Earth’s soil is drying up, and it could be irreversible

The amount of water stored on lands across Earth’s continents has declined at such staggering levels that changes are likely irreversible while humans are alive, a study published Thursday found.

The losses in soil moisture - a result of the planet’s climate conditions and prolonged droughts - already pose issues for farming, irrigation systems and critical water resources for humans. But it also affects sea-level rise and Earth’s rotation - datasets the research team used to better track water storage for decades longer than previous studies.

“What we were looking for was evidence of changing hydrology around the world,” said Jay Famiglietti, co-author of the study published in Science. “What we found was this unprecedented decrease in soil moisture in the early part of the 21st century, which took us by surprise.”

The team found that from 1979 to 2016, the biggest soil moisture losses occurred between 2000 and 2002 - losing around 1,614 gigatons of water from land. The team estimated that it added to global mean sea-level rise at a rate of about 1.95 millimeters a year.

The startling contribution to rising sea levels was larger than Greenland’s ice loss around that time. Greenland contributed about 0.8 millimeters a year in recent decades. From 2002 to 2006, it lost about 900 gigatons.

“The rate of water dumping into the oceans was bigger from terrestrial water storage than from what we normally think of as the biggest source, which was the melting of Greenland,” said Clark Wilson, a co-author and geophysicist at University of Texas at Austin.

Famiglietti agreed that “soil moisture depletion is playing a bigger role in sea level rise than we previously thought.”

The biggest drops in soil moisture during that period occurred in large regions in East and Central Asia, Central Africa, and North and South America. The study showed the decline was primarily driven by changes in precipitation patterns and more drying power from the atmosphere because of rising temperatures.

“Remarkably, the global-scale drought that occurred between 2000 and 2002 was largely unnoticed at the time,” said Ki-Weon Seo, lead author and geophysicist at Seoul National University. “This study suggests that greater attention should be paid to drought events.”

The drop in soil moisture from 2000 to 2002 is interesting because it’s not well depicted in computer models depicting Earth’s past water storage, Wilson said. One well-known computer model indicated a global drought, but it was unclear if it was accurate. The study’s findings though, he said, confirm the observation and will help better refine models.

Soil moisture continued to decrease following 2002, although not at that same intense rate. Satellite observations from NASA’s Gravity Recovery and Climate Experiment showed about another 1,287 gigatons of water on land depleted from 2005 to 2015, which is about the equivalent of 3.52 millimeters of global mean sea-level rise.

“There is continuing loss of moisture of water stored on land,” Wilson said. “In our perspective of human timescales, we may not have seen the kind of rainfall events that would be important in recharging the land.”

Tracking Earth’s global water storage on land is not an easy feat.

Scientists were long restricted to regional measurements and models, but then the launch of GRACE allowed for new global views of water storage from the surface to below ground. But it only launched in 2002 - leaving scientists in the dark on what it looked like in prior decades.

In the new study, Seo and his colleagues extend this record back to 1979 - and provided the first “evidence of a permanent shift in Earth’s hydrological cycle due to climate change,” said hydrologist Luis Samaniego, who was not involved in the study and wrote a review article on the research.

Since direct observations of global water storage on land were scarce before 2002, the team looked at two other longer datasets as indicators: global sea-level rise and Earth’s tilt.

Global sea-level rise is largely fed by the melting of glaciers and ice sheets, but it is also affected by changing the amount of water on land. When water leaves the continents, it ends up in the oceans, Famiglietti said.

At the same time, moving water from one part of Earth to another can affect the planet’s rotation around its axis. Earth spins on an imaginary line between the North and South poles, but the exact position of the line isn’t fixed. The points where Earth’s axis of rotation meets Earth’s surface wobbles and drifts a few meters each year - called polar motion. (These changes aren’t noticeable to people but is perceptible by GPS systems on phones.)

Previously, Famiglietti and his same colleagues found removing groundwater shifted Earth’s tilt 31.5 inches eastward. In the new study, the team also found polar motion underwent observable changes from global groundwater loss from 1993 to 2010.

Samaniego said the new study validates a long-term trend of Earth’s water cycle seen in models with not just one, but three completely independent global datasets - “an impressive scientific feat.”

These declines have been seen on a regional level, but he said this is the first “conclusive” evidence of a global shift in water storage.

As of 2021, the team said soil moisture still has not recovered and likely won’t under the current climate conditions. Prolonged droughts, which are increasing in a warming world, will prevent soil moisture from bouncing back on the given course - at least in our lifetimes.

Soil moisture “that has left the soil layers and hasn’t been replenished for decades is unlikely to return to its original levels,” said Samaniego, a researcher at Helmholtz Centre for Environmental Research.

The conclusion, researchers say, is that societies must learn to practice smarter and sustainable water resource management.

“Climate change is not only about rising temperatures, but also about long-term impacts on water availability - affecting agriculture, ecosystems and societies alike,” Samaniego said.