The predictions are more dire than other forecasts, though they use different assumptions. While the study did not specify a time frame for the melting and sea-level rise, the authors suggested much of it can play out between now and the year 2100.
“The point is, we need to plan for that ice as if it weren’t on the ice sheet in the near future, within a century or so,” William Colgan, a study co-author who studies the ice sheet from its surface with his colleagues at the Geological Survey of Denmark and Greenland, said in a video interview.
“Every study has bigger numbers than the last. It’s always faster than forecast,” Colgan said.
One reason that new research appears worse than other findings may just be that it is simpler. It tries to calculate how much ice Greenland must lose as it recalibrates to a warmer climate. In contrast, sophisticated computer simulations of how the ice sheet will behave under future scenarios for global emissions have produced less alarming predictions.
A one-foot rise in global sea levels would have severe consequences. If the sea level along the U.S. coasts rose by an average of 10 to 12 inches by 2050, a recent report from the National Oceanic and Atmospheric Administration found, the most destructive floods would take place five times as often, and moderate floods would become 10 times as frequent.
Other countries — low-lying island nations and developing ones, like Bangladesh — are even more vulnerable. These nations, which have done little to fuel the higher temperatures that are now thawing the Greenland ice sheet, lack the billions of dollars it will take to adapt to rising seas.
The paper’s lead author, Geological Survey of Denmark and Greenland scientist Jason Box, collaborated with scientists based at institutions in Belgium, Denmark, Finland, Norway, the Netherlands, Switzerland and the United States to assess the extent of ice loss already locked in by human activity.
Just last year, the U.N. Intergovernmental Panel on Climate Change — which generally forecasts lower figures for total ice loss from Greenland by the end of the century — projected around half a foot of sea-level rise from Greenland by the year 2100 at the high end. That scenario assumed humans would emit a large amount of greenhouse gases for another 80 years.
The current study, in contrast, does not factor in any additional greenhouse gas emissions or specify when the melting would take place, making the comparison with the U.N. report imperfect.
The finding that 3.3 percent of Greenland is, in effect, already lost represents “a minimum, a lower bound,” Box said. It could be much worse than that, the study suggests, especially if the world continues to burn fossil fuels and if 2012, which set a record for Greenland ice loss, becomes more like the norm.
But that aspect of the study offers hope: even if more sea-level rise is locked in than previously believed, cutting emissions fast to limit warming close to 1.5 degrees Celsius (2.7 degrees Fahrenheit) would prevent things from getting much worse.
Greenland is the world’s largest island and is covered with a sheet of ice that, if it melted entirely, could raise sea levels by more than 20 feet. That is not in doubt — nor is the fact that in past warm periods in Earth’s history, the ice sheet has been much smaller than it is today. The question has always been how much ice will thaw as temperatures rise — and how fast.
Melt rates have been increasing in the past two decades, and Greenland is the largest single ice-based contributor to the rate of global sea-level rise, surpassing contributions from both the larger Antarctic ice sheet and from mountain glaciers around the world. Greenland lies in the Arctic, which is warming much faster than the rest of the world.
Higher Arctic temperatures cause large amounts of ice on Greenland’s surface to thaw. While the island’s oceanfront glaciers are also shedding enormous icebergs at an accelerating pace, it is this surface melt — which translates into gushing ice rivers, disappearing lakes and giant waterfalls vanishing into crevasses — that causes the biggest ice losses.
In the past, scientists have tried to determine what Greenland’s ongoing melting means for the global sea level through complex computer simulations. They model the ice itself, the ocean around it, and the future climate based on different trajectories of emissions.
In general, the models have produced modest figures. For instance, according to the latest IPCC assessment, the most “likely” loss from Greenland by 2100 under a very high emissions scenario equates to about 5 inches of sea-level rise. This represents the disappearance of about 1.8 percent of Greenland’s total mass.
Most models and scenarios produce something much lower. In a low-emissions scenario, which the world is trying to achieve right now, the IPCC report suggests Greenland would contribute only a few inches to sea-level rise by the century’s end.
The new research “obtains numbers that are high compared to other studies,” said Sophie Nowicki, an expert on Greenland at the University at Buffalo who contributed to the IPCC report. Nowicki observed, however, that one reason the number is so high is that the study considers only the last 20 years — which have seen strong warming — as the current climate to which the ice sheet is now adjusting. Taking a 40-year period would yield a lower result, Nowicki said.
“This committed number is not well-known and actually quite hard to estimate, because of the long response time scale of the ice sheet,” Nowicki said.
Box, for his part, argues that the models upon which the IPCC report is based are “like a facsimile of reality,” without enough detail to reflect how Greenland is really changing. Those computer models have sparked considerable controversy recently, with one research group charging they do not adequately track Greenland’s current, high levels of ice loss.
In Greenland, the processes triggering ice loss from large glaciers often occur hundreds of meters below the sea surface in narrow fjords, where warm water can flick at the submerged ice in complex motions. In some cases, these processes may simply be playing out at too small a scale for the models to capture.
Meanwhile, while it is clear that hotter air melts the ice sheet from the surface, the consequences of all that water running off the ice sheet — and sometimes, through and under it — raises additional questions. Much of the water vanishes into crevasses, called moulins, and travels through unseen pathways through the ice to the sea. How much this causes the ice itself to slick and lurch forward remains under debate and might be happening at a finer scale than what the models capture.
“Individual moulins, they are not in the models,” Colgan said.
The new research assesses Greenland’s future through a simpler method. It tries to calculate how much ice loss from Greenland is already dictated by physics, given the current Arctic climate.
An ice sheet — like an ice cube, but at a vastly larger scale — is always in the process of melting, or growing, in response to the temperature surrounding it. But with an ice body as large as Greenland — picture the entire state of Alaska covered with ice that is one to two miles thick — adjustment takes a long time. This means that a loss can be almost inevitable, even if it has not actually happened yet.
Still, the ice sheet will leave clues as it shrinks. As it thaws, scientists think the change will manifest itself at a location called the snow line. This is the dividing line between the high altitude, bright white parts of the ice sheet that accumulate snow and mass even during the summer, and the darker, lower elevation parts that melt and contribute water to the sea. This line moves every year, depending on how warm or cool the summer is, tracking how much of Greenland melts in a given period.
The new research contends that in the current climate, the average location of the snow line must move inward and upward, leaving a smaller area in which ice would be able to accumulate. That would yield a smaller ice sheet.
“What they’re saying is that the climate we already have is in the process of burning away the edges of the ice,” said Ted Scambos, an ice sheet expert at the University of Colorado at Boulder who did not work on the paper.
Scambos, however, said it could take much longer than 80 years for 3.3 percent of the ice sheet to melt: the study says “most” of the change can happen by 2100.
“A lot of the change they forecast would happen in this century, but to get [that level of retreat] would require several centuries, more perhaps,” he said.
Future ice losses will be greater than that amount if global warming continues to advance — which it will. If the massive melt year of 2012 became the norm, for instance, that would likely lead to about two-and-a-half feet of committed sea-level rise, the study says.
Pennsylvania State University professor Richard Alley, an ice sheet expert, said the fact that researchers remain uncertain about how the planet’s ice sheets will change and raise global sea levels shows the need for more research.
“The problems are deeply challenging, will not be solved by wishful thinking, and have not yet been solved by business-as-usual,” he said.
But Alley added that it is clear that the more we let the planet warm, the more the seas will rise.
“[The] rise can be a little less than usual projections, or a little more, or a lot more, but not a lot less,” Alley said.