The Antarctic ice sheet and its response to climate change has received increased attention over the last couple of years. Now it becomes clear, that choices that the world makes this century could determine the fate of this massive layer of ice covering Antarctica. A study published in Nature finds that continued growth in greenhouse-gas emissions over the next several decades could trigger an unstoppable collapse of Antarctica’s ice — raising sea levels by more than a meter by 2100 and more than 15 meters by 2500.

The Antarctic ice sheet covers more than 98 percent of Antarctica and is the largest aggregation of freshwater in the world. The glaciers protrude far into the surrounding oceans in form of massive ice shelves. If all of the Antarctic ice would melt, sea levels would globally rise more than 60 meters. Picture: Michael WengerThe Antarctic ice sheet covers more than 98 percent of Antarctica and is the largest aggregation of freshwater in the world. The glaciers protrude far into the surrounding oceans in form of massive ice shelves. If all of the Antarctic ice would melt, sea levels would globally rise more than 60 meters. Picture: Michael Wenger

The findings add to a growing body of research that suggests that Antarctic ice is less stable than once thought. In its 2013 report, the Intergovernmental Panel on Climate Change estimated that Antarctic melting would contribute just a few centimeters to sea-level rise by 2100. But as scientists develop a better understanding of how the ocean and atmosphere affect the ice sheet, their projections of the continent’s future are growing direr. “That is literally remapping how the planet looks from space,” says study co-author Rob DeConto, a geoscientist at the University of Massachusetts Amherst. The good news, he says, is that it projects little or no sea-level rise from Antarctic melt if greenhouse-gas emissions are reduced quickly enough to limit the average global temperature rise to about 2 °C.

The authors modelled the behaviour of the Antarctic ice sheet in 2500 using 3 case scenarios according to the IPCC. The top graph shows the sea level rice under the 3 scenarios plus the result of the authors. In the best case, sea levels will remain largely below 1 meter (black line). In the worst case, sea levels will rise up to almost 15 meters according to the latest model by DeConto and Pollard. The lower maps show Antarctica under the different scenarios. Different shades of blue represent different ice heights, brown are ice-free areas. Graph: taken from DeConto & Pollard, Nature 531, 2016 and modified.
The authors modelled the behaviour of the Antarctic ice sheet in 2500 using 3 case scenarios according to the IPCC. The top graph shows the sea level rice under the 3 scenarios plus the result of the authors. In the best case, sea levels will remain largely below 1 meter (black line). In the worst case, sea levels will rise up to almost 15 meters according to the latest model by DeConto and Pollard. The lower maps show Antarctica under the different scenarios. Different shades of blue represent different ice heights, brown are ice-free areas. Graph: taken from DeConto & Pollard, Nature 531, 2016 and modified.

DeConto and co-author David Pollard, a palaeoclimatologist at Pennsylvania State University in University Park, developed a climate model that accounts for ice loss caused by warming ocean currents — which can eat at the underside of the ice sheet — and for rising atmospheric temperatures that melt it from above. Ponds of meltwater that form on the ice surface often drain through cracks; this can set off a chain reaction that breaks up ice shelves and causes newly exposed ice cliffs to collapse under their own weight. They found that by including all of these processes, they could better simulate key geological periods that have long puzzled scientists. Before the last ice age began 130,000–115,000 years ago, for instance, sea levels were 6–9 meters higher than today — yet atmospheric carbon-dioxide levels were about 30% lower. And 3 million years ago, when CO2 levels roughly equaled today’s, the oceans may have been 10–30 meters higher.
Dr. Nick Golledge, an ice-sheet modeller at Victoria University, Wellington (NZ) whose study had been published in Nature just recently, says: “I think their process are still a bit speculative, but it’s good work.” Several works published over the last several months forecast a sea-level rise of 30 – 39 centimeters by 2100 and approximately 3 meters by 2300. Dr. Golledge states, that “We don’t really have a great handle on what the climate was like in the past.” David Holland, a physical-climate scientist at NY University also says: “On the observational side, I see the things they are talking about. There’s a lot of observation and modelling to go, but they are adjusting people’s thinking in a very scientific way.”

Tabular icebergs are broken off pieces of ice shelves in Antarctica. According to many climate scientists, more of these bergs will drift in the currents around the continent because small and medium-sized ice shelves will disintegrate faster than larger shelves. Picture: Michael Wenger
Tabular icebergs are broken off pieces of ice shelves in Antarctica. According to many climate scientists, more of these bergs will drift in the currents around the continent because small and medium-sized ice shelves will disintegrate faster than larger shelves. Picture: Michael Wenger

For DeConto, the new model results underscore the choice that humanity is facing. If he and Pollard have the physics correct, this process of ice-shelf disintegration, followed by ice-cliff collapse, will be nearly impossible to stop once it gets under way. “Once the ocean warms up, that ice will not be able to recover until the oceans cool back down,” he says — a process that could take thousands of years. “It’s a really long-term commitment.”

Source: Jeff Tollefson, Nature, Issue 531