Drought Index Calculations Are Flawed
November 18, 2012

Worsening Drought Conditions Due To Global Warming Overestimated

redOrbit Staff & Wire Reports - Your Universe Online

Reports claiming that the climate change has caused global drought conditions to intensify over the past several decades are flawed and based on faulty calculations, according to a new study published Thursday in the journal Nature.

According to Anna Salleh of ABC News Australia the researchers report in their study that the 2007 4th Assessment Report (AR4) by the UN Intergovernmental Panel on Climate Change (IPCC) "overestimated" reports of an increase of drought conditions throughout the world.

That report had claimed that droughts had increased in intensity and duration, and had been observed in more and more locations, especially in the tropics and subtropics, Salleh explained.

The IPCC report was compiled using information from the Palmer Drought Severity Index (PDSI), which lead author Dr Michael Roderick of the Australian Research Council (ARC) Centre of Excellence for Climate System Science and colleagues said is flawed.

“Many climate change researchers use the Palmer Drought Severity Index (PDSI) and calculate evaporative demand as a sole function of temperature," Dr. Roderick said in a statement. "The use of the PDSI has led to a bias in results that indicated an increase in the area of global drought where none has actually occurred."

"It is curious that the long term use of the PDSI by climate impact researchers has persisted, when it has been recognized repeatedly as not being a realistic indicator of historical drought conditions," he added. "Even the section on droughts in the IPCC AR4 report was substantially revised by the more recent IPCC report on extremes because of its over reliance on the PDSI and the potential for overestimating the increases in global and regional drought."

In order to compile what they dub a more realistic calculations of areas affected by drought, Dr. Roderick and his colleagues, Dr. Justin Sheffield and Professor Eric Wood of Princeton University used a formula in which evaporation is dependent not just upon temperature, but also sunlight, humidity and wind speed.

"Water doesn't necessarily evaporate faster as the temperature goes up. It depends on the details. And the details are all important here," Roderick said. "What determines the evaporation rate of water predominantly is solar radiation, the humidity of the air and the wind."

Using all four of those criteria, they state that there has been "no statistically significant increase" in drought area from 1950 through 2008, the ARC Centre explained.

"The previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades," the study authors wrote, according to AFP reports.

"More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years," they added.