Hotter temps, not less rain, are biggest triggers of West's drought, study says

Warming temperatures, not declining precipitation, have by far been the biggest triggers of the drought that has overpowered the Western climate since 2000, a new study finds.

The study looked at drought trends over 75 years in 11 Western states, including Arizona, since 1948. It concluded that until 1999, declining rainfall and other precipitation was the principal cause of drought conditions, but that temperature increases have been the biggest driving force behind the 21st-century drought.

Like many before it, the study directly tied the warming temperatures this century to human causes — the emission of greenhouse gases such as carbon dioxide due to the burning of fossil fuels, including oil and natural gas.

“The bottom line is the warmer it gets the stronger the drought gets. How much warmer it gets depends on how much C02 is in the atmosphere,” said Rong Fu, a co-author and a UCLA professor of oceanic and atmospheric sciences.

Assuming temperatures keep rising, and that keeps increasing evaporation, “droughts in the Western U.S. will be more intense, last longer, and become more frequent and widespread despite uncertainties in future precipitation,” the study said.

The study examined both temperature and precipitation records and computer model findings to reach its conclusions. Researchers from UCLA, the National Oceanic and Atmospheric Administration and the University of Colorado-Boulder collaborated on the study. It was published in the journal Science Advances.

Since pre-industrial times, temperatures in the Western U.S. have risen faster than those worldwide, the study noted — about 3.6 degrees Fahrenheit in this region compared to nearly 2.7 degrees worldwide.

The study’s key implication is that society needs to control greenhouse gas emissions enough to keep global temperatures from rising more than that 2.7-degree figure or at most the 3.6-degree figure worldwide, Fu said.

Specifically, the study measured drought conditions not by measuring rainfall and snowfall, but by how water is being received by the West’s surface soils.

“If you have less rainfall, you lose more moisture. If you have higher evaporation demand, you also lose more moisture,” Fu said.

Another implication of the study is that if continued temperature increases — which are forecast by many climate scientists — continue to aggravate Southwestern droughts, that means flows on the Colorado and other Western rivers will continue declining, she said.

The Colorado River provides drinking water to the Tucson and Phoenix areas, as well as drinking and irrigation supplies for 40 million people living in and near the entire seven-state river basin. Its annual average flow is down nearly 20% since 2000.

Overall, “precipitation may go up and down (over the years), but so far precipitation over the Western U.S. has not really changed significantly from 1948 up to now,” said Fu.

“Some decades are dry. Some decades are wet. But there’s not a strong decreasing (precipitation) trend and we didn’t find any significant increases in the trend, and this is for the entire 11 Western states,” Fu said.

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“But when the temperature increases, with rainfall, it evaporates before it makes it to the reservoir or river,” Fu said. “Therefore, these river basins will be dry.”

“The Colorado River will receive less and less flow, because the rainfall and the catchment area (that receives it) are more lost to evapotranspiration. Therefore, less could turn into runoff or streamflow to feed the Colorado,” Fu said.

Historically, precipitation deficit has been the dominant factor in Western U.S. droughts, whereas evaporative demand, largely controlled by surface temperature, has been a minor factor until recent decades, when “droughts have become not only more pervasive but also more impactful over the Western U.S.,” said the study.

The study also found that an increasing “concurrence” in recent years between precipitation deficits and heat waves amplifies “water and energy demands; intensify heat and water stresses for humans, animals, and plants; and worsen air and water pollution.”

Particularly since 2020, the Western drought has triggered frequent power shortages from hydropower production, intense and widespread wildfires, low reservoir inflows, reduced agricultural productivity, and ecosystem degradation, said the new study, citing previous research by NOAA.

The study cited the most recent severe drought in this region from 2020 through 2022 as a prime example of hotter weather overtaking precipitation as a principal cause of droughts. That was the West’s worst drought since instrumental records started being kept in 1895.

The drought for the entire period since 2000 has been the worst drought in at least the past 1,200 years, based on tree ring records, another study found.

The new study found that a precipitation shortfall was the biggest contributor to the recent dry period until summer 2021. But for the entire three years, evaporation triggered by hot weather was responsible for 61% of the region’s dry conditions, the study concluded.

The study also concluded the 2020-22 Western drought was essentially a 1,000-year drought, meaning it has a 1 in 1,000 chance of happening in a given year. But if greenhouse gas emissions increase at their highest possible forecast rate in the future, the chances of a drought period like that will increase to 1 in 60, the study found. By 2100, it said, the odds will be 1 in 6.

Brad Udall, another climate scientist, of Colorado State University, said this study’s findings amplify his long-held view that the West can experience droughts even when there is more rainfall.

“I keep saying to decision-makers, ‘precipitation is not runoff. Do not confuse precipitation with runoff,’” said Udall, who has long raised strong concerns about increasing temperatures’ impacts on river flows. “To get runoff, you have a series of challenges that water from the sky has to overcome, for water to end up in your rivers and creeks.

“All those processes are related to the demand the atmosphere has for water, the demand plants have for water, the demand soil wants for water,” Udall said.

He said he’s not even sure the 2020-22 drought was as rare as a 1,000-year event.

“Trying to put return periods on these events is damn near impossible because the climate is changing. To have return period numbers requires a long stable climate, to be precise in calculating them. So I question that number.

“What’s important there is that that they are telling us these kind of events will be far more frequent in the future,” Udall said of the new study’s researchers. “That makes perfect sense in a warming and drying climate.”