If Antarctica and Greenland were just big ice cubes, predicting future sea level rise would be a breeze. The physics of ice melting is simple when you know the temperature. But these frozen continents aren’t nearly as dull as an ice cube, so the task of calculating how fast they will melt in the next century (and beyond) is enormous.
Antarctic ice flows like slow putty, sliding away from the high interior of the continent and toward the sea. Some of the “exhaust glaciers” on the rim break apart before hitting the shoreline, but most push into the water to produce floating ice shelves. The point where the ice lifts off the ground and begins to float is called the “ground line” and is incredibly important to a glacier’s stability.
A new study led by Hannes Konrad of the Alfred Wegener Institute significantly expands the map of Antarctica’s ground lines, tracking areas of change and areas of stability.
To the ground
That ground line migrates seaward or landward as the glacier advances or retreats — processes 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 the ice, increasing the force holding it to the ocean floor. As a result, the movement of Antarctic ice does not respond uniformly to global warming, but instead behaves very differently from place to place.
Finding the location of the ground line is somewhat tricky in itself, as it is hidden under the ice. Sensitive laser measurements from aircraft have been used to find the location where the ice no longer subtly moves up and down with the tides, but this is expensive work that must be repeated if you want to track the ground line migration. That means that this technique has mainly been limited to parts 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 height of the ice sheet’s surface since 2010. For each location where the ground line was mapped in the past, they used changes in ice surface to calculate where the ground line should be now. That’s actually a complex calculation that has to take into account the density of ice, snow and seawater, as well as maps of rock topography beneath the ice.
Animation illustrating how horizontal motion of glacier groundlines is detected using satellite measurements of their elevation change. Credit: Hannes Konrad et al., University of Leeds.
Because of all the things you need to know for this calculation to work, the researchers were only able to apply it to about a third of the Antarctic coastline. However, that triple the previous cover. So for a third of Antarctica’s glaciers, we can now see how quickly their ground lines moved between 2010 and 2016, offering a much better “big picture” perspective.
Refuge!
Just over 10 percent of the length of the ground lines migrated significantly inland as those glaciers retreated. There are, however, strong regional differences. For the relatively stable portion of the ice sheet known as East Antarctica, the number was just three percent, while nearly a quarter of West Antarctica’s vulnerable groundlines receded. In all, Antarctica lost about 1,500 square kilometers of previously grounded ice, all of which is now floating.
Specific glaciers also showed some interesting trends, demonstrating the importance of local conditions. The Pine Island Glacier, for example, has been intensively studied in recent years, as it is responsible for a significant portion of Antarctica’s lost ice. Between 1992 and 2011, the ground retreated an incredible mile every year. But between 2010 and 2016, this new study estimates it slowed to about 40 meters per year. The delay is likely due to changes in ocean currents, which had previously brought in warmer waters.
Pine Island’s neighbor, Thwaites Glacier, is another glaciologists fear could cause much sea-level rise in the future. There, the researchers saw an acceleration of the withdrawal rate from 1996-2011 from about 340 meters per year to 420 meters per year.
In general, much of the area that first received ground line migration estimates was stable, although places like the Getz Ice Shelf in West Antarctica were rapidly retreating. In addition, the researchers discovered a surprisingly generalizable relationship between ground line retreat and decreasing ice thickness at faster-flowing glaciers. Regardless of their differences, these glaciers thin by about a meter for every 110 meters that the ground line retreats – with the missing ice contributing to sea level rise. That relationship could help us estimate grounding movements elsewhere.
Zooming out for a larger context allows researchers to highlight places that need further investigation. In this case, if the local laser mapping is used to locate the ground line in additional locations, the data from the overhead zooming CryoSat-2 satellite can be easily applied to add even more glacier-tracking coverage to the frozen continent.
Natural Geosciences2018. DOI: 10.1038/s41561-018-0082-z (About DOIs).