Study finds dramatic increase in concurrent droughts, heat waves

Droughts and heat waves are happening simultaneously with much greater frequency than in the past, according to research by climate experts at the University of California, Irvine. Their findings appear in Proceedings of the National Academy of Sciences.

A team from UCI's Center for Hydrometeorology & Remote Sensing examined data gathered from ground sensors and gauges during a 50-year period beginning in 1960. Applying a statistical analysis to the half-century data set, the researchers observed a significant increase in concurrent droughts and heat waves.

''Heat waves can kill people and crops while worsening air quality, and droughts exacerbate those serious impacts,'' said senior author Amir AghaKouchak, assistant professor of civil & environmental engineering. ''With these two extremes happening at the same time, the threat is far more significant.''

For the purposes of the study, heat waves were defined as three to seven consecutive hot days, with temperatures in the 90th percentile of the historical record. Droughts were described as extended periods during which precipitation was 20 percent or less of the norm, as measured by the Standardized Precipitation Index.

While the researchers did not look into human-caused global climate change in this study, AghaKouchak said, an overall increase in the mean temperature worldwide is raising the probability of heat waves. He cited the recent record-breaking high in Iran: NASA satellites and National Oceanic & Atmospheric Administration data sets documented a 115-degree Fahrenheit surface temperature – with a ''comfort index'' of 165 degrees – on July 31 in the city of Bandar-e Mahshahr.

More than 2,500 people died during a heat wave that gripped India in late May. A European heat wave in 2003 claimed about 70,000 victims, many of them seniors and children.

UCI graduate student and lead author Omid Mazdiyasni and AghaKouchak explain in the paper that while commonly used analysis methods do not show any trend in concurrent droughts and heat waves, a unique statistical approach called the Cramer-von Mises test reveals a statistically significant change in the simultaneous occurrence of these climate extremes.