09/02/2016

Scientists Just Uncovered Yet Another Troubling Fact About Antarctica’s Ice

Washington Post - Chris Mooney

Highly crevassed ice front of Ferrigno Ice Stream in the Bellingshausen Sea in 2011. (Matthias Braun, University of Erlangen-Nuremberg, Germany)


In the past two years, major scientific developments have suggested that all of our eyes should be on a place that only a tiny fraction of us will ever visit — Antarctica, the frozen South Pole continent that's larger than the continental United States and contains the majority of the planet's land-based ice.
In 2014, scientists revealed that key parts of remote West Antarctica may have been destabilized by warm ocean waters reaching the bases of vast submarine glaciers and melting them from below. (West Antarctica, as a whole, contains nearly 11 feet of potential sea level rise.) And last year, research hinted that a similar vulnerability may exist for the truly gigantic Totten Glacier of East Antarctica.
Now, in a study in Nature Climate Change, researchers provide a new way of looking at how vulnerable Antarctica's ice is — and the approach, unfortunately, largely reinforces the conclusions of the prior studies.
To understand the new research, you first have to understand a truly astonishing feature of Antarctica that is virtually without rival anywhere else — it is ringed with gigantic ice shelves. These are sometimes country-sized sheets of ice extending out over the surface of the ocean and floating on top of it.
Antarctic ice shelves play a critical role in ensuring that Antarctica's inland ice, which flows toward the sea through multiple vast glaciers, moves relatively slowly. They are sometimes likened to the flying buttresses of Gothic cathedrals, because they in effect act as a brace, holding back the flow of glaciers — a role they exert because they tend to be attached to islands or seafloor rises.
"These ice shelves, they are hundreds of meters thick, it's different than sea ice," says Johannes Fürst of the French National Centre for Scientific Research, lead author of the new study. "The ice shelves are really, really huge, and that's why they can support this buttressing basically."
But ice shelves sometimes break off or "calve" large pieces of ice that can be as big as cities. Indeed, they can also collapse entirely, as happened with the Larsen A ice shelf in 1995 and the Larsen B ice shelf in 2002. Meanwhile, the huge Larsen C ice shelf has a large and advancing crack across it.
After Larsen A and B collapsed, scientists documented a rapid acceleration of the glaciers behind them, pouring much more ice into the sea. Ice shelves do not themselves raise sea level if they collapse, since they are already afloat. But the land based glaciers behind them do raise seas if they flow into the water.
Therefore, the stability of Antarctic ice shelves has key implications for global sea level. Enter the new study: Fürst and a group of colleagues from France and Germany examined the totality of ice shelves around Antarctica to see how much ice they could lose before the glaciers that they hold back become destabilized. Ice shelves, they say in the paper, are Antarctica's "safety band."
And they concluded that in the Amundsen Sea region of West Antarctica in particular, the situation is "alarming." This is a region where ice shelves can lose very little ice if they are still to play their buttressing role of holding back larger glaciers.
Calving is shown in 2011 front of Fleming Glacier, an outlet glacier which fed the former Wordie Ice Shelf, which broke up toward the end of the 1990s. (Matthias Braun, University of Erlangen-Nuremberg, Germany)


Fürst and his colleagues use satellite imagery and model simulations to calculate what percentage of each Antarctic ice shelf is "passive," meaning that it isn't playing any major buttressing role. Antarctica-wide, they determined, about 13.4 percent of the total area of ice shelves is comprised of passive ice, and therefore can be lost without worrying about the consequences for the glaciers held back behind them.
In some key regions that "passive" percentage was considerably larger, while in others, it was much smaller. The latter are the danger regions for Antarctica — and for coasts across the planet.
Visually, ice shelves that don't have much passive ice tend to look quite different from those that do, the study notes. They tend to have a concave shape, sinking inward toward land rather than being convex and pushing outward into the ocean. Before they disintegrated, the study notes, both the Larsen A and Larsen B ice shelves took on a concave shape.
So what parts of Antarctica look the most vulnerable? In particular, key regions of West Antarctica were found to have very little margin of safety. Getz ice shelf was comprised of just 4.6 percent passive ice; Cosgrove ice shelf had just 2.7 percent; and Dotson ice shelf had just 1.5 percent. Similarly, along the Bellingshausen Sea between West Antarctica and the Antarctic Peninsula, there were many regions of high vulnerability (although this area contains considerably less total vulnerable ice than West Antarctica does).
"The ice shelves in the Amundsen and Bellingshausen seas have limited or almost no 'passive' portion, which implies that further retreat of current ice-shelf fronts will yield important dynamic consequences," write the authors.
"The regions which don't show so much passive shelf are the ones that already dynamically react, so we there see already a big dynamic signal. That's where we find mass loss as well," says Fürst.
Finally, far over in East Antarctica, Totten Glacier was again identified as a vulnerability zone — with just 4.2 percent of its ice shelf able to be lost without consequences. Destabilization of Totten could ultimately lead to about as much sea level rise as the entirety of West Antarctica.
"It's a confirmation of what some of the vulnerable sectors are, and it's an eye opener on some of the other places that we haven't thought through completely that need a little bit more attention," says Eric Rignot, an expert on Antarctica's ice at the University of California-Irvine, who was not involved in the research. "On Totten, I was a little bit surprised to see a 4.2. Apparently it's a very sensitive one."
"In those places where ice-shelf shrinkage and ice-stream acceleration have been observed, there is little or no passive ice shelf, confirming the inference that these places are sensitive to additional warming and ice-shelf loss, and likely have been subject to ice-shelf loss in the past," added Richard Alley, a glaciologist at Penn State University who was not involved in the study.
Granted, the study also found some more stable zones — including Larsen C. While it looks like this ice shelf may soon lose a large portion in a major calving event, the research suggested that the loss would not speed up glaciers behind it, because although the event would be very dramatic, the ice lost would mostly be passive.
The vulnerability of ice shelves is a separate issue from the question of what might ultimately cause them to lose ice in an amount that pushes beyond a critical threshold, Rignot notes. That could be either warmer air temperatures, which have been blamed for the collapse of Larsen A and Larsen B, or warmer oceans melting ice from below, which seems to be the case in West Antarctica.
But either way, an ice shelf with more passive ice will be more able to weather a warming trend, while one with less passive ice will be more easily pushed over the brink.
What we still don't understand — and the next challenge that arises in the wake of this research — is how to predict when an ice shelf is going to calve a large piece, or collapse, says Rignot.
"It's like studying plate tectonics and earthquakes," he says. "We have good control on the rate of motion of the plates, and the calving is like the next earthquake. It's a bit of different physics, it's more chaotic, it's difficult to understand."
For lead study author Fürst, in the end the research allows scientists to know which regions of Antarctica really need monitoring. "Now we kind of know where to look," he says. It remains to be determined if scientists, studying these key areas, will see what many fear — continued loss of ice and acceleration of the glaciers behind ice shelves.

Links
at

No comments:

Post a Comment