If Antarctica and Greenland were just big ol’ ice cubes, projecting future sea-level rise would be a snap. The physics of ice melting are simple when you know the temperature. But these frozen continents are not nearly as boring as a cube of ice, so the task of working out how fast they will melt over the next century (and beyond) is herculean in scope.

Antarctic ice flows like slow putty, sliding away from the high interior of the continent and toward the sea. Some “outlet glaciers” at the edge fall apart before they hit the shoreline, but most push out into the water to produce floating ice shelves. The point where the ice lifts off the ground and begins to float is called the “grounding line,” and it’s incredibly important to a glacier’s stability.

A new study led by Hannes Konrad of the Alfred Wegener Institute greatly expands the map of Antarctica’s grounding lines, tracking areas of change and areas of stability.

On the ground

That grounding line migrates seaward or landward as the glacier advances or retreats—processes that are controlled by water temperatures and currents, air temperatures, snowfall, and the topography of the bedrock beneath the ice. Some of these effects are indirect. For example, snowfall can pile weight on top of the ice, increasing the force that holds it to the ocean bottom. As a result, the movement of Antarctic ice isn’t responding to global warming uniformly but instead behaves quite differently from place to place.

Finding the location of the grounding line is, itself, somewhat tricky since it is hidden beneath the ice. Sensitive laser measurements from airplanes have been used to find the location where the ice stops subtly moving up and down with the tides, but this is expensive work that must be repeated if you want to track migration of the grounding line. That means this technique has mainly been limited to sections of the ice sheet that we know are changing rapidly.

To expand coverage, Konrad and his colleagues used the European Space Agency’s CryoSat-2 satellite, which has been measuring the elevation of the surface of the ice sheet since 2010. For every location where the grounding line was mapped in the past, they used changes in ice-surface elevation to calculate where the grounding line must be now. That’s actually a complex calculation that has to account for the density of ice, snow, and seawater, as well as maps of bedrock topography beneath the ice.

Because of all the things you need to know to make this calculation work, the researchers could only apply it to about a third of Antarctica’s coastline. However, that triples the previous coverage. So for a third of Antarctica’s glaciers, we can now see how fast their grounding lines moved between 2010 and 2016, granting a much better “big picture” perspective.

Retreat!

A little over 10 percent of the length of the grounding lines migrated significantly landward as those glaciers retreat. There are strong regional differences, though. For the comparatively stable part of the ice sheet known as East Antarctica, the number was just three percent, while almost a quarter of vulnerable West Antarctica’s grounding lines were in retreat. In total, Antarctica lost around 1,500 square kilometers of previously grounded ice, all of which is now floating.

Specific glaciers showed some interesting trends as well, highlighting just how important local conditions are. The Pine Island Glacier, for example, has been the subject of intensive study in recent years because it has been responsible for a considerable share of Antarctica’s lost ice. Between 1992 and 2011, its grounding retreated at an incredible kilometer each year. But between 2010 and 2016, this new study estimates that it slowed to about 40 meters per year. The slowdown is probably due to changes in ocean currents, which had previously delivered warmer water.

Pine Island’s neighbor, Thwaites Glacier, is another one that glaciologists worry could produce a lot of sea-level rise in the future. There, the researchers saw an acceleration from the 1996-2011 retreat rate of about 340 meters per year up to 420 meters per year.

Overall, much of the area receiving grounding-line migration estimates for the first time were stable, although places like the Getz Ice Shelf in West Antarctica were retreating rapidly. Additionally, the researchers discovered a surprisingly generalizable relationship between grounding-line retreat and shrinking ice thickness at faster-flowing glaciers. Regardless of their differences, these glaciers thin by about one meter for every 110 meters that the grounding line retreats—with the missing ice adding to sea-level rise. That relationship could help us estimate grounding-line movements elsewhere.

By zooming out for big-picture context, researchers can highlight places that require closer examination. In this case, if the local laser-mapping is used to find the grounding line in additional locations, the data from the CryoSat-2 satellite whirring overhead can readily be applied to add even more glacier-tracking coverage to the frozen continent.

Nature Geoscience, 2018. DOI: 10.1038/s41561-018-0082-z  (About DOIs).

https://arstechnica.com/?p=1288785