Scientists studying the melting of Greenland’s ice have made a deeply worrying discovery: it’s melting four times faster than they previously thought, which means sea levels are going to rise more quickly.

“The only thing we can do is adapt and mitigate further global warming — it’s too late for there to be no effect,” said Ohio State University geodynamics professor Michael Bevis, the lead author of a paper published Monday. “This is going to cause additional sea level rise. We are watching the ice sheet hit a tipping point.”

The researchers say coastal cities are now threatened, as are island nations. Data suggests that sea levels rose by 0.03 inches each year between 2002 and 2016, based on ice melt from Greenland alone. If the ice on the world’s largest island is melting more quickly than previous realized, that spells trouble.

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Using satellite and GPS data, Bevis and the other researchers discovered that the rate of Greenland’s ice loss increased almost four-fold between 2003 and 2012.

The fact that Greenland’s ice is melting and contributing to sea-level rises is already well known, but researchers have so far largely focused on Greenland’s glaciers. Chunks are breaking away, floating off in the sea and eventually melting.

The new research focuses on Greenland’s southwest region, which is where the greatest ice loss between 2003 and mid-2013 took place. But this region doesn’t have many glaciers to begin with, meaning the ice there is melting inland and flowing out in rivers.

The culprit appears to be climate change, which combines with a natural air-current phenomenon — the North Atlantic Oscillation — to pummel Greenland with warm air.

“In the case of Greenland, global warming has brought summertime temperatures in a significant portion of Greenland close to the melting point, and the North Atlantic Oscillation has provided the extra push that caused large areas of ice to melt,” Bevis said.