Overlapping droughts and heat waves now happen more frequently than in the past, a phenomenon that researchers said could pose more significant threats.

Climate experts from the University of California, Irvine looked at 50 years-worth of data that were collected from ground sensors and gauges starting 1960. By conducting a statistical analysis on this data set, they found a significant increase in simultaneous drought and heat waves.

"Despite an apparent hiatus in rising temperature and no significant trend in droughts, we show a substantial increase in concurrent droughts and heatwaves across most parts of the United States," wrote the study researchers Amir AghaKouchak and Omid Mazdiyasni, from UC Irvine.

The odds of the two events occurring together were found to be highest across the southern part of the U.S spanning from New Mexico to Alabama while the two climate events were least likely to occur simultaneously in the northern states and parts of the Midwest.

Although the researchers did not take into account global climate change caused by man-made activities in the study, they said that an overall increase in the mean temperature globally boosts the likelihood of heat waves.

The National Centers for Environmental Information said that the warmest year on historical record is 2014 but 2015 appears bound to break this record.

 "We show that the tail of the distribution of concurrent drought and heatwave conditions has shifted toward more frequent and extreme concurrent extremes," the researchers wrote in their study. "Although commonly used trend analysis methods do not show any trend in concurrent droughts and heatwaves, a unique statistical approach discussed in this study exhibits a statistically significant change in the distribution of the data."

AghaKouchak and Mazdiyasni described heat waves as between three to seven consecutive hot days marked by temperatures in the 90th percentile of the historical record. Droughts, on the other hand, are described as extended periods marked by 20 percent or less of the normal precipitation.

AghaKouchak said that heat waves can kill people and damage crops as well as worsen the quality of air, and drought can worsen these serious impacts. He said that when these two extreme events happen simultaneously, the threats posed to the society and the environmental are far more significant.

In late May, for instance, over 2,500 people died during a heat wave that hit India. In 2003, a European heat wave also killed about 70,000 people many of them children and the elderly.

The study was published in the Proceedings of the National Academy of Sciences on Aug. 31.

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