Until now, most glacier research has focused on how warming air melts ice sheets, but warming oceans play a crucial role.
Flying over eastern Greenland, the NASA scientists stared down from a Gulfstream jet as it followed the precise course they had flown in previous years - using radar to map the loss of ice.
“In the tube,” flight engineer David Elliott said as the team locked into their route over the ice sheet covering 80 per cent of the world’s largest island. Out the window, massive chunks of broken ice looked like salt flakes on the water.
The March mission was part of NASA’s Oceans Melting Greenland (OMG) project, a five-year, $30 million effort aimed at improving sea-level rise projections by understanding how warming oceans are melting ice sheets from below - the most ambitious research on the subject to date.
Rising seas threaten low-lying cities, islands and industries worldwide.
But projections for how high and how soon the rise will come to vary wildly in part because scientists lack clarity on how fast warming oceans are melting polar ice sheets.
The uncertainty confounds the preparations of governments and businesses and fuels the arguments of climate-change sceptics.
A draft report by the United Nations’ Intergovernmental Panel on Climate Change (IPCC), for example, predicts that seas are likely to rise between 33 centimetres and 1.33 meters by 2100 - a wider range than the 28-to-98 centimetres estimate in the last IPCC assessment from 2013.
The IPCC projections, which were reviewed by Reuters, have not been previously reported.
Until now, most glacier research has focused on how warming air melts ice sheets, but warming oceans play a crucial role, said OMG’s principal investigator, Josh Willis.
“It’s not just an ice cube and a hairdryer,” he said, offering an oft-used metaphor for how warmer air melts glaciers. “We’re really just beginning to grapple with how these ice sheets are going to behave in a warming world.”
The OMG project aims to clarify how Greenland itself contributes to rising seas, but also to apply that knowledge to the study of the much larger region of Antarctica, which has far more ice and could ultimately play a much bigger role in sea-level rise. And while most of Greenland’s ice is on land above sea level, large parts of the Western Antarctic ice sheet are below sea level, making them more vulnerable to warming oceans.
Melting ice in Greenland currently adds 0.8 millimetres of water to global ocean levels annually, more than any other region, according to NASA.
That’s enough water to fill 115 million Olympic-sized swimming pools.
Scientists authoring the IPCC study, expected to be the most authoritative sea-level assessment to date, declined to discuss the preliminary findings on the record.
Hans-Otto Poertner, a leader of the IPCC report, said the document will be “subject to review and further revision” before its planned release in September 2019.
The forecast range for sea-level rise, however, is unlikely to get more precise, one scientist working on the draft said on condition of anonymity.
“The range for sea-level rise is getting wider,” the scientist said.
DEEPER, WARMER WATERS
Some of Greenland’s glaciers are disappearing more rapidly than others and understanding why is a key goal of NASA’s mission.
New NASA data on water temperature, depth and salinity have helped explain why the rate of ice loss at northwestern Greenland’s Tracy glacier is almost four times the rate of the nearby Heilprin glacier. That’s because the freshwater flowing from Tracy, which sits on deeper bedrock, is mixing with a layer of warm, salty water off Greenland’s coast, accelerating the melting process, the researchers found.
Many more Greenland glaciers are in similar trouble. Researchers discovered last year that 67 glaciers were connected to the warmer, deeper layer at least 200 meters below sea level – at least twice as many as previously known.
In June, an iceberg four miles wide broke away from Greenland’s Helheim Glacier and cracked into pieces in a process called calving, an event that can be caused by warming oceans. Scientists worry that calving will happen on a disastrous scale in Antarctica, where the much larger Thwaites glacier, for instance, is believed to be a linchpin holding back the West Antarctic ice sheet.
“If the Thwaites was to behave in the way of Helheim, we have no idea” how fast it could erode, said Robert DeConto, who is a professor of geosciences at the University of Massachusetts-Amherst and a member of the IPCC team producing the sea-level rise report expected next year.
The IPCC draft suggests that Antarctica alone could contribute up to half a meter of sea-level rise this century and cites increasing evidence that the process may be irreversible.