This story was updated at 12:12 p.m. EDT.
A large area of the Greenland ice sheet once considered stable is actually shedding massive amounts of ice, suggesting that future sea-level rise may be worse than expected, a team of scientists warned yesterday in a new study.
The research in Nature Climate Change signals that many climate models may be too conservative in their projections through this century, as they are not considering ice loss from the northeast portion of Greenland. The discovered ice loss in the northeast -- which has been largely overlooked to date -- is worrisome because the drainage basin there covers 16 percent of the ice sheet and holds the capability of funneling ice deep from Greenland's interior out to sea, the scientists said.
"This is an area we should be worried about because it's huge. Who knows, maybe this is starting something very big," said Shfaqat Khan, senior researcher at Technical University of Denmark, and the lead author of the study. The ice loss found in northeast Greenland is accelerating, and it is uncertain how many years it will continue, he said.
The scientists reported that northeast Greenland was stable -- with a zero ice mass loss -- until about 2003, when summer temperatures spiked. The northeast was considered the last remaining stable part of the ice sheet, according to the researchers. Within a few years, the main outlet glacier draining the region -- Zachariae Isstrom -- retreated about 20 kilometers, and regional ice mass loss jumped from zero to roughly 10 metric gigatons a year. "That is huge," Khan said.
Ice loss from northeast Greenland into the Fram Strait abutting the Arctic Ocean is now closer to 15 to 20 metric gigatons a year and is still increasing, said Khan.
In comparison, it took the Jakobshavn Isbræ ice stream -- a southwest Greenland region with a fast-moving glacier that has been a focal point of scientific examination of ice sheet melt -- 150 years to retreat 35 kilometers, said Khan. "We haven't seen something like this anywhere else," Khan said about the northeast.
He predicted northeast Greenland would overtake the Jakobshavn ice stream within a few years as Greenland's largest contributor to sea-level rise. The Zachariae ice stream is about twice the size of Jakobshavn, extending some 373 miles into the interior.
Climate models may understate risks
Previously, many scientists had believed that northeast Greenland was too cold to be a significant contributor to ice loss, said Khan. With so much focus on Jakobshavn and the northwest, there was less attention to the northeast, said study co-author Michael Bevis, a professor at Ohio State University.
Sea-level rise estimates through the end of the century range widely, with the Intergovernmental Panel on Climate Change (IPCC) suggesting a possible number below 1 foot to around 3 feet, while other scientists suggest it could be higher than 3 feet. The ultimate result could make a huge difference with storm surge and flooding in coastal states like North Carolina.
Over the past 20 years, Greenland melt contributed about 16 percent of the global total of sea-level rise annually, according to the study. Thermal expansion of water, glacier and Antarctica ice melt, and aquifer changes on land drove most of the rest.
Considering that existing climate models typically do not consider northeast Greenland with future sea-level projections, the findings suggest that sea-level rise estimates may err on the high side, close to 3 feet or higher, said Khan.
There remains a high level of uncertainty, because of unknowns and cyclical dynamics with future air and water temperature, said Bevis. Ocean currents bringing unusually warm water, for instance, could shift away more from Greenland, or move in closer, he said.
Greenland's ice streams drain the ice sheet via meltwater runoff much in the same way that water basins drain rivers, explained Bevis. For years, floating ice in the bay abutting the ice sheet in the northeast acted as a barrier to ice loss. The ice pack of loose debris used to be year-round, making it previously "hard for glacier to push its way through that," he said.
With significant warming in the past decade, there was significant melt from the ice sheet. Further, the floating bay ice decreased, allowing easier flow from the ice sheet into the water.
Additionally, the Zachariae glacier at the ice sheet margin began its retreat and moved into deeper water, which exacerbated the melt, said Bevis. Water touching the glacier got warmer, and "there was more interface between the water and the ice," said Bevis. The paper notes that ocean warming around Greenland may be almost double the global mean by 2100.
Exact numbers are a work in progress
The scientists used a combination of surface elevation data from satellites and planes between 1978 and 2012 and a GPS network that weighs the ice sheet like a scale, according to Ohio State. The Greenland GPS Network, or GNET, utilizes more than 50 stations on the ice sheet coast to detect ice loss.
Ice experts not involved in the research offered mix reviews.
Richard Alley, a glaciologist at Pennsylvania State University, said it is an "interesting paper" that shows that thinning has started in a region thought resistant, in response to warming that is much smaller than what is projected for the future. It suggests that a small temperature change helped trigger a notable ice loss.
Tad Pfeffer, a professor at the University of Colorado, Boulder, said the results appear "pretty plausible" and "carefully done." It will be extremely important to track glacier changes in coming years to see whether the rapid ice loss slows down in the northeast, he said.
Yet Ian Joughin, a scientist at University of Washington, said he is skeptical of the result. He said it's not impossible that northeast Greenland is losing that much ice, but he would be surprised if the numbers hold up with additional research.
The speed of the glacier doesn't seem to match the measurements from surface elevation and GPS data, said Joughin. He said he suspects the researchers "overextrapolated" some of the data, although it was clear that there was some real change going on in the northeast. However, data from that region are really sparse for the studied period, he said.
However, the big picture for Greenland is clear, in that warming melts ice, said Alley. There is good evidence throughout history that the Greenland ice sheet has shrunk with warming to raise sea levels, but putting precise numbers on how much and how fast is a work in progress, he said.
He said the new paper highlights many things that are still not well-known to scientists, such as extensive information about water temperature reaching the grounding line of the outlets of the northeast Greenland ice stream, where water is probably most influential with melt.
"The paper still doesn't lead to 'Run for the hills!' sea-level rise, but it may lean toward the high side of the IPCC projections. ... [T]here is work to do to really nail down the numbers," said Alley.
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