Antarctica’s exit glaciers: The drunk drivers of climate change

Richard Alley’s studies of the role of ice sheets in climate change have earned him various awards, a PBS special, and have made him a repeat performer at the meetings of the American Association for the Advancement of Science. When I first saw him speak a few years ago, he argued that the ice sheets of Antarctica and Greenland play a huge role in controlling sea levels. Mountain glaciers don’t hold nearly as much water, while the thermal expansion of water in the oceans is a slower and more predictable process.

The ice sheets, in contrast, have been a big unknown. At the time, we didn’t yet fully understand how much of them might melt, or how quickly they might dump water into the oceans.

Alley was back at this year’s meeting, and his news was mixed. As he predicted in his earlier appearance, a few years of intensive study have helped narrow down some of the uncertainties. And, while the news is (relatively) good in Greenland, some of the results from Antarctica are decidedly worrying.

Going with the flow

Alley described an ice sheet as a big pile of ice that wants to spread slowly, a spreading that would put more of its mass into the oceans and raise sea levels. Anything that speeds up that spreading is bad news for us, since it would give us less time to get carbon emissions under control and create a more sudden rise in sea levels that would be harder to adapt to.

Two of the big unknowns that Alley’s been studying can help accelerate the spread of the ice cap into the oceans. In Greenland, the summer melt creates large lakes of meltwater on the ice cap’s surface, which drain suddenly, pouring huge volumes of water to the base of the ice. This can lubricate its flow over the underlying rocks, and accelerate the ice cap’s spread. In Antarctica, this sort of melting isn’t as much of an issue; instead, exit glaciers carry ice from the continent’s interior to the ocean. Because these flow slowly through narrow outlets, he compared them to flying buttresses, the architectural features that hold up the pile of rocks that comprise Medieval cathedrals.

In the time since his last talk, Alley felt that we’d come to terms with Greenland. The water from the summer melt does reach the bottom of the ice sheet, and it does accelerate its movement to some degree. But the terrain underneath the ice is very bumpy, which limits how easily the ice can flow even when water is present. As such, he thinks that Greenland’s contribution to future sea level rises will be relatively slow and steady—it will melt, but it won’t do anything sudden and unexpected.

The news was worse from Antarctica. Here, work by Alley and others has focused on the dynamics of exit glaciers that hold back the flow of large glaciers near the West Antarctic Peninsula. The key thing that regulates the flow of these glaciers is what’s called a “grounding line,” or the place where the glacier’s end is in contact both with the ocean floor and with the ocean itself (this is in contrast to the floating ice that sometimes spreads past this site).

While a glacier is on the grounding line, Alley said there are a lot of feedbacks that tend to keep it there. The sediment it carries gets dumped there, raising the ocean bottom. The Earth itself, with less ice above it, rebounds from the weight that was present during the last ice age, also keeping the contact between the ice and ocean floor intact. These and a few other feedbacks help keep the grounding line stable even as rising temperatures would otherwise tend to force the glacier to break up and retreat.

The problem is that, when the feedbacks are finally overcome, the grounding line fails catastrophically, and the ice tends to retreat rapidly to the next potential grounding line. This behavior shows up in models of the glacier’s behavior, and it’s apparent in imaging of the ground under the ice, where there’s little sign of retreat from past melts outside a handful of grounding lines.

What does this mean for the particular glacier Alley chose to focus on? If its current grounding line fails, there’s another a bit behind it that it will likely retreat to. If that one fails, however, there’s enough ice between it and the one behind that to raise sea levels by two meters. And, from a geological perspective, that retreat could occur in a flash—fast enough to obviate any long term plans for adaptation.

Science has a head-on collision with policy

That’s worrying on its own. But it’s a special problem, Alley argued, because of the way policy makers have approached sea level rise. To begin with, they’ve tended to assume that any rise will be gradual and slow. Alley blamed this in part on the reports from the Intergovernmental Panel on Climate Change—its most recent report provided a conservative estimate, and failed to convey the large uncertainties in what we know about ice sheet behavior.

The other problem, in his view, is that economists also assumed that people would be rational actors, and either take preventative measures or stop investing any money in property that would inevitably wind up under water. Neither of those have happened. Alley said that he and many others had been teaching introductory geology classes that discuss how conditions in New Orleans made something like Katrina inevitable, but that didn’t actually result in any investment in infrastructure to handle the problems. Similarly, some of the problems New York City faced during Sandy had been accurately predicted a number of years in advance.