A close examination of nearly 2,000 of the world’s largest lakes found they are losing about 5.7 trillion gallons (21.5 trillion liters) a year. That means from 1992 to 2020, the world lost the equivalent of 17 Lake Meads, America’s largest reservoir, in Nevada. It’s also roughly equal to how much water the United States used in an entire year in 2015.

Even lakes in areas getting more rainfall are shriveling. That’s because of both a thirstier atmosphere from warmer air sucking up more water in evaporation, and a thirsty society that is diverting water from lakes to agriculture, power plants and drinking supplies, according to a study in Thursday’s journal Science.

Authors also cited a third reason they called more natural, with water shrinking because of rainfall pattern and river runoff changes, but even that may have a climate change component. That’s the main cause for Iran’s Lake Urmia to lose about 277 billion gallons (1.05 trillion liters) a year, the study said.

The declining lakes don’t mean places are suddenly going to go without drinking water, but it may lead to more competition for lake water, which is also used in hydroelectric power and recreation such as boating, the study authors said.

“More than half of the decline is primarily attributable to human consumption or indirect human signals through climate warming,” said study lead author Fangfang Yao, a climate scientist at the University of Colorado.

The diversion of water from lakes — a direct human cause of shrinkage — is probably larger and more noticeable because it is “very acute, very local and it has the capability of really changing the landscape,” said co-author Ben Livneh, a University of Colorado hydrologist.

But the indirect human shrinking, from warmer air due to climate change, “is this global blanketing effect that kind of affects everything or more places,” Livneh said. California’s Mono Lake is a good example of this type of shrinking, Yao said.

Even areas that are getting wetter because of climate change are losing lake water because hotter air is sucking more moisture out of the lakes. And that means more water in the air, which can fall as rain or snow but “may end up falling as rain far away, outside the basin where it evaporated or even over the ocean,” Livneh said in an email.

Yao, Livneh and colleagues used almost 30 years of satellite observation, climate data and computer simulation to figure out what’s happening to lakes and found more than half of them have shrunk so much that it is statistically significant and not random.

In the United States, Lake Mead lost two-thirds of its water between 1992 and 2020, while the Great Salt Lake also shrank noticeably, Yao said. The Great Lakes dropped considerably from 1992 to 2013 then plateaued and then increased.

Another problem is that lakes are filling with sediment or dirt from upstream rivers.

Scientists have long known about the problems of climate change, diversion and sedimentation, “however the complete quantification of water storage variations for large lakes that Yao and colleagues provide is new” and it creates “a much more complete picture” than past research has, said University of North Carolina hydrology professor Tamlin Pavelsky, who wasn’t part of the study.

“I’m generally most worried about lakes that are ecologically important and in populated areas without a lot of other good sources of water,” Pavelsky said in an email. “Lake Urmia in Iran, the Dead Sea, the Salton Sea … these are all worrisome.”

It’s likely to get worse as society looks for more water and more reservoirs with a growing population and a warmer Earth, said UCLA climate hydrologist Park Williams, who wasn’t part of the study.

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