Two new studies, each looking at different measurements, have reached a similar conclusion: The impact of global warming on the Greenland ice sheet is being underestimated.

Andrew Shepherd, the coauthor of a paper published by scientific journal Nature Climate Change, says that his study, along with another by Beata M. Castho, shows that the ice sheet is behaving differently than scientists had previously thought.

“I think that people tend to think they know what's going on with the climate system,” Shepherd says. “Every time a study comes out, that solves the problem. But it's not the case.”

Shepherd, who is the director of the Center for Polar Observation and Modeling, and the principal scientific advisor for the CryoSat satellite mission, looked at "supraglacial lakes," bodies of water that form in summer each year around the edges of Greenland, where the ice is low and close to sea level.

“They're really quite beautiful things,” Shepherd says. “If you fly across the Atlantic between America and Europe you may get a chance to look at them if you look down from the plane … They’re a few kilometers across, maybe 50 meters deep. They look like really nice swimming pools."

The ice in Greenland grows in winter and melts each summer. If everything was in balance, the huge island would stay the same shape and size. But, as we now know, things are not in balance anymore. Now there's more ice melting in the summer than winter snowfalls can replenish, so Greenland is shrinking and the ice is causing sea levels to rise.

Previous studies have shown Greenland now has more supraglacial lakes than it did 20 years ago. But it’s not really the number of lakes that’s important, Shepherd says, it's the huge area over which they spread.

Today the lakes reach about 100 kilometers inland from the edges of Greenland. According to Shepherd’s models, “the lakes will stretch to about 200 kilometers inland within the next 50 years. That means the area over which they're spread is really twice as large.”

Shepherd expects this trend to continue and perhaps even accelerate. “The air temperatures are warming and there's more melting, so these lakes [will] form in places where it's really not hot enough for them to form today — but in the future it will be,” he says. “That means different parts of the ice sheet will be exposed to the effects that these lakes have on the ice sheet flow, which has already been shown to be quite considerable.”

The lakes not only drain water more into the sea, Shepherd explains. “It’s that on their way to the sea they reach the bottom of the ice sheet, which causes the [sheet] to slip more quickly into the sea … So it's a double whammy: in addition to melting the ice on the surface, you slide more ice into the oceans as well.”

The other study, run by Beata M. Csatho of Ohio State University, looked at NASA measurements taken over the past 10 or 15 years via aircraft and satellites. Shepherd says the results show the Greenland ice sheet is in fact thinning in places that hadn't been identified before —  and behaving differently than scientists had thought.

“When we look at different parts of the ice sheet, we see different things, and it matters that we're able to examine the ice sheets everywhere,” Shepherd explains.

The upshot is simple: “We don't really know everything that's going on in the climate system, and the models that people make, no matter how good they are, have to include all of the processes that are happening on Earth, and if we don't see those processes, people tend to forget to include them.”

This story is based on an interview that aired on PRI's Science Friday with Ira Flatow

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