Most meltwater in Greenland fjords likely comes from icebergs, not glaciers

Greenland, the world's largest island, is almost entirely covered by a permanent ice sheet that has been shrinking and melting as global warming increases temperatures. In fjords, narrow inlets where glaciers meet the sea, ice breaks off from glaciers to form dense packs of icebergs.

The new study finds 10 to 50 percent of iceberg melting happens in the fjords, not in the open ocean as assumed by previous research. As a result, more than half of all meltwater entering Greenland's glacial fjords can come from these dense packs of icebergs, outweighing the amount of freshwater coming from the island's glaciers, according to the new study.

"We should now be able to better measure the freshwater fluxes that are coming off of Greenland," said Ellyn Enderlin, a glaciologist at the University of Maine in Orono, and lead author of the new study published in Geophysical Research Letters, a journal of the American Geophysical Union. "That could be really important when we're thinking about how Greenland melts, how that influences ocean circulation and climate."

The new study could help scientists better understand what happens at the ice-ocean interface where glaciers meet the water, according to Jason Amundson, a geophysicist at the University of Alaska Southeast in Juneau, who was not involved in the new study.

"The reason that's interesting is that there's been quite a few studies in the past 20 years that have shown that the stability of ... glaciers depends on what happens at the ice-ocean interface," he said.

Melting point

The Greenland Ice Sheet releases more than 1,000 cubic kilometers (240 cubic miles) of meltwater per year, according to the authors. Previous research found half of this meltwater comes from icebergs and half comes from glaciers, but the amount icebergs melted in fjords before they reached the ocean remained a mystery.

In the new study, Enderlin and her colleagues used satellite images of two Greenland fjords to calculate the total volume of icebergs within them. Tracking the icebergs over the course of days, weeks and months allowed the researchers to calculate how much volume the icebergs lost through melting before they reached the ocean.

The researchers determined that from October to April, melting icebergs dominate the freshwater flux into the fjords losing up to half of their volume -- glaciers barely melt during the cold winter months. At their peak, these dense packs of icebergs melted at a rate of around 1,000 cubic meters per second (260,000 gallons per second), the equivalent of filling an Olympic-sized swimming pool every two and a half seconds. Even during the warm summer months, underwater glacier melting only occurs at a maximum rate of 400 cubic meters per second (about 100,000 gallons per second).

Icebergs are tiny in size compared to glaciers, but they contribute such a large fraction of meltwater to fjords because their large surface area allows them to melt more quickly, according to Enderlin.

"If you took an ice cube and put it in your drink, one solid ice cube would melt pretty slowly, but if you took it out, hit it with a hammer and put it back in, it would melt a lot faster," she said.

Enderlin and her team also used satellite images to estimate the iceberg distribution in the two fjords, which they used to calculate the icebergs' total underwater surface area. Multiplying the melting rate by the total underwater surface area, Enderlin and her colleagues found more meltwater in the fjords was coming from icebergs than from glaciers throughout most of the year.

"What I see now is that iceberg melting is huge, and so if you don't take that into account you're going to come up with some crazy high estimates for glacier melting that might not be representative," Enderlin said.

Melting icebergs in the fjords create a layer of cold freshwater near glaciers. This freshwater can affect water circulation in the fjords, which can affect how glaciers melt and recede, Enderlin said.

Ocean circulation patterns could also be disrupted depending on where the icebergs melt and release their freshwater, according to Enderlin. Ocean circulation is a major driver of heat movement from the tropics to the poles, and disruptions to it could cause chaotic and unpredictable changes to weather and climate, she said.

Enderlin hopes to expand her studies on iceberg meltwater flux to other areas, like Antarctica. She said she will continue the work she started with Gordon Hamilton, who was a glaciologist at the University of Maine in Orono, and a co-author of new study. Hamilton died in October while doing research in Antarctica.

"I would say that really this was sort of our joint brainchild," Enderlin said. "I bounced lots of ideas off of him ... He was really instrumental to [the research] and it was sad that he couldn't see it come to be finally."