This story has been updated.
The global warming problem seemed to take on a new level of urgency last year, when a NASA study suggested that a key region the massive West Antarctic ice sheet may have been destabilized. “We conclude that this sector of West Antarctica is undergoing a marine ice sheet instability that will significantly contribute to sea level rise in decades to centuries to come,” the NASA study concluded.
Research suggests that if all of West Antarctica were to melt, global sea level could potentially rise by 3.3 meters, or about 11 feet. However, the NASA study did not directly address how quickly this could occur — a question with implications from Miami to Bangkok.
The core reason for worry about West Antarctica is clear — its oceanfront ice shelves, buttressing regions for the larger ice sheet, rest on what is termed a “retrograde” bed, not only below sea level but sloping downhill as you move further inland. Hence the fear that once warm ocean water starts melting them from below, the process just continues and continues.
But it also takes time to move gigantic volumes of ice. And now, in a new study recently published in The Cryosphere, a large team of researchers from a bevy of universities and research institutes across 6 countries — including the University of Bristol in the UK and Lawrence Berkeley National Laboratory in the U.S. — have applied a sophisticated computer modeling approach to try to determine West Antarctica’s potential melt rate.
The researchers began with different scenarios for global emissions, and for corresponding changes to the atmosphere and oceans around Antarctica. Then they used those scenarios to fire up a high resolution ice sheet model. The model, depending on its inputs, then computed a range of predictions for what would happen to West Antarctica going forward.
The result was neither good news, nor utterly catastrophic.
The study found that at the most extreme, the entire ice sheet could move fast enough to raise sea level by 200 millimeters (20 centimeters) by the year 2100, and 47.5 centimeters by 2200. That would be an acceleration in comparison with what it’s doing today. For instance, one recent study found that West Antarctica is currently losing 134 gigatons of ice per year (a gigaton is a billion metric tons). That may sound like a very large amount, but it’s only enough to raise sea level by about a third of a millimeter per year (it takes 360 gigatons to produce 1 millimeter of sea level rise).
Nonetheless, 20 centimeters of sea level from West Antarctica by 2100 is not too dramatic of a scenario. In fact, it’s in line with the U.N.’s Intergovernmental Panel on Climate Change’s 2013 consensus projections, which had sea level rising by, at most, about a meter by the year 2100.
“It turns out that the numbers we get are consistent with the IPCC,” says study author Tony Payne, a glaciologist with the University of Bristol in the UK.
The new research arrives, however, at an interesting time – one when a number of scientists are doubting the IPCC’s projections, as well as the ability of current modeling efforts to fully capture the dynamics of ice sheets. For instance, a recent modeling study that added two additional mechanisms into the mix, dubbed hydrofracturing and ice cliff failure, found “a very rapid collapse of West Antarctic ice, on the order of decades” in a scenario of increased warming. The new study does not include those mechanisms.
Other research has suggested that so-called “process based models,” like the one used in the latest research, tend to produce lower estimates of future sea level rise than other approaches like “expert elicitation,” in which you simply get a lot of experts on the subject and ask them what they think is going to happen.
Indeed, two noted ice sheet experts — Eric Rignot of NASA and the University of California, Irvine, and Isabella Velicogna of the University of California, Irvine — recently joined former NASA scientist James Hansen as co-authors of a controversial paper, currently undergoing public peer review, which asserts that “several meters” of sea level rise due to ice sheet loss could occur within 50, 100, or 200 years, depending on how fast the rate of ice loss is able to double.
As Hansen put it to me by email:
There is a huge difference between atmosphere-ocean (climate) models and ice sheet models. The atmosphere-ocean models explicitly include the fundamental processes that determine the dynamics of atmosphere and ocean flow, and the models do a demonstrably good job of simulating the real world. With ice sheets, the modelers are still fooling around with adding key processes.
In other words, some scientists, like Hansen, think there’s pretty high uncertainty in the ice sheet modeling world, and that the real world may be moving considerably faster than the models.
Granted, just because it has a more conservative result than these studies doesn’t mean the new research suggests there’s nothing to worry about. “I wouldn’t say we’re not alarmed,” says Dan Martin, a coauthor of the paper at Lawrence Berkeley National Laboratory in the U.S. He adds that “the ice sheets are definitely out of balance, but it’s a question of how fast they get there, and how fast you can deliver ice to the ocean.”
Martin also points out that West Antarctica is not the only source of sea level rise — we’ll also be getting plenty from Greenland, glaciers around the world, and the expansion of sea water as it warms.
Moreover, even 20 centimeters from West Antarctica will mean that that “a particular size of flood will happen a lot more often” in the future, says co-author Payne. And the new study finds — as all studies do — that it’s not like melting ends at 2100. It goes on, and on, and on after that.
Still, the uncertainty here, and the quite different views among experts about the rate of change that we might see, is not very consoling for those worrying about coastlines. It is very clear that we need to know a great deal more — and fast — about what is happening with West Antarctica.
No wonder the U.S. National Academy of Sciences, in a new report advising the National Science Foundation on how to set Antarctica research priorities, said that determining “how fast and by how much” seas rise should be the number one priority. “Evidence is building that portions of the Antarctic ice sheet are becoming unstable and beginning to collapse, and that the pace of change has accelerated in recent years,” noted the National Academies report. “There is an urgent need to understand this process in order to better assess how future sea level rise from ice sheets might proceed.”
In 1978, in a now famous paper in climate science (because of its prescience), the late Ohio State University glaciologist John Mercer postulated that “deglaciation of West Antarctica would probably be the first disastrous result of continued fossil fuel consumption.” Nearly 40 years later, Mercer’s concerns seem even more pressing – but scientists still have a long way to go in forming a consensus about how quickly they could be realized.