)
The research team proceeded to calculate 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.
Another unrelated recent study by Aiguo Dai, an atmospheric scientist at the State University of New York at Albany puts into question this particular conclusion by Sheffield and his team, indicating that the 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 He also claims that the new study relies on outdated weather records and questionable radiation data.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. (via ScienceNews)
One thing that most scientists agree on is that the Palmer Index will become less and less accurate over time.
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