New Paper: Antarctica Melt Could Be Higher, Faster
March 30, 2016
Video from 2 years ago, as so often happens, further reinforced by the newest studies.
A new study from climate scientists Robert DeConto at the University of Massachusetts Amherst and David Pollard at Pennsylvania State University suggests that the most recent estimates by the Intergovernmental Panel on Climate Change for future sea-level rise over the next 100 years could be too low by almost a factor of two. Details appear in the current issue of Nature.
DeConto says, “This could spell disaster for many low-lying cities. For example, Boston could see more than 1.5 meters [about 5 feet] of sea-level rise in the next 100 years. But the good news is that an aggressive reduction in emissions will limit the risk of major Antarctic ice sheet retreat.”With mechanisms that were previously known but never incorporated in a model like this before, added to their ice-sheet model to consider the effects of surface melt water on the break-up of ice shelves and the collapse of vertical ice cliffs, the authors find that Antarctica has the potential to contribute greater than 1 meter (39 inches) of sea-level rise by the year 2100, and greater than 15 meters (49 feet) by 2500 if atmospheric emissions continue unabated. In this worst case scenario, atmospheric warming (rather than ocean warming) will soon become the dominant driver of ice loss.
The revised estimate for sea-level rise comes from including new processes in the 3-dimensional ice sheet model, and testing them against past episodes of high sea-levels and ice retreat.
The researchers find that “ocean-driven melt is an important driver of Antarctic ice shelf retreat where warm water is in contact with shelves, but in high greenhouse-gas emissions scenarios, atmospheric warming soon overtakes the ocean as the dominant driver of Antarctic ice loss.” Further, they find that if substantial amounts of ice are lost, the long thermal memory of the ocean that will inhibit the ice sheet’s recovery for thousands of years after greenhouse-gas emissions are curtailed.
DeConto and Pollard’s study was motivated by reconstructions of sea level rise during past warm periods including the previous inter-glacial (around 125,000 years ago) and earlier warm intervals like the Pliocene (around 3 million years ago). These high sea levels, ranging from a few meters to 20 meters above today, imply that the Antarctic Ice Sheet is highly sensitive to climate warming.
“So, at a time in the past when global average temperatures were only slightly warmer than today,” says DeConto (above), “sea levels were much higher. Melting of the smaller Greenland Ice Sheet can only explain a fraction of this sea-level rise, most which must have been caused by retreat on Antarctica.”
The situation would grow far worse beyond 2100, the researchers found, with the rise of the sea exceeding a pace of a foot per decade by the middle of the 22nd century. Scientists had documented such rates of increase in the geologic past, when far larger ice sheets were collapsing, but most of them had long assumed it would be impossible to reach rates so extreme with the smaller ice sheets of today.
“We are not saying this is definitely going to happen,” said David Pollard, a researcher at Pennsylvania State University and a co-author of the new paper. “But I think we are pointing out that there’s a danger, and it should receive a lot more attention.”
The research, published by the journal Nature, is based on improvements in a computerized model of Antarctica and its complex landscape of rocks and glaciers, meant to capture factors newly recognized as imperiling the stability of the ice.
The new version of the model allowed the scientists, for the first time, to reproduce high sea levels of the past, such as a climatic period about 125,000 years ago when the seas rose to levels 20 to 30 feet higher than today.
That gave them greater confidence in the model’s ability to project the future sea level, though they acknowledged that they do not yet have an answer that could be called definitive. It is still possible that unrecognized factors could emerge that might help to stabilize the ice sheet in a warming climate.
“You could think of all sorts of ways that we might duck this one,” said Richard B. Alley, a leading expert on glacial ice at Pennsylvania State University. “I’m hopeful that will happen. But given what we know, I don’t think we can tell people that we’re confident of that.”
Dr. Alley was not an author of the new paper, though it is based in part on his ideas about the stability of glacial ice. Several other scientists not involved in the paper described it as significant, with some of them characterizing it as a milestone in the analysis of huge ice sheets and the risks they pose in a warming world.
But those same scientists emphasized that it was a single paper, and unlikely to be the last word on the fate of West Antarctica. The effort to include the newly recognized factors imperiling the ice is still crude, with years of work likely needed to improve the models.
Peter U. Clark of Oregon State University helped lead the last effort by a United Nations panel to assess the risks of sea level rise; he was not involved in the new paper. He emphasized that the research, like much previous work, highlighted the urgency of bringing emissions of carbon dioxide and other greenhouse gases under control.
Dr. Clark described the new work as “a really important paper that adds to the growing recognition that in the absence of rapid and strong mitigation of carbon emissions, we are in store for a large sea level rise at rates that may be even faster than has been considered.”