Increasingly, Droughts and Heat Waves Are Happening at the Same Time

Across much of the United States, regional droughts and heatwaves are appearing simultaneously more frequently, imposing more-extreme conditions than either would deliver separately, according to a new study.

California's four-year drought is a case in point, says Amir AghaKouchak, a civil engineer at the University of California at Irvine and the new study's senior author.

Looking only at 2014 and using precipitation as the indicator, "it is a serious drought, but it is not that extreme," he says.  It would be classed as a drought that would occur about once every 25 years.  But temperatures in 2014 were well above average, he adds.  Put the two extremes together and the drought becomes a once-in-200-year event. 

In the past, climate scientists typically have looked at extreme conditions individually – focusing on trends in droughts or heat waves, for example.  However that approach can understate the risk that global warming adds for extreme conditions, he adds.

A new, nationwide analysis of this overlap, which appears in the current issue of the Proceedings of the National Academy of Sciences, was prompted by recent studies that yielded conflicting results on global warming's impact on the length or intensity of droughts around the world.

In exploring the causes for such discrepancies, a team led by Kevin Trenberth, with the National Center for Atmospheric Research in Boulder, Colo., noted in a paper published in late 2013 that while global warming doesn't cause droughts, rising temperatures would boost the intensity of droughts and lead to quicker onset of droughts.

Read more at Increasingly, Droughts and Heat Waves Are Happening at the Same Time