Greenland Ice Sheet Melts and Refreezes, Speeding its Flow to the Sea

It turns out that Greenland's ice sheet may have a richer story than scientists once thought. Using ice-penetrating radar, scientists have discovered ragged blocks of ice as tall as city skyscrapers and as wide as the island of Manhattan at the very bottom of the ice sheet, shedding light on how this massive ice sheet is currently melting.

"We see more of these features where the ice sheets starts to go fast," said Robin Bell, one of the researchers, in a news release. "We think the refreezing process uplifts, distorts and warms the ice above, making it softer and easier to flow."

Using the radar, the researchers found that the massive, ragged-block structures cover about a tenth of northern Greenland. These features become bigger and more common as the ice sheet narrows into ice streams, or glaciers, headed for the sea. As the meltwater at the bottom of the ice sheet refreezes over hundreds to thousands of years, it radiates heat into the surrounding ice sheet, which makes it pick up its pace as the ice becomes softer and flows more easily.

In fact, the scientists linked the bottom features to faster ice flow. In fact, the Petermann Glacier in the north of Greenland, which shed a 100-sqare-mile chunk of ice in 2010, is sweeping about a dozen large features with it toward the coast as it funnels off the ice sheet. This is a somewhat worrying phenomenon as our climate continues to warm.

"Overall, these observations suggest that basal freeze-on is a key control on the large-scale flow of Petermann Glacier, a possibility that has not been explored previously," said Joseph MacGregor, one of the researchers, in a news release.

It looks as if lakes and rivers flow over the edges of the ice sheet. Then, the water from these features falls through the crevasses of the ice to reach the base of the ice sheet, where some of it refreezes. This refreezing and deformation may actually contribute to faster melting.

The findings are published in the journal Nature Geoscience.