by Abrahm Lustgarten, Graphics by Lucas Waldron, Illustrations by Olivier Kugler for ProPublica

As the planet gets hotter and its reservoirs shrink and its glaciers melt, people have increasingly drilled into a largely ungoverned, invisible cache of fresh water: the vast, hidden pools found deep underground.

Now, a new study that examines the world’s total supply of fresh water — accounting for its rivers and rain, ice and aquifers together — warns that Earth’s most essential resource is quickly disappearing, signaling what the paper’s authors describe as “a critical, emerging threat to humanity.” The landmasses of the planet are drying. In most places there is less precipitation even as moisture evaporates from the soil faster. More than anything, Earth is being slowly dehydrated by the unmitigated mining of groundwater, which underlies vast proportions of every continent. Nearly 6 billion people, or three quarters of humanity, live in the 101 countries that the study identified as confronting a net decline in water supply — portending enormous challenges for food production and a heightening risk of conflict and instability.

The paper “provides a glimpse of what the future is going to be,” said Hrishikesh Chandanpurkar, an earth systems scientist working with Arizona State University and the lead author of the study. “We are already dipping from a trust fund. We don’t actually know how much the account has.”

The research, published on Friday in the journal Science Advances, confirms not just that droughts and precipitation are growing more extreme but reports that drying regions are fast expanding. It also found that while parts of the planet are getting wetter, those areas are shrinking. The study, which excludes the ice sheets of Antarctica and Greenland, concludes not only that Earth is suffering a pandemic of “continental drying” in lower latitudes, but that it is the uninhibited pumping of groundwater by farmers, cities and corporations around the world that now accounts for 68% of the total loss of fresh water in those areas, which generally don’t have glaciers.

Groundwater is ubiquitous across the globe, but its quality and depth vary, as does its potential to be replenished by rainfall. Major groundwater basins — the deep and often high-quality aquifers — underlie roughly one-third of the planet, including roughly half of Africa, Europe and South America. But many of those aquifers took millions of years to form and might take thousands of years to refill. Instead, a significant portion of the water taken from underground flows off the land through rivers and on to the oceans.

The researchers were surprised to find that the loss of water on the continents has grown so dramatically that it has become one of the largest causes of global sea level rise. Moisture lost to evaporation and drought, plus runoff from pumped groundwater, now outpaces the melting of glaciers and the ice sheets of either Antarctica or Greenland as the largest contributor of water to the oceans.

Most of the water lost from drying regions is from groundwater pumping, which ultimately shifts fresh water from aquifers into the oceans.

The study examines 22 years of observational data from NASA’s Gravity Recovery and Climate Experiment, or GRACE, satellites, which measure changes in the mass of the earth and have been applied to estimate its water content. The technique was groundbreaking two decades ago when the study’s co-author, Jay Famiglietti, who was then a professor at the University of California, at Irvine, used it to pinpoint where aquifers were in decline. Since then, he and others have published dozens of papers using GRACE data, but the question has always lingered: What does the groundwater loss mean in the context of all of the water available on the continents? So Famiglietti, now a professor at Arizona State University, set out to inventory all the land-based water contained in glaciers, rivers and aquifers and see what was changing. The answer: everything, and quickly.

Since 2002, the GRACE sensors have detected a rapid shift in water loss patterns........

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