Why future droughts will not be about rain

Dry wells. Dwindling reservoirs. Parched ground. Forest fires. The American West has gotten awfully familiar with drought in the 21st century.

And it wouldn’t be the same without the heat. This summer, like many before, set new benchmarks for heat, as big swaths of the West, including Arizona and California, lived through their hottest summers on record.

These temperatures, even more than the amount of rainfall, are key to understanding drought in the climate change era, according to a new study in the journal Science Advances. The research found that since 2000, rising temperatures - leading to greater evaporation - have done more to contribute to the severity and extent of droughts in the West than a lack of rainfall.

“This is quite different from our grandma’s drought,” said Rong Fu, a professor in atmospheric and oceanic science at UCLA who is one of the authors of the study.

Before hotter temperatures - driven by humans burning fossil fuels - droughts were caused by shifting weather patterns and fluctuating rainfall as part of a natural cycle, she said. But now, as long as the planet keeps getting warmer, evaporation is playing a dominant role and is “only going to increase.”

“Even if you have rainfall,” she said, much of it won’t reach western reservoirs because it will evaporate along the way.

“That has quite profound implications,” she said.

The long-term trend toward a drier West is not a new idea. Much has been written on this process of “aridification” as warming temperatures lead to longer and more intense droughts across the region. Others have documented how the two-decade megadrought in the West since 2000 amounted to the driest period for the region in more than 1,000 years.

The rest of the country is dealing with it as well. At the moment, all but two states - Alaska and Kentucky - are enduring drought conditions.

Warmer temperatures allow the atmosphere to hold more water and suck moisture from the ground. This “evaporative demand” grows as temperatures rise.

The role of evaporation has been an important issue as western states debate how to divvy up the dwindling Colorado River, which provides water for tens of millions of people. Scientists have long known that temperatures play a big role in the declining flows of the river.

“They have taken this idea that has been out there and done, from what I can tell, a really bang-up job of looking at it through a slightly different lens,” said Brad Udall, a water and climate scientist at Colorado State University who was not involved in the study. “It makes total sense to me that these hotter temperatures actually can cause drought. It’s not just precipitation. It’s heat.”

The research by Fu and others sought to quantify evaporation’s role in producing droughts over the past several decades - and how it’s changing. To do this, she and her colleagues analyzed weather patterns going back decades and found differences between how recent droughts played out compared to those of last century.

In the second half of the 20th century, the paper found, the severity of droughts in the West was primarily explained by a lack of rain. Around 2000, the situation changed, and now “evaporative demand” - due to warming - plays a dominant and increasing role. That evaporation accounted for 61 percent of the severity of the exceptional 2020-2022 drought in the West, which helped deplete major reservoirs of the Colorado River down to crisis levels, the researchers calculated.

These types of crippling droughts will become increasingly common by the end of the century if temperatures continued to rise. What had been a 1-in-1,300 year event could happen once every six years, the study found.

“It’s not just an increase in drought,” Fu said. “The whole western U.S. will be a lot more arid.”