Rivers of meltwater on Greenland’s ice sheet contribute to rising sea levels

The team was particularly interested in learning how much of the meltwater would be retained, how much of it moved to the ocean, and how quickly and through what means it did so.

“The question was whether the ice sheet acts like a sponge or like Swiss cheese,” Smith said.

Ultimately, the researchers determined the answer to be a little bit of both. At least on top of its surface, the sheet was like Swiss cheese: All of the 523 actively flowing streams and rivers across a 2,000-square-mile area of the ice sheet drained into moulins, or sink holes, that promptly carried meltwater under the ice sheet and into the ocean.

Meanwhile, a modest “sponge” effect occurred somewhere below the surface — either inside or, more likely, at the base of the ice sheet. Within this area, the researchers determined, the ice sheet drained at a rate of 55,000 to 61,000 cubic feet per second, a rate more than double the average flow of the Colorado River, produced from less than one-tenth of its watershed area.

The group also specifically examined the Isortoq River, which exits the ice sheet on land and drains about one-fifth of the mapped networks. Its output is critical because it’s a key element of the Modele Atmospherique Regional, or MAR, a climate model used by the Intergovernmental Panel on Climate Change to develop worldwide responses to global warming. The researchers found that the Isortoq’s amount of discharge was more than 25 percent less than the model predicted.

“There’s a lot of water coming out of the ice sheet but a certain amount can’t be accounted for,” said study co-author Vena Chu, a UCLA doctoral candidate in geography.

The discrepancy suggests that at least some of the meltwater is being captured in a subterranean manner that has yet to be accounted for, the researchers said.

“The model automatically assumes that the meltwater is going directly to the ocean,” said Marco Tedesco, a co-author and head of City College of New York’s Cryospheric Processes Laboratory, which operates MAR. “Some can get retained under the ice. More research is definitely needed.”

Still, researchers found that the flow in the Isortoq River carried meltwater from the ice sheet to the ocean at an average flow rate of 23,000 to 46,000 feet per second — a massive flow rate that is all the more awe-inspiring because the Isortoq is but one of at least 100 large terrestrial rivers connecting the melting Greenland ice sheet to the world’s oceans.

The new research will allow scholars to refine existing climate models, which appear to modestly overestimate the direct contribution from runoff to rising sea level, said Tedesco, an associate professor of earth and atmospheric sciences at CCNY.

“If we can get better estimates, then we can have better projections for the extent and the impact of global warming,” he said. “Greenland is really the big player for sea level rise in the future, so improving climate models is extremely crucial.”

— Read more in Laurence C. Smith et al., “Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet,” Proceedings of the National Academy of Sciences (13 November 2014) (doi: 10.1073/pnas.1413024112)