In a finding offering another sea-level rise warning, scientists yesterday reported the existence of deep, canyon-like valleys in the bedrock underneath the Greenland ice sheet that could facilitate the extensive flow of ice into the ocean.
The analysis of Greenland's topography from NASA and University of California, Irvine, scientists suggests that estimates of Greenland's contribution to sea-level rise may have to be revised upward for this century and beyond. It also follows on the heels of earlier studies this year indicating that parts of Greenland and Antarctica may be far more vulnerable to warming ocean temperatures than previously believed.
"For sure, this will have a strong impact on future modeling of sea-level rise. We just don't know how much yet," said Mathieu Morlighem, associate project scientist at the University of California, Irvine, and lead author of the study, published in Nature Geoscience.
Many older models of Greenland assumed that its massive ice sheet sat on bedrock that was relatively flat, even though scientists did not know the full thickness of the ice. There also was an assumption that many melting glaciers on the ice sheet's periphery eventually would retreat to higher ground on this flat bedrock, cutting off contact with warm ocean waters and slowing down the ice sheet's shedding.
The discovery of the valleys—which stretch more than 100 kilometers (62.1 miles) into Greenland's interior in some cases—suggests instead that some of Greenland's most vulnerable outlet glaciers will just keep retreating and retreating along deep canyons. About 100 of the valleys sit far below sea level and are attached to glaciers on Greenland's periphery that already are shedding ice, like Jakobshavn Isbræ glacier, said Morlighem.
That means that as these glaciers retreat, their fronts will remain in contact with warm ocean water that melts ice, rather than hitting higher ground anytime soon. "As long as the vertical face of the glaciers remains in contact with the ocean, they are vulnerable," said Morlighem.
Models will need to be recalibrated
"We once thought it would be decades for some of the glaciers to reach higher ground, and now it's maybe a thousand years for the longest troughs," he added. Previous mapping often did not capture the depth of the valleys, when their existence was known at all, he said.
The bed underneath Jakobshavn Isbræ glacier, for instance, is "several hundreds of meters deeper than in previous reconstructions," the paper states. Many of the discovered valleys also stretch farther into the interior of the ice sheet than previous measurements.
The paper reports that 107 marine-terminating glaciers are underlain by fjords extending on average 67 kilometers (41.6 miles) inland below sea level, a number 300 percent greater than one previous assessment. If these glaciers retreat at a similar rate to what they did in the past decade, 30 of them would disconnect from warm ocean waters by the end of the century with that kind of travel distance, it says.
"The beds at the margins of the ice sheet ... are probably one of the least explored parts of Earth's surface," said Ian Howat, a glaciologist at Ohio State University who did not participate in the research. He called the paper a "major breakthrough" culminating years of careful work that would give scientists much more data for assessing Greenland's changes.
"This is going to prompt a flurry of rerunning of models" projecting future sea-level rise, he said.
Andrew Shepherd, a professor of Earth observation and Greenland expert at the University of Leeds, said the new data are "robust and will be trusted by the wider scientific community." Like Howat, he said the findings could have major implications for predictions of how the Greenland ice sheet evolves.
"This could lead to an upwards revision of predicted ice losses," he said.
Ice thickness is deceptively hard to measure, Howat said. In many locations, the ice is deep and wet, and the surface is crevassed and rough, making it difficult to fully assess with radar, he noted. It's taken decades to determine "basic things" like where the interface between underlying rock and ice is.
How high? An ongoing debate
Since the 1970s, there have been measurements of Greenland's ice thickness via radar pinging of the interface between ice and underlying foundation. The problem with those measurements, Morlighem said, is that they were scattered, and the data were limited to areas where there were flights over the ice sheet.
There had been some previous detections of the depths underneath the ice, but there was little understanding whether it was a limited depression or part of a much more extensive canyon, he said. The researchers used a range of data of other ice sheet dynamics, like snowfall and surface velocity of ice, to create a more detailed overview of Greenland's topography.
Greenland's ultimate contribution to sea-level rise is a subject of ongoing debate among scientists. A report from the Intergovernmental Panel on Climate Change last year found that between 1993 and 2010, the Greenland ice sheet contributed less than 10 percent to global sea-level rise. Thermal ocean expansion of the ocean and glaciers contributed the most.
Six or seven years ago, some initial projections suggested that sea levels could rise quite high by 2100, said Howart. Subsequent papers from multiple scientists pulled initial numbers far back and reported figures closer to 3 feet, he said. Now the trend may be going in the other direction with new data about ice sheets, although a lot more research is needed, he said.
Last week, scientists reported that glaciers in West Antarctica may be on a pathway of irreversible decline, although it could take centuries for their collapse (ClimateWire, May 13). In March, another paper reported that northeast Greenland—long considered relatively stable—was shedding significant amounts of ice (ClimateWire, March 17).
Shepherd said, though, that there still is still a lot of uncertainty about how much additional melt in some locations of the Greenland ice sheet will actually be lost to the ocean.
Inland, the surface of Greenland is made up of low-density firn, something halfway between snow and solid ice, he noted. In these places, meltwater can percolate through the firn and refreeze.
"It's not easy to measure or model how much additional meltwater refreezes rather than runs off into the ocean," he said.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500