Earth’s storage of water in soil, lakes and rivers is dwindling. And it’s especially bad for farming

University of Melbourne hydrology professor Dongryeol Ryu and his collaborator Ki-Weon Seo were on a train to visit Ryu's family when they found something startling. Stopped at a station for technical issues, Seo had pulled out his computer to pass the time with some work when a result popped up in their data that Ryu could hardly believe: It suggested a “remarkable” amount of Earth's water stored on land had been depleted.

“At first we thought, ‘That’s an error in the model,’” Ryu said.

After a year of checking, they determined it wasn't.

Their paper, published Thursday in the journal Science, finds that global warming has notably reduced the amount of water that’s being stored around the world in soil, lakes, rivers, snow and other places, with potentially irreversible impacts on agriculture and sea level rise. The researchers say the significant shift of water from land to the ocean is particularly worrisome for farming, and hope their work will strengthen efforts to reduce water overuse.

Earth's soil moisture dropped by over 2,000 gigatons in roughly the last 20 years, the study says. For context, that's more than twice Greenland's ice loss from 2002 to 2006, the researchers noted. Meanwhile, the frequency of once-in-a-decade agricultural and ecological droughts has increased, global sea levels have risen and the Earth's pole has shifted.

Ryu and his colleagues used three different data sources to verify that Earth storing less water on land than it once did. He also said their results reveal a deeper truth about the land, one farmers have to contend with frequently: When a big, dramatic rainfall event comes after a drought, sometimes leading to huge floods, that doesn't mean the water stored underground has recovered.

“It seems that lands lost their elasticity to recover the previous level,” he said.

Whether that elasticity ever returns will depend on whether humans take action on climate change and significantly change water use, the researchers say. The increasing heat stress on plants means they need more water. Agriculture, particularly irrigated agriculture, continues to draw up more water than it can afford. And humans are continuing to emit greenhouse gases without a strong effort to reverse course.

"There are long-term climate changes that have happened in the past and presumably could occur in the future that could reverse the trend described, but probably not in our lifetimes,” said Katharine Jacobs, a University of Arizona professor of environmental science who wasn't involved in the study. “Because greenhouse gases will continue to cause global warming well into the future, the rate of evaporation and transpiration is not likely to reduce any time soon.”

The study also confirms an explanation for a slight wobble in the rotation of the Earth — it’s being driven by the changing moisture levels of the planet.

“When I read this thing, I was very excited,” said Luis Samaniego, a professor of hydrology and data science at the University of Potsdam who wrote an overview commentary discussing the findings in Science. “It's a fascinating puzzle of all disciplines that came at the right moment to verify something that was not possible before.”

But Samaniego stressed that the finding isn't only fascinating; it's a wake-up call. Imagine the planet's wobble like an electrocardiogram for the Earth, he said. Seeing this result is like detecting an arrhythmia.

Choosing not to listen to the doctor — “that's what we are playing around with at the moment,” he said.

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