When humans over-exploit underground water supplies, the ground collapses like a huge empty water bottle. It’s called subsidence, and it could affect 1.6 billion people by 2040.
AS CALIFORNIA’S ECONOMY skyrocketed during the 20th century, its land headed in the opposite direction. A booming agricultural industry in the state’s San Joaquin Valley, combined with punishing droughts, led to the over-extraction of water from aquifers. Like huge, empty water bottles, the aquifers crumpled, a phenomenon geologists call subsidence. By 1970, the land had sunk as much as 28 feet in the valley, with less-than-ideal consequences for the humans and infrastructure above the aquifers.
San Joaquin Valley Subsidence. Photo credit: USGS
The San Joaquin Valley was geologically primed for collapse, but its plight is not unique. All over the world—from the Netherlands to Indonesia to Mexico City—geology is conspiring with climate change to sink the ground under humanity’s feet. More punishing droughts mean the increased draining of aquifers, and rising seas make sinking land all the more vulnerable to flooding. According to a recent study published in the journal Science, in the next two decades, 1.6 billion people could be affected by subsidence, with potential loses in the trillions of dollars.
“Subsidence has been neglected in a lot of ways because it is slow moving. You don’t recognize it until you start seeing damage,” says Michelle Sneed, a land subsidence specialist at the U.S. Geological Survey and coauthor on the paper. “The land sinking itself is not a problem. But if you’re on the coast, it’s a big problem. If you have infrastructure that crosses long areas, it’s a big problem. If you have deep wells, they’re collapsing because of subsidence. That’s a problem.”
For subsidence to become a problem, you need two things: The right kind of land, and an over-exploited aquifer. Aquifers hold water in between bits of sand, gravel, or clay. When the amount of clay in an aquifer is particularly high, the grains arrange themselves like plates thrown haphazardly in a sink—they’ve basically got random orientations, and the water fills in the spaces between the grains. But if you start extracting water from an aquifer, those spaces collapse and the grains draw closer together. “Those plates rearrange themselves into more like a stack of dinner plates that you put in your cupboard,” says Sneed. “It takes a lot less space, obviously, to stack the plates that way. And so that’s the compaction of the aquifer system that then results in land subsidence at the surface.”
But wouldn’t pumping more water back into the aquifer force the clay plates back to their random, spacey orientations? Unfortunately, no. “It’ll press those grains apart a little bit—you’ll get a little bit of expansion in the aquifer system represented as uplift on the land surface. But it’s a tiny amount,” says Sneed. We’re talking maybe three quarters of an inch of movement. “They’re still stacked like the plates in your cupboard,” she continues.
So at this point you’ve got a double-barreled problem: The land has sunk and it won’t reinflate, and the aquifers won’t hold as much water as they once did, because they’ve compressed. “And that’s an important point,” says Sneed. “As places around the world, including California, are starting to use aquifer systems as managed reservoirs, the compaction of them prior to now has reduced their ability to store water.”
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But scientists haven’t modeled global risks of subsidence—until now. To build their model, Sneed and her colleagues scoured the existing literature on land subsidence in 200 locations worldwide. They considered those geological factors (high clay content), as well as topology, as subsidence is more likely to happen on flat land. They factored in population and economic growth, data on water use, and climate variables.
The researchers found that, planet-wide, subsidence could threaten 4.6 million square miles of land in the next two decades. While that’s just 8 percent of Earth’s land, humanity tends to build big cities in coastal areas, which are prone to subsidence. So they estimate that, in the end, 1.6 billion people could be affected. The modeling further found that worldwide, subsidence exposes assets totaling a gross domestic product of $8.19 trillion, or 12 percent of global GDP.
True, gradual subsidence isn’t as destructive as a sudden earthquake or volcanic eruption. “But it will cause these indirect effects or impacts that, in the long term, can produce either damages to structures or infrastructure, or increase floodable areas in these river basins or coastal areas,” says geoscientist Gerardo Herrera-García of the Geological and Mining Institute of Spain, lead author on the paper.
Subsidence is uniquely sensitive to climate change—at least indirectly. On a warmer planet, droughts are longer and more intense. “This is very important,” says Herrera-García. “Because no matter the amount of annual rainfall you have, the most important issue is that you have a prolonged drought period.” Dry reservoirs will lead cities to pump even more water out of their aquifers, and once you collapse the structure of an aquifer by neatly stacking those plates of clay grains, there’s no going back. For the 1.6 billion people potentially affected by subsidence—and that’s just by the year 2040—the consequences could be dire, leading to both water shortages and the flooding of low-lying land…
At the end of the day, subsiding cities are up against unstoppable physical forces. “Geology is geology,” says Sneed. “We can’t do anything about that.”
