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 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 skeptics.
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 OMGs principal investigator, Josh Willis.
“Its not just an ice cube and a hair dryer,” he said, offering an oft-used metaphor for how warmer air melts glaciers. “Were 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 Greenlands 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. Thats 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.
HOLES IN THE DATA
Better projections would be invaluable to governments worldwide. Britains Environment Agency, for instance, foresees upgrades to a barrier on the Thames River to protect London against 90 centimetres of sea level rise by 2100, but could modify the plans to account for a catastrophic 2.7 meters.
One 2017 study published by US scientists in a journal of the American Geophysical Union estimated a rise of 50 centimetres by 2100 would submerge land that is now home to about 90 million people, mainly in China, Vietnam and Bangladesh. A rate of 1.5 meters would swamp the homes of more than 150 million people worldwide, the study estimated.
A 2016 study published in a journal of the US National Academy of Sciences predicted that an event comparable to Superstorm Sandy, which flooded large swaths of the New York region in 2012, would be up to 17 times more likely by 2100 if seas rise by 1 meter.
Higher seas are already creating more dangerous storm surges, the IPCC says, exacerbating flooding or coastal erosion from the US Gulf Coast to the Maldives to China. Some low-lying island nations, such as Fiji and Vanuatu, have moved some coastal communities to higher ground.
The IPCC draft says that the rate of sea level rise in 2100 will depend heavily on the effectiveness of governments in reducing fossil fuel pollution, which the UN panel says has been the dominant cause of global warming since the mid-20th century.
But holes in scientists understanding of ice sheet dynamics also make it difficult to predict how glacier melt will contribute to rising seas, said David Holland, a New York University oceanographer who has studied Greenlands glaciers for 12 years.
“The modeling of it is like Swiss cheese,” Holland said. “Its getting into the more challenging projections and unknown physics.”
DEEPER, WARMER WATERS
Some of Greenlands glaciers are disappearing more rapidly than others, and understanding why is a key goal of NASAs mission.
New NASA data on water temperature, depth and salinity has helped explain why the rate of ice loss at northwestern Greenlands Tracy glacier is almost four times the rate of the nearby Heilprin glacier. Thats because the fresh water flowing from Tracy, which sits on deeper bedrock, is mixing with a layer of warm, salty water off Greenlands 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 Greenlands 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.
SOUNDING THE ALARM
The importance of glacier change for global sea-level rise this century was not widely grasped in the scientific community until recently. In 1995, the IPCCs second assessment said “little change in the extent of the Greenland and Antarctic ice sheets is expected over the next 50–100 years.”
By 2007, it said new data showed “losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003” but did not factor in future ice-sheet melt because of a lack of published research.
“In the beginning people thought, lets just assume that those things are stable,” said climate scientist Gavin Schmidt, who directs the NASA Goddard Institute for Space Studies. Even today, he said, “there are very few climate models that have a credible simulation of any kind of dynamic ice sheet behaviour at all.”
The scientific uncertainty is a political problem, said Tad Pfeffer, a professor at the University of Colorado and former IPCC author. Skeptics including US President Donald Trump attack the science of climate change as an unproven and politically motivated attack on fossil fuel industries, and Trump last year said he would pull the United States out of the 2015 Paris climate agreement that includes almost 200 nations.
“The public, especially if they are getting nudged by anti-science views, take uncertainty as saying, These scientists dont know what theyre doing,” Pfeffer said.