From The Nevada Independent (Daniel Rothberg):
We talk about it a lot: Across the Southwest, human-caused warming is changing the way that water falls as snow or rain, creating uncertainty around the regional water supplies we rely on.
Yet precipitation tells only one part of the story. Climate scientists expect another less-discussed variable to increase the risks of wildfires and droughts in Nevada and California over the coming decades. That variable is known as “evaporative demand,” and it’s effectively a measure of how thirsty the atmosphere is — the extent to which the atmosphere is trying to evaporate water.
That all might sound technical, but evaporative demand has real-world consequences. In a new paper, Nevada and California researchers showed how greater atmospheric thirst, mainly driven by warming temperatures, could have major implications for drought and wildfire risk.
“We saw this steady increase in evaporative demand through the end of the century,” said Dan McEvoy, a researcher with the Desert Research Institute and Western Regional Climate Center.
What McEvoy and the paper’s co-authors found was that greater seasonal evaporative demand — a roughly 13 to 18 percent increase by the end of the century — could dry out the landscape, creating conditions that are likely to increase the danger of intense fire and multiyear droughts.
Recent fires have already been linked to extreme days of evaporative demand. Those extreme days are expected to increase, the paper found, and that could result in more wildfire danger.
McEvoy said the results showed “steadily increasing extreme days.” By the late century, from 2070 to 2099, the paper forecasted a four to ten-fold increase in the number of extreme days.
The paper, published in Earth’s Future last month, helps to fill in a gap around predicting the effects of climate change across the state. Drought involves both precipitation trends and evaporative demand. But as the state’s newly released climate strategy explains, there remains a degree of uncertainty around how climate change will affect precipitation. That’s not the case when it comes to evaporative demand, which has risen in Nevada over the past four decades.
Julie Kalansky, a co-author of the paper and a researcher based out of the Scripps Institution of Oceanography at the University of California, San Diego, said both variables must be weighed.
“When you think about drought just as the lack of precipitation — and without the evaporative demand — you are missing a relatively large piece of the puzzle,” Kalansky said in an interview.