New York City is, slowly but surely, sinking into the Atlantic Ocean. Researchers recently showed that the city creeps .04 to .08 inches toward sea level every year. Combined with rising sea levels and intensifying storms, this could raise the risk of devastating floods.
While “most of the headlines are saying the skyscrapers are the problem, they really aren’t,” says Tom Parsons of the United States Geological Survey, who led the new study.
Though parts of the city are built over artificial land (made by filling water with sediment), most of the Big Apple’s heaviest skyscrapers are built on sturdy bedrock; the city’s slow shoreward slump has far more to do with geology than hefty construction. Sinking aside, the sea is actually rising faster in New York City than the land is falling, Parsons adds.
“Without the sea level rise, this wouldn't really be that big of an issue,” he says.
However, this isn’t the case for many other cities around the world. From Jakarta, Indonesia to New Orleans, plenty of cities really are sinking much faster than the tides are rising to swallow them up—and here’s why.
Sinking and flooding
Every year, the average global sea level rises by about .1 in. But in some parts of Jakarta, the annual relative sea level rise can be closer to 10 inches. That’s because the tides aren’t just rising. The city is sinking.
“We have to sum up these two effects,” says Pietro Teatini, a civil engineer at the University of Padova in Italy and chair of the UNESCO Land Subsidence International Initiative. “This is what people call relative sea level rise: it means the sea level rise plus land subsidence.”
Land subsidence happens when human activity or natural forces cause parts of the Earth’s surface to lower. It can potentially cause problems both inland and at the coasts. And while subsidence has many causes (including human activities like pumping groundwater and building cities over soft sediments), it isn’t usually a direct result of climate change.
Thanks to a perfect storm of human factors and unfortunate geography, Jakarta is among the world's fastest-sinking cities. With more than 40 percent of the city now below sea level and storms possibly intensifying due to climate change, flooding in Jakarta has become so frequent and severe that the Indonesian government plans to relocate its capital to an entirely different island.
Extracting water and other resources underground
People need water. But when cities turn to underground aquifers, they can inadvertently create enormous subsidence problems. “I can say without any doubt that [groundwater pumping] is the main cause of land subsidence all over the world. For sure the most important one affecting cities,” says Teatini.
The rocks and sediments in certain underground aquifers act like sponges, pockmarked with empty spaces called pores that are filled with water. If the water is removed, the pores can squish or shrink under the weight of the overlying earth. This is why pumping groundwater can cause the ground to compact.
In parts of Mexico City, scientists say that groundwater extraction is causing up to 14 inches of subsidence every year.
“The population grew and they have a demand for water. So they take the water from the aquifer,” says geophysicist Shimon Wdowinski of Florida International University. “And the surface responds by subsiding.”
Within the last century, the Mexican capital has sunk by about 33 feet, warping buildings and disrupting infrastructure. Groundwater extraction is largely to blame, but the city’s unusual geography also contributes. Much of Mexico City was built over a filled-in lake, and the old lakebed is soft, waterlogged, and easily squashed.
Development on and near ground level can cause subsidence, too.
In parts of the Netherlands, for instance, the land is sinking .15 to .2 inches per year. Geologist Gilles Erkens of Utrecht University and Deltares, a nonprofit research institution, says that’s largely because humans are meddling with wetlands.
Draining wetlands to gain farmland can cause soil to vanish. When wetlands dry, oxygen from the air creeps into the soil and oxygen-breathing microbes start to munch up peat and convert it to carbon dioxide for energy. This shrinks and weakens the soil, leading to subsidence.
Though this is mostly a rural problem, some Dutch cities like Gouda are built right on top of peat soils, says Erkens. Because peat is easily compacted, he adds, it is “susceptible to subsidence because of loading. That happens in the cities.”
Building heavy cities over fluffy sediments isn’t just a problem in peatlands. Many large cities, from Shanghai to Jakarta to Cairo, sit atop river deltas—flat, often fertile plains at the mouths of rivers that, like peatlands, are naturally squishy.
Delta sediments naturally compress under their own weight over time, but regular river floods would normally compensate for this by delivering fresh sediments. Cities often confine their rivers to embankments to avoid flooding, which stops new sediments from reaching the delta. Damming upstream rivers can also halt sediment delivery, leading to subsidence—and floods.
“A delta is [only] a delta if there is enough sediment from [inland]… if there is not too much sea level rise and not too much land subsidence,” says Erkens. “Those factors—all three of them—are currently forming a danger for a lot of deltas and around the world.”
Natural movement of the earth
The Earth’s surface is constantly shifting: sediment naturally compacts, the ground deforms, and tectonic plates rift and collide. Indeed, New York City’s sinking skyline has its roots in the Ice Age—not in modern construction.
“In the last Ice Age, there was a huge, heavy ice sheet that pressed down the center of the continent,” says Parsons. Just as the sides of an air mattress inflate when someone lies down in the middle, this extra glacial weight at the center of North America caused its coasts to pop up.
“Then when the ice melted, that whole process reversed,” Parsons adds. “That's what's giving us two millimeters a year [of subsidence] all along the eastern coast, including in New York.”
Thanks to this post-glacial rebound, the land below New York City would continue to sink at mostly the same rate even if every one of the metropolis’ million buildings was disassembled and carried off.
While geological forces dramatically reshape the Earth’s surface over long periods, human activity is almost always the main reason for severe subsidence, emphasizes Teatini. Even in places like New York City, where subsidence is minor and mostly beyond human control, tiny dips in elevation add up—especially when combined with accelerating sea level rise due to climate change.