The Sea Level Threat To Cities Depends On Where The Ice Melts — Not Just How Fast

The world's oceans are rising. Over the past century, they're up an average of about eight inches. But the seas are rising more in some places than others. And scientists are now finding that how much sea level rises in, say, New York City, has a lot to do with exactly where the ice is melting.

A warming climate is melting a lot of glaciers and ice sheets on land. That means more water rolling down into the oceans.

But the oceans are not like a bathtub. The water doesn't rise uniformly.

To understand why, think of the earth as a spinning top. When huge ice sheets — some are two miles thick — start to melt, it actually affects the Earth's rotation.

"What happens is when you change the mass of the ice," explains Eric Larour, who studies the frozen parts of the planet, "the modification itself makes the wobble change, and this in turn changes the shape of the ocean on the Earth."

When the wobble shifts, the oceans shift as a whole, as if you were shaking a mound of Jello at the Thanksgiving table.

That's part of the story, but something else happens too.

Many ice sheets and glaciers are so massive, they produce a significant gravitational field, almost as if they were small versions of the moon. The force is tiny, but it does attract nearby ocean water.

"So what happens when the ice melts," says Larour, "is that there is less of it, so the ocean recedes away from the mass of ice."

Larour's team at NASA's Jet Propulsion Lab has mapped how changes in these giant ice fields influence sea levels both nearby and thousands of miles away. They published their results in the journal Science Advances. For example, the ocean along Norway's coast could actually drop a tiny bit if nearby ice sheets in eastern Greenland melt. Meanwhile, those Greenland ice sheets could raise sea levels by inches on the other side of the planet — in places like Tokyo.

Larour says this is useful information: "Out of all the masses of ice around the Earth — Alaska glaciers, Greenland, Antarctica, Patagonia, Himalaya — which ones are going to be contributing to sea level in New York?"

It turns out that in New York City, the sea level would be affected more by melting ice on the northern end of Greenland than much closer ice in southern Greenland, or even ice in Canada.

Scientists are now using this information to predict the future for American cities, but they're also building in a lot of local geographical information. Oceanographer William Sweet is one of those scientists, and he has a personal interest in that future; he lives along the Chesapeake Bay.

"Right here on the Severn River," he says as he sits on a dock on the riverside, "we are somewhere that's very likely to experience 25 to 50 percent more than the global average" of sea level rise.

Sweet works at the National Oceanic and Atmospheric Administration, which is putting together a sea level rise grid for the country, one that will factor in local conditions as well as the effects of faraway melting ice. The land along Louisiana's coast is sinking, for example, as are parts of the East Cost. And off the East Coast in the Atlantic, a huge river of warm water — the Gulf Stream — could influence sea level as the Atlantic warms.

"Every 60 miles or so along the United States coastline," he says, the grid will lay out for local authorities what their coastline will look like over the rest of the century under various warming scenarios. "So it really matters when you start planning ... 'I'm going to be prepared for one meter of sea level rise.' Well, you might want to be prepared for four or five feet."

He adds that one thing's for certain: "Once you wait until you realize you have a problem, it's going to be chronic rather quickly."

Already, scores of coastal cities are flooding much more often than they used to. As the climate warms, that kind of flooding could become the norm.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.