Over many years, glaciers helped form Greenland’s fjords, those narrow and deep inlets in the sea that are often surrounded by steep cliffs and serve as exit routes for the vast ice sheet’s sea-terminating outlet glaciers.

And, according to new research, fjords in West Greenland are much deeper than previously thought. That means the world’s sea levels could rise faster than anticipated, because those outlet glaciers are more exposed to warm water. The findings have been peer-reviewed and accepted for publication in Geophysical Research Letters.

The shape and depth of fjords have big implications for the ice sheet, which has been melting both from the top and the bottom and contains 20 feet of potential sea level rise in total. Warm air erodes ice above the water, but warmer waters — which reside at deep levels in some parts of the polar regions — undercut glaciers and melt ice from below. “As they melt faster, they can slide out to sea,” said Eric Rignot, leader researcher and a glaciologist at the University of California at Irvine.

Deeper fjords means there are “a lot more places where the warm water, subsurface water, can reach the glaciers,” Rignot said. Shallow fjords don’t pose as much of a threat.

On average, the fjords in this region are about 200 to 300 meters deeper than previously thought in some areas, he added. Glaciers undercut by warm water can melt twice as fast as those in colder waters, all other things being equal.