The water that pours out of your tap, or that’s unnecessarily packaged in a single-use bottle, or that helped grow the produce in your fridge—all of it may well have come from aquifers somewhere. These are layers of underground material that hold water, and can be made up of porous rock or sediments like sand and gravel. When it rains, some water collects in lakes and rivers and eventually flows out to sea, but some soaks deep into the ground, accumulating in these subterranean stores.
We dig shallow wells or drill deeper boreholes to tap into aquifers to hydrate our civilization, but that extraction has gotten way out of hand. An alarming new paper published today in the journal Nature looked at available data on 1,700 aquifer systems worldwide and found that groundwater is dropping in 71 percent of them. More than two-thirds of these aquifers are declining by 0.1 meters (0.33 feet) a year, while 12 percent are notching a rate of 0.5 meters. (Think of this decline as like looking down into a well, then coming back the next year and seeing that the water level is 0.1 meters lower.) Nearly a third of the aquifers are experiencing accelerated depletion, meaning the decline is speeding up, in particular where the climate is dry and there’s a lot of agriculture that needs watering.
“Real-world observations—300 million of them in hundreds of thousands of wells around the globe—show two main findings,” says water scientist Scott Jasechko of UC Santa Barbara, co-lead author of the new paper. “One is that rapid groundwater declines are unfortunately widespread globally, especially in dry places where croplands are extensive. And then second, even worse, groundwater declines have, if anything, accelerated over the last four decades in a disproportionately large share of the global landmass.”
Aquifers are supposed to be reliable banks of water, safely locked underground where the liquid can’t easily evaporate away. They’re a rainy-day fund—or, more accurately, a dry-day fund—available to tap into in times of need, like during a drought. But from Chile to Afghanistan to India to China, and back to the United States, humans are emptying these water stores at an unsustainable pace. (In the maps below, the deep red indicates groundwater declines of a meter a year, with lighter reds showing less decline.) In areas where an already dry climate is getting drier because of climate change, people have less aboveground water to rely on, and so they’re forced to over-extract aquifers.
Where these drying areas host extensive agriculture, the problem gets even worse: The more the world warms, the more water crops will need. And if there’s less rainfall in these areas because of climate change, there’s less water to naturally refill the aquifers, which might normally offset some of the extraction by humans. “A very large share of the aquifer systems that are experiencing accelerated groundwater level declines are also places where precipitation has declined over the last 40 years,” says Jasechko.
The threat isn’t just eventually running out of water, but also triggering peculiar geological and hydrological side effects. In some places, groundwater feeds into rivers—drain the aquifers, and you shrink those water bodies too. Along coastlines, losing groundwater allows seawater to flow subterraneously into the aquifer, contaminating water supplies for people and crops.
Even more dramatically, the relentless extraction of groundwater is causing land to sink, a phenomenon known as subsidence: Drain an aquifer and it’ll collapse, like an empty water bottle. According to one estimate, in the next two decades this subsidence could affect 1.6 billion people and cause trillions of dollars of damage.
In California, for example, agriculture has made the land sink dozens of feet in some places. (Notice the state’s deep red in the map above.) Parts of Jakarta, Indonesia, are sinking almost a foot each year, because residents and industries have been draining aquifers. Earlier this month, researchers reported that up to 74,000 square kilometers (29,000 square miles) of the East Coast of the US are exposed to subsidence of up to 2 millimeters (0.08 inches) annually, and over 3,700 square kilometers are sinking more than 5 millimeters. (However, the problem isn’t just a matter of dwindling groundwater: Other scientists have found that NYC is sinking up to 4 millimeters a year due in part to all those skyscrapers pushing down on the earth.) That sort of coastal sinking is happening just as seas are rising, greatly exacerbating the problem.
An additional pressure is that coastal cities are rapidly growing, meaning they have more people who will require more water, which may only be available from an underlying aquifer. So while the solution here is simple—stop pumping so much groundwater, and actually add water back to the earth where you can—that’s easier said than done if you’ve got thirsty humans and crops to take care of. “There are cases, although they are rare, where groundwater level declines have turned around,” says Jasechko. Bangkok, Thailand, is one, and the aquifer in Arizona’s Avra Valley has been refilling nicely, thanks to a diversion from the Colorado River.
This sort of “aquifer recharge” technique is spreading around the world. California is dangling giant sensors from helicopters to find the best aquifers to direct stormwater into, for example. Los Angeles, Pittsburgh, and other major cities are deploying urban infrastructure that soaks up rainwater, allowing it to trickle underground. Such sponge cities buck centuries of traditional urban planning, in which streets were designed to ferry away water as quickly as possible to prevent flooding. Now more than ever, we need community gardens and other urban green spaces to absorb rainwater, which both keep the area from flooding and recharge the local aquifer. (If you grow crops in those gardens under solar panels, you produce food and renewable energy for your urbanites.)
Municipalities are also launching water recycling programs to relieve pressure on aquifers: New technologies can now turn very-much-not-consumable wastewater into ultrapure liquid for drinking or watering crops. So instead of flushing wastewater from homes and businesses out to sea, we can keep on recycling it, reducing the demand for local aquifer water. “Taking wastewater, putting it through a highly regimented treatment process, and then putting it directly back into a drinking water system, that really could be a game changer,” says Katherine Kao Cushing, who studies sustainable water management at San José State University but wasn’t involved in the new research.
So in this clear and present threat to the world’s aquifers, we actually have opportunities—to make our cities greener, to keep coastal areas from sinking, and to renegotiate our fraught relationship with water. “What the paper encourages us to think about,” says Cushing, “is that at a global scale, major changes could be coming in terms of groundwater availability, and that we should prepare accordingly.”
