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 and will contribute “significantly” to sea level rise.
Research suggests that if all of West Antarctica were to melt, global sea level could potentially rise by 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 — have applied a sophisticated computer modeling approach to try to determine West Antarctica’s potential melt rate.
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 7 inches by the year 2100, and 15 inches 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, 7 inches of sea level from West Antarctica by 2100 is not too dramatic. 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 3 feet by the year 2100.
“It turns out that the numbers we get are consistent with the IPCC,” said 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 “process based models,” like the one used in the latest research, tend to produce lower estimates of future sea level rise than other approaches such as “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 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.
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. As Hansen wrote in an email: “There is a huge difference between atmosphere-ocean [climate] models and ice sheet 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,” said Dan Martin, a co-author of the paper at Lawrence Berkeley National Laboratory. 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 seawater as it warms.
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.