Hot summer days could be fueling Utah droughts even more than dry spells.
Before 2000, drought severity in the West was mostly driven by the absence of precipitation, rather than the presence of heat. That’s flipped in the past couple of decades.
Research from UCLA and the National Oceanic and Atmospheric Administration indicates that temperature has become the dominant factor in determining how intense and widespread drought becomes.
“For the same precipitation deficit, drought is much stronger,” said UCLA professor Rong Fu, who worked on the study. “This is the main reason we've seen drought so much stronger.”
Utah is already feeling the effects of this shift as a megadrought parches the Southwest, and climate change — fueled by greenhouse gas emissions — makes warmer temperatures the new normal. That has led to far-reaching impacts, from strained water supplies and stressed ranchers to destructive wildfires.
Drought has steadily increased across Utah this summer as temperatures have risen. As of mid-August, 100% of the state is experiencing some level of drought — up from just 9% last fall. Extreme drought now covers nearly 11% of the state from Box Elder County to San Juan County.
“Utah is in the area where the temperature impact is strong,” Fu said of the study’s findings. “That means you're going to see drought more frequently. Because over time, drought does not require a lack of precipitation anymore.”
By mid-century, she said, around half of droughts may not have a precipitation deficit at all. So, a Utah community could see normal rainfall totals compared to its 20th-century averages, but extreme heat could still create dire conditions.
The reason is an atmospheric phenomenon called evaporative demand — the thirst of the atmosphere.
When people think about water demand, lawn sprinklers and farm irrigation may first come to mind. But humans aren’t the only force putting pressure on the West’s water supply.
The atmosphere also extracts water around the clock. And as the air gets hotter, it wants more.
“Drought isn't just about water coming in,” said Dan McEvoy, a climatologist with the Western Regional Climate Center. “It's really becoming evident — and even more important, in some of these water-limited areas — to pay attention to the water that's going out of the system as well.”
There’s still a lot of focus on the supply side of that equation, McEvoy said, as people pay attention to snowpack during the winter. But the summers that drive evaporative demand are often hotter and drier than they were a few decades ago.
“It is changing the way we're thinking about drought in the West,” McEvoy said. “We're seeing more acceleration and more expansion of drought conditions during those summer months than what we were in the past.”
As temperatures heat up, the atmosphere will continue to suck more and more moisture from the surface through evapotranspiration. That’s why summer air tends to feel so thick and sticky in the Southeast. But when there’s no easily accessible water left to meet the atmosphere’s demands, it will keep drawing moisture from soils and vegetation until they are dried out.
That dynamic can create a vicious cycle, McEvoy said. Dry dirt heats up faster, warming the air above it and fueling more evaporative demand.
It also means less water from snowpack makes it to rivers and reservoirs, as runoff loses moisture to evaporation and to the dry soil that soaks it up.
“If you have the same amount of water in the winter in 2025 compared to 1950, that's not going to translate to the same amount of water into the system that you had in 1950,” McEvoy said.
Heat-driven evaporative demand is a growing concern worldwide. A recent University of Oxford study suggests that the thirsty atmosphere has increased global drought severity by 40% since the early 1900s. That means even in rainy regions, droughts are getting worse.
If a future Utah community has a summer with above-average precipitation, Fu said, that rainfall would likely not be enough to offset the increasing power of evaporative demand. It’s not that precipitation will be decoupled entirely from drought, just that temperature will be the primary force determining its magnitude.
Water managers need to make sure their decisions are based on data that accounts for the increasing clout of heat-driven evaporation, she said. And Utahns should work to reduce water use, including by rethinking irrigation practices.
But this shift also means that the severity of Utah’s future droughts could be at least partially in our hands.
“Throughout human history, drought was always at the mercy of God,” Fu said, but this element of it is now tied to humanity’s decisions about curbing fossil fuel emissions.
“The less warming we have, you will have less increase of drought,” she said, “but under a higher emission scenario, it will just continue increasing.”
