Earth’s landmasses are holding onto a lot less water than they used to — and this loss is not just due to melting ice sheets. Terrestrial water storage, which includes water in underground aquifers, lakes, rivers and the tiny pore spaces within soil, declined by trillions of metric tons in the early 21st century, researchers report in the March 28 Science.
This sharp decrease in freshwater stores is driven by rising temperatures on land and in the oceans, which in turn are linked to an increased global incidence of drought. And given the projected warming of the planet, this trend isn’t likely to change any time soon, say geophysicist Ki-Weon Seo of Seoul National University and colleagues.
The team used several independent methods to assess terrestrial water loss from 2000 to 2020. These methods — each covering slightly different time spans in this period — included satellite gravity observations over land, satellite assessments of soil moisture, measurements of global sea level rise and observations of variations in Earth’s rotation due to changes in mass distribution around the planet. As water has moved from land to sea, Earth’s pole has drifted by about 45 centimeters.
Altogether, the researchers say, the data agree on a fundamental point: Water storage on land has dropped off sharply in the early 21st century. From 2005 to 2015, the period during which these assessments overlap, terrestrial water storage decreased by nearly 1.3 trillion metric tons. That’s equivalent to a global sea level rise of 3.5 millimeters, the team found.
Soil moisture loss was particularly stark and worrisome, because of its link to drought, the team says. Satellite data show that water stored in soils dropped especially steeply from 2000 to 2002, shrinking by about 1.6 trillion tons. That water ultimately helped increase sea levels by almost two millimeters in each of those years. In contrast, Greenland’s melting ice sheets contributed about 900 billion tons, or about 0.8 millimeters a year — less than half as much — to sea level rise from 2002 to 2006. Greenland, previously, was thought to be the largest contributor to sea level rise annually.
The decline in soil moisture continued throughout the period for which those data are available, although at a lower rate. From 2003 to 2016, an additional roughly 1 trillion tons of water were lost from soils.
The primary culprits behind the water loss are rising temperatures, in both the atmosphere and the ocean. The increase in Earth’s average temperature over the last few decades has changed precipitation patterns while intensifying evaporation and transpiration — the process by which water vapor is released into the atmosphere from plants.
More water vapor from evaporation and transpiration can lead to brief bouts of intense rainfall, but that water doesn’t tend to infiltrate into the soil; much of it becomes runoff that flows overland to the sea.
And, as Earth’s temperatures rise, “the areas of the globe that are drying due to higher temperatures and changes in precipitation are getting larger than the areas that are getting wetter” from any increases in precipitation, says environmental scientist Katharine Jacobs of the University of Arizona in Tucson.
Meanwhile, there’s also increasing demand for groundwater, Jacobs adds. “It is fair to say that most people who work on water issues are unaware of the connections between groundwater pumping and sea level rise, and if they do know that there is a connection they probably do not understand that the changes are measurable and that they are affecting the tilting of the Earth’s axis as well.”
That’s why using these different data sets is so important, she says: Without them, “the majority of researchers might miss the connections.”
The big picture, the researchers say, is that the total amount of water in Earth’s soils has been declining since the turn of the century. And given future projections for the planet’s temperatures, that water isn’t likely to replenish.
It’s a worrisome finding, says climate modeler Benjamin Cook of the NASA Goddard Institute for Space Studies in New York City. “Everything needs water. If you don’t have enough, you’re in trouble.”