Look down from a plane at farms in the Great Plains and the West and you’ll see green circles dotting the countryside, a kind of agricultural pointillism.
They’re from center-pivot irrigation systems. But some farmers are finding older versions, many built 10, 15 or even 20 years ago, aren’t keeping up with today’s hotter reality, said Meetpal Kukal, an agricultural hydrologist at the University of Idaho. “There’s a gap between how much water you can apply and what the crop demands are,” he said.
By the time the sprinkler’s arm swings back around to its starting point, the soil has nearly dried out. The main culprit? Atmospheric thirst.
“A hotter world is a thirstier one,” said Solomon Gebrechorkos, a hydroclimatologist at the University of Oxford. He led a new study, published on Wednesday in the journal Nature, which found that atmospheric thirst, a factor that fills in some of the blanks in our understanding of drought, over the last four decades has made droughts more frequent, more intense and has caused them cover larger areas.
In general, droughts happen when there’s an imbalance between water supply and demand.
Rain delivers water to the surface. The atmosphere removes water from the surface through evaporation, with temperature, wind, humidity and radiation from the sun controlling how much water is evaporated. It’s a complicated physical process that is hard to capture in models and, for a long time, studies of global droughts only focused on precipitation.
“It just really wasn’t detailed enough,” Dr. Gebrechorkos said, likening it to trying to balance a checkbook while only looking at income and leaving out expenses.
The new study aimed to figure out how atmospheric thirst has changed over more than one hundred years, including how to best model it and how it can improve monitoring and predictions of drought.
Dr. Gebrechorkos and his co-authors used multiple precipitation data sets, climate models and ways of calculating drought from 1901 to 2022 to assess how to capture atmospheric thirst and how it has been affecting droughts.
They found that it played an even bigger role than previously thought, drying out historically arid and wet regions alike.
Drought has been spreading and getting more intense since the 1980s almost everywhere around the world except for southeastern Asia, the study found. Atmospheric thirst, a direct result of global warming, made those droughts about 40 percent more severe, the study found.
The Western United States, large areas of Africa and South America, Australia and Central Asia are particularly prone to drought because of increased atmospheric thirst, the study found.
“We were very much shocked when we saw the results,” Dr. Gebrechorkos said. A sharp increase in drought activity in the last five years of the study, from 2018 to 2022, particularly alarmed him.
The area affected by droughts during that time was on average 74 percent larger than in the previous four decades. The drought area in the Western United States more than doubled during this time, as well as in Australia and southern South America. Atmospheric thirst was to blame.
And 2022 was striking. About one-third of the world experienced moderate or extreme drought at some point. Lake Mead, an important reservoir on the Colorado River, nearly dried up. Europe saw a record-breaking combination of drought and extreme heat that led to limits on water use. Millions of people faced food insecurity in the Horn of Africa.
Mike Hobbins, a hydrologist at the University of Colorado Cooperative Institute for Research in Environmental Sciences who was not involved in the study, said the findings were in line with his expectations.
“But I think it’s very important to quantify,” Dr. Hobbins said. “The demand side of drought has been ignored for so long, and we don’t have to ignore it any more. We can get it right.”
This year, Dr. Hobbins and Dr. Kukal added a new term to the weather dictionary: thirstwaves, for when evaporative demand is exceptionally high for at least three days, putting crops at risk.
The study’s broader results matched up with Dr. Kukal’s experience working with farmers in the West, as well as national patterns in irrigation.
“There’s a huge shift,” Dr. Kukal said. Some farmers in the west are giving up on irrigation, while more farmers in the upper Midwest and eastern United States are beginning to invest in expensive irrigation systems as atmospheric thirst causes more and more flash droughts.
“All this, we see because of warming,” Dr. Gebrechorkos said. “Looking ahead, unfortunately, the trend is set to continue.”
The study ends in 2022, but the following years brought record heat. The summer of 2024 was the warmest on record in the Northern Hemisphere; 2023 was second. And as warming continues, atmospheric thirst will grow, as will droughts. Landscapes that experience droughts again and again struggle to recover, creating a vicious cycle of desiccation.
Better models of atmospheric thirst should improve predictions for drinking water, irrigation and hydropower, allowing better adaptation to an increasingly drought-stricken world. The study used a six-month average; future work could focus on shorter time scales that are more helpful for farmers and water managers, Dr. Kukal said.