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.
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.”
As the growing human population and more intense droughts brought on by climate change are putting ever more stress on water supplies, land is subsiding all over the world. Some parts of the Indonesian capital of Jakarta, for instance, are sinking as much as 10 inches a year, all while the seas are rising around it. Models estimate that in just three decades, 95 percent of North Jakarta could be underwater. The situation is so dire, Indonesia is planning to move its capital. [Continue reading…]