SCIENCE

Greenland's contribution to sea-level rise may be much larger than expected, studies warn

Two new studies are offering new warnings about the Greenland ice sheet and its future contribution to rising sea levels that could wreak havoc on coastal areas.

In one, published in Proceedings of the National Academy of Sciences, researchers report that current computer models are not fully capturing how Greenland's glaciers are changing with warming and adding to ocean levels, particularly in the southeast part of the ice sheet. The other study, in Nature Climate Change, reveals that "supraglacial" lakes that can speed up ice loss could double their reach on Greenland by 2060 as they move further inland.

While they are from different journals, both papers have a parallel subtext that current projections of Greenland and sea-level rise are likely underestimating ice loss, and perhaps to a large degree.

"The pattern of changes are much more complex than we expected ... and the complexity is not considered in the projections yet," said Beata Csatho, an associate professor of geology at the University of Buffalo and author of the PNAS paper.

The Greenland ice sheet holds enough ice to raise sea levels by more than 20 feet. Recent projections suggest that Greenland could contribute approximately 20 centimeters (7.87 inches) to sea-level rise under some emissions scenarios by century's end -- a level put into question with the new studies.

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Csatho and other scientists analyzed surface height changes on Greenland's ice sheet at more than 100,000 locations between 1993 and 2012, using data from both NASA satellites and air flights. Those surface height measurements in turn allowed for new assessments of overall annual ice loss.

While satellites have provided an accurate picture of overall ice loss from the ice sheet, there was less understanding of what was happening glacier by glacier. According to Csatho, four large well-glaciers that typically are relied on to assess how much ice from broader Greenland will be dumped into the ocean this century -- Jakobshavn, Helheim, Kangerlussuaq and Petermann -- may not be representative at all.

'Big changes' in little-studied area

Rather, ice loss from small glaciers in the southeast part of the ice sheet constituted more than 40 percent of Greenland's ice loss during the study period, even though much of the area has often been ignored in modeling. "We are detecting very rapid and big changes there," said Csatho. That is a concern because the shedding is likely to continue, even though it's not adequately considered now in projections, she said.

Because the southeast has small glaciers, it has been difficult to study in many locations despite its importance, added Cornelius van der Veen, a geography professor at University of Kansas and study co-author.

The team also found increasing mass loss in north and northeast Greenland, where ice projections have been based on the Petermann Glacier, which actually did not undergo much change during the study period, said Csatho. That further signals that projections of the future of ice loss from that part of the ice sheet may be on the low side, she said.

"Here, a few decades of continuing thinning could unground large regions, which could ultimately cause significant mass loss from the deep, central part of the ice sheet," she said.

Overall, the paper reports that the Greenland ice sheet lost about 243 gigatons of ice annually from 2003 to 2009, adding about 0.68 millimeter of water annually to the oceans. The scientists divided 242 Greenland glaciers into a new set of seven groups based on how they behaved during the study. They plan to monitor representative glaciers in each group.

Csatho said she is working on a new project analyzing the different characteristics of the seven groups, via analysis of factors such as the amount of water beneath glaciers. Some of the glaciers actually thickened during the study period, or thickened and thinned in rapid reversals.

Lakes from melting ice speed up glacier flow

The second study, in Nature Climate Change, further warns that current projections of sea level this century likely are too low, as they do not consider the effects of supraglacial lakes increasingly creeping toward Greenland's interior.

"This is important, as the lakes are known to trigger increased ice sheet lubrication, flow and loss, and yet their spreading was neglected in IPCC models used to predict ice sheet losses," said Andrew Shepherd, a professor of earth observation at the University of Leeds and co-author of the research.

The lakes -- formed annually on the surface during the melt season -- can drain to the bottom of the ice sheet and help create a lubricant effect for sliding glaciers, while also allowing more absorption of heat because of their dark color. In the study, scientists from the United Kingdom, Belgium, Ohio State University and the University of California, Irvine, reported that the area covered by these lakes on Greenland could increase 48 to 53 percent by 2060, depending on future emissions.

The lakes have crept 56 kilometers (34.8 miles) inland in a ring near the coast since the 1970s, the study says, and could move inland up to 110 kilometers (68.4 miles) by 2060 as they migrate to areas currently too cold for lake formation. The inland migration rate of the lakes also sped up after 1995 with warming, the study finds.

Amber Leeson, a scientist at University of Leeds, explained in a statement that the inland location of these future lakes is important because they won't drain as easily into the ocean as current lakes near the coast. "In contrast, water draining from lakes further inland could lubricate the ice more effectively, causing it to speed up," Leeson said.

Many unknowns remain

Shepherd said new modeling of sea-level rise likely would occur soon. Earlier estimates from the IPCC that were later revised may "be back on the table," he said.

Scientists who reviewed one or both papers praised their details but cautioned that much remains unknown about Greenland.

"The authors make the point that it's much harder to predict what's going to happen in the future if the processes driving ice-sheet mass balance happen on such small scales," said Ben Smith, a principal physicist at University of Washington about the Csatho paper, which he said was "nice work."

Similarly, Richard Alley, a glaciologist at Pennsylvania State University, noted the complex set of factors playing into ice motion, including the character of subglacial mud that can act on ice much in the same way that mud lubricates wheels of a stuck car.

Work is ongoing to map where the bed of Greenland is frozen versus thawed, a dynamic that could affect the ultimate impact of the supraglacial lakes, he said. The lakes will "matter more" if they move over now-frozen areas, he said.

"I don't think either of these papers is THE answer, but both are solid steps," he said in an email.

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