Until now, the accepted predictions for future sea-level rise were those presented by the UN’s Intergovernmental Panel on Climate Change back in 2013. That report concluded that the likely contribution of Antarctica to sea-level rise by the end of the century would be in the range of 4 to 5 centimeters, barring the potential (and now increasingly likely) collapse of certain marine-based sections of the ice sheet — in which case the likely contribution was expected to be a few tenths of a meter.
In contrast, the new Nature study, which leaned on advanced computer models and an improved understanding of the processes that have affected sea-level rise in the past, concluded that sea-level rise from Antarctic contributions alone could exceed a full meter by the year 2100 in a high greenhouse gas emissions scenario.
Now, according to some scientists, it’s possible that the IPCC’s predictions for Greenland — which contains 6 meters (or 20 feet) of potential sea level rise — are also too conservative. The 2013 report concluded that Greenland’s contribution to sea-level rise through the end of the century would likely be about 12 centimeters (with a range of 7 to 21 centimeters) in a business-as-usual, or high greenhouse gas emissions, scenario. Under more moderate scenarios, where governments take significant steps to control carbon output, the predictions are more modest — anywhere from 4 to 13 centimeters.
Currently, data from NASA indicate that Greenland is losing about 287 billion metric tons of ice per year — a staggering amount already and close to the total of 360 billion that is necessary to raise seas a millimeter annually. But what’s less certain is how much this loss rate will accelerate in the future as the climate continues to change — and how much scientists may need to adjust projections for the area’s future ice loss.
So we checked in with a few experts to find out what the scientific community thinks about whether these projections are likely to still stand.
According to some, the processes most significantly affecting ice loss in Antarctica may not be so pronounced in Greenland. One of the most notable characteristics about Antarctica — particularly the West Antarctic ice sheet, which is the region considered the most unstable at the moment — is that much of the ice sheet lies below sea level.
These “marine glaciers” can be more unstable than those grounded above sea level for a variety of reasons. Interaction with warm ocean water can cause increased ice melt and can also destabilize glaciers from the bottom up, making them more likely to break into pieces or collapse. As glaciers lose ice and retreat into even deeper water, they have a tendency to lose ice even more quickly.
“We know these things can be unstable, but the question is always how rapidly they can actually disintegrate,” said Martin Truffer, a physics professor and glacier expert at the University of Alaska Fairbanks, by email. “The Nature paper makes a good case that it can be quite a bit more rapid than previously assumed. Greenland is not a marine ice sheet (most of its base is above sea level), so it is not subject to the same instability.”
Ian Joughin, a glacier expert at the University of Washington, made a similar observation by email. He reiterated the fact that as marine glaciers retreat into deeper water, they tend to “flow” faster, meaning they lose ice more quickly. An improved understanding of this tendency is responsible for much of the new Nature paper’s updated predictions about the Antarctic contributions to sea-level rise. But because much of the Greenland ice sheet rests above sea level, this factor may not be so important there, so there’s less uncertainty about Greenland — although, as Joughin noted, “there are bound to be some surprises.”
But in some parts of the Greenland ice sheet, at least, the same processes feared to affect Antarctica are already at play, said Eric Rignot, a University of California Irvine glaciologist and senior research scientist at NASA’s Jet Propulsion Laboratory. Although much of the ice sheet rests above sea level, there are some marine glaciers, which serve as outlets flowing from the vast and mostly land-based ice sheet, that are currently losing massive amounts of ice to the ocean and may continue to do so at accelerating rates.
“It would be a natural extension of [the Antarctica study] to examine how the rapid break up of icebergs would impact Greenland’s ice mass balance,” Rignot said by email. “We know that in the case of Jakobshavn Isbrae, this is indeed the dominant mechanism by which the glacier is retreating. So it is completely relevant and the new paper certainly calls into question the projection for Greenland.”
The Jakobshavn glacier is based more than a kilometer below sea level and is one of the parts of Greenland of most concern to scientists, as it’s believed to be the ice sheet’s fastest flowing glacier. Past research has suggested that Jakobshavn may be losing up to 35 billion metric tons of ice each year. Jakobshavn is just one out of more than 200 major outlet glaciers flowing from the Greenland ice sheet, not all of whose vulnerability to ocean-melting is even known.
As for the rest of the ice sheet, other researchers added that even the areas grounded above sea level are subject to different and equally significant influences. They say there are other processes affecting the ice in Greenland than those that are dominant in Antarctica, in large part because Greenland sees warmer air temperatures — and these factors may contribute significantly to accelerated melt on the Greenland ice sheet in the near future.
Marco Tedesco, a researcher with Columbia University’s Lamont-Doherty Earth Observatory, pointed out that a process called “ice darkening” is one significant process at play in Greenland. Tedesco was the lead author on a recent study that focused on this effect in northern Greenland. The idea is that as the climate warms and the ice continues to melt, it begins to lose its ability to reflect the sun’s radiation away from the surface. Radiation that doesn’t get reflected is absorbed instead, causing the ice sheet to warm even further and melt faster — a vicious cycle.
“More melting creates more darkening and accelerates the melting itself — a positive feedback effect,” Tedesco said.
He also noted that in some parts of Greenland, less meltwater is being absorbed back into the ice sheet than in the past. A January study in the journal Nature Climate Change addressed this issue, focusing on a section of the ice sheet known as “firn” — a porous layer of built-up snow that’s capable of trapping meltwater as it flows over the surface and helping it refreeze.
The problem is that as a warming Greenland melts more quickly, this porous space is starting to fill up with refrozen water, meaning there’s less room for new meltwater to trickle in. So the excess meltwater has nowhere to go but run off the surface of the ice into the ocean, where it becomes yet another contribution to rising sea levels.
In an email to The Washington Post, Jason Box of the Geologic Survey of Denmark and Greenland affirmed that many of these processes ongoing in Greenland have not been reflected in past models — meaning that previous predictions are likely too modest.
“Numerous sensitivities are not included in the IPCC model sea level projections from land ice,” he noted. “Some of the sensitivities are feedbacks that mean the warmer climate gets, the ice will be lost increasingly faster.”
The question that remains is how much greater we can expect Greenland’s future contributions to be — and it’s one that remains to be answered. But scientists seem to agree that both Antarctica and Greenland still have some surprises in store, which will become more clear the better we understand the physical processes affecting the ice.
“Both Antarctica and Greenland are these wild cards in the sea-level rise,” Tedesco said. “And the contributions from both is likely underestimated in any of the projections provided by the IPCC.”