Climate change is warming the Arctic more than twice as fast as anywhere else on the planet. One of the most serious consequences is sea level rise, which threatens nations from Bangladesh to the U.S. But exactly how does melting Arctic ice contribute to sea level rise? Scientific American asked Eric Rignot, professor of earth system science at the University of California, Irvine, and Andrea Dutton, assistant professor of geology at the University of Florida, how changes in this particular northern region are driving the oceans to dangerous heights.

Seas are now rising an average of 3.2 millimeters per year globally, and are predicted to climb a total of 0.2 to 2.0 meters by 2100. Rignot and Dutton say that in the Arctic, the Greenland Ice Sheet poses the greatest risk for ocean levels because melting land ice is the main cause of rising seas—and “most of the Arctic’s land ice is locked up in Greenland,” Rignot explains. That’s 2.96 million cubic kilometers of ice now covering land areas—and it’s melting into the ocean. If the entire Greenland Ice Sheet thawed, Dutton says, it would raise sea levels by an average of seven meters. That would significantly flood coastal megacities such as Mumbai and Hong Kong.

Greenland’s land ice is already thawing fast enough to raise worldwide seas 0.74 millimeter per year. “The melt rate has been increasing,” in large part because the ice sheet’s surface thawing has picked up as global temperatures warm, Dutton says. “This acceleration of surface melt has doubled Greenland’s contribution to sea level rise” compared with the period from 1992 to 2011, Dutton adds.

And the Arctic has other frozen land areas—mountain glaciers and ice caps—in places like Iceland, the Canadian and Russian Arctic, Alaska and Norway’s Svalbard Islands. These hold nowhere near as much water as the Greenland Ice Sheet but are still a significant part of the sea level equation. Together with glaciers and ice caps in the Southern Hemisphere (excluding the Antarctic Ice Sheet), their complete meltdown could potentially raise oceans nearly half a meter, according to the U.S. Geological Survey. But the northern areas have many more icy features than southern ones, Rignot notes. “Melting glaciers and ice caps in Patagonia and the other southern places don’t contribute as much as those in the Arctic,” he says.

In addition to simply adding water to the ocean, thawing Arctic land ice can raise sea levels even more via a mechanism called thermal expansion. “In a warmer climate the ocean absorbs a lot of extra heat from the climate system, and as a result it becomes less dense,” Rignot explains. As Arctic land ice melts into the sea, there is more ocean water overall—and thus more water to heat up and expand as the climate warms, which drives up sea levels even more. “The amount it expands is significant, enough that we can measure it,” Dutton says. From 1993 to 2010 thermal expansion added an average of 1.1 millimeters of sea level rise per year, according to the International Panel on Climate Change Fifth Assessment Report (pdf).

The Arctic has a lot of sea ice, too—at least 6.5 million square kilometers—and it is about two meters thick on average, according to the National Snow and Ice Data Center. Dutton and Rignot say that although the sea ice is shrinking, it does not add to water levels as it melts because it is already part of the ocean’s mass. “You can freeze and melt the sea ice as much as you want—it’s not going to change the sea level,” Rignot says. “You’re just changing the state of the water.” Dutton provides an analogy: “If you’re sitting in a bar with a drink with ice cubes, and your cup is full and the ice cubes melt, it’s not going to overflow. That ice is already floating in the water, so it has displaced the volume of its own space.”

But thawing sea ice still plays a role in sea level rise. “The sea ice acts as a blanket on top of the ocean,” protecting the water from incoming solar energy and atmospheric heat, Rignot says. As that frozen coating disappears, its white surface is no longer there to reflect sunlight back into the atmosphere—so the ocean absorbs much more solar energy. “It’s like going from receiving no heat from the sun to receiving the sun’s full heat,” he explains. “It’s a tremendous difference.” This effect accelerates overall warming, which in turn melts more land ice and drives up sea levels. So even though all that melting sea ice may not seem like a big deal because it is not directly adding to sea levels, Rignot says “it matters greatly—the disappearance of that blanket disturbs the whole Arctic system.”