Many—if not most—of the Earth’s aquifers are in trouble.
That’s the finding of a group of NASA scientists, who published their study of global groundwater this week in the journal Water Resources Research. Water levels in 21 of the world’s 37 largest known aquifers, they report, are trending negative.
The study is the first major accounting of groundwater change over time on the planetary scale. It was accomplished, not with wells or surveys, but with satellites.
Groundwater reserves are one of the environmental phenomena that’s hardest to conceptualize. Droughts are systemic and complex, sure, but a curious person can always go stand in a reservoir and see where the water is supposed to be. Dry lawns and fallow fields present another view on to what a drought is. But aquifers remain hidden, hard to measure, hard even to imagine: What does it mean that, beneath much of the United States, there are invisible seas full of drinkable freshwater? How can we think usefully about both their vastness and their finitude?
Already, 2 billion people worldwide rely on groundwater for daily use. That will have slosh-over effects: A 2012 study reported that water from aquifers, moved to the surface by human activity like farming and mining, would constitute 25 percent of sea level rise before 2050, and possibly even more after that. Relocated groundwater, by that paper’s estimate, would be the third-most significant cause of sea level rise this century, after the melting ice sheets of Antarctica and Greenland.