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A double whammy of climate change and human-caused land subsidence means global coastal residents are experiencing extreme sea-level rise.

The world’s coastal residents are experiencing more extreme sea level rise than is widely appreciated because they are concentrated in places where the land is sinking rapidly, a study published Monday in Nature Climate Change has found.

Sea levels are rising globally as Earth’s ice sheets melt and as warming sea water expands. But on a local scale, subsidence, or sinking land, can dramatically aggravate the problem. Cities like New Orleans and Jakarta are experiencing very rapid sea level rise relative to their coastlines—the land itself is sinking as the water is rising.

Now, an international team of researchers has demonstrated that this one-two punch is more than a local problem. Sinking land makes coastal residents around the world disproportionately vulnerable to rising seas: The typical coastal inhabitant is experiencing a sea level rise rate three to four times higher than the global average.

“We are talking about not a forecast; we are saying this is happening today,” says lead study author Robert Nicholls of the University of East Anglia’s Tyndall Centre for Climate Change Research. “And it’s quite significant.”

The silver lining, such as it is: Where coastal land is sinking, it’s in large part because of human activities, like groundwater withdrawal, that coastal cities can do something about.

Some of the factors contributing to the fall (or rise) of Earth’s coastlines are beyond human control. Parts of Earth are still adjusting to the disappearance of glaciers that blanketed it during the last ice age, springing up in some places and sinking in others. In coastal river deltas, land subsides slowly as freshly deposited sediments are compacted.

But in addition to those natural processes, human activities, including groundwater withdrawal, oil and gas extraction, sand mining, and the construction of flood barriers around rivers, can all cause the ground to sink. Preventing river flooding, a good thing in itself, also stops rivers from spreading sediments that slowly rebuild the land.

In places where people are concentrated, these activities, particularly groundwater removal, often cause the land to subside much more quickly than it would via geological processes alone: Over the 20th century, parts of Jakarta, New Orleans, Shanghai, and Bangkok sank between six and 10 feet.

The problem of subsidence and its effects on sea level rise—where the land sinks, the oceans rise relative to the shore by the same amount—is well documented for certain cities. But prior to the new study the effect hadn’t been assessed at a global scale. “We wanted to actually understand what is the human experience of relative sea level rise” by taking subsidence into account worldwide, Nicholls says.            

To estimate the rate of sea level rise experienced along thousands of sections of coastline worldwide, Nicholls and his colleagues compiled data from four key sources: satellite observations of climate change-fueled sea level rise; model estimates of how land is adjusting from the last ice age; data on natural subsidence in 117 river deltas, and estimates of human-caused subsidence in 138 large coastal cities.

The findings were dramatic. Satellite measurements put climate-driven sea level rise at about 3.3 millimeters per year (around an eighth of an inch). Nicholls and his colleagues found that on average, Earth’s coastlines actually experienced a slightly lower relative lift of about 2.6 millimeters per year (0.1 inch) between 1993 and 2015, because so much land is still rising due to glacial rebound. But that’s not where the majority of people live: Over the same time period, Earth’s coastal inhabitants saw the seas rise by an average of 7.8 to 9 millimeters annually (about half an inch).

This, the authors say, reflects the fact that coastal inhabitants are concentrated in rapidly subsiding areas, including sinking deltas and sinking coastal cities. The problem is especially acute in southeast Asia, where in 2015, 185 million people lived in coastal floodplains—around 75 percent of the global total. Such people live with the threats of both river flooding and sea level rise.

If subsidence continues at current rates, far more coastal residents could be at risk in the next few decades. Projected population growth alone will cause the number of people living in coastal floodplains to rise from 249 million in 2015 to 280 million in 2050, the study found. Climate change-driven sea level rise will place another 25 to 30 million people in that flood zone; ongoing city subsidence adds 25 to 40 million more people on top of that.

University of Miami sea level rise expert Harold Wanless warns that it might not be accurate to suppose that today’s high rates of city subsidence will continue into the middle of the century.

“Shanghai, for example, has been trying for some time to limit theirs,” Wanless writes in an email. “And rising sea level over the next 30 years will force abandonment of portions of these low-lying cities.”

Indeed, a key implication of the study is that coastal cities around the world should be taking immediate steps to limit subsidence, before the combined effects of sinking land and rising seas force residents to retreat inland.

For many cities, Nicholls says that the “fundamental problem” is groundwater extraction, which causes the sediments in aquifers to compact together and the land above to sink. That was the situation in Shanghai, where land subsidence was first recognized as a problem in the 1920s and has been mitigated greatly in recent decades through better groundwater management. The same is true for Tokyo, where parts of the city sank more than 13 feet during the 20th century due to rapid groundwater depletion. Today, the city has all but eliminated subsidence through strict water-pumping regulations.

In other places, like coastal Louisiana’s fast-sinking Mississippi River Delta, undoing some of the damage caused by historic flood control measures will be key to reducing subsidence. There, a multibillion-dollar plan is underway to help restore the natural cycle of wetland growth by punching holes in the river’s 20th-century levee system. That would allow sediments to spill over into nearby marshes once again.

“Every locality needs to understand its situation,” says University of Florida oceanographer and coastal engineer Arnoldo Valle-Levinson, who wasn’t involved with the new paper. Valle-Levinson says the study offers “a nice way of reminding coastal municipalities that it’s not only climate-induced sea level rise that they should pay attention to.” Ultimately, he says, adaptation strategies will have to be tailored to local challenges.

Nicholls agrees that understanding and addressing the local causes of subsidence is essential. But he hopes that by framing sinking coastal real estate as a global issue, the new paper will encourage knowledge sharing between regions that have developed successful mitigation strategies, as well as more consideration of the issue by climate policymakers.

“You can think of subsidence mitigation just like climate mitigation,” Nicholls says. “It’s not ‘do one or the other’—it’s ‘do both.’”

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