Three major lines of evidence point to the glaciers' eventual demise: the changes in their flow speeds, how much of each glacier floats on seawater, and the slope and depth of the terrain they are flowing over. In a paper in April, Rignot's research group discussed the steadily increasing flow speeds of these glaciers over the past 40 years. This new study examines the other two lines of evidence.
As glaciers flow out from land to the ocean, large expanses of ice behind their leading edges float on the seawater. The point on a glacier where it first loses contact with land is called the grounding line. Nearly all glacier melt occurs on the underside of the glacier beyond the grounding line, on the section floating on seawater. The Antarctic glaciers studied have thinned so much they are now floating above places where they used to sit solidly on land, which means their grounding lines are retreating inland.Above: Move bar to compare the grounding line of the Smith Glacier from 1996 (left) to the location in 2011 (right) which has retreated inland 35 km during this time. The green line indicates the location of the 1996 grounding line. Download HTML to embed this in your web page.
The bedrock topography is another key to the fate of the ice in this basin. All the glacier beds slope deeper below sea level as they extend farther inland. As the glaciers retreat, they cannot escape the reach of the ocean, and the warm water will keep melting them even more rapidly.
Below are two edited versions of narrated stories released by JPL to explain this research. In addition are the two versions of the unedited animations provided to JPL to support the release. The unedited animations show the region of study by the JPL researchers, identifying by name the glaciers that terminate in the Amundsen Sea. One of the animations includes data showing the velocity of the glaciers in the region, flow vectors showing the movement of the glaciers colored by their velocity and a difference image showing the change in velocity between 1996 and 2008. The second animation does not include these datasets.
Both versions of the animation draw close to the Smith Glacier and show how the grounding line of this glacier has moved inland 35 kilometers between 1996 and 2011. As the surface of the ice sheet is peeled away, showing the height and depth of the bedrock topography. Regions below sea level are shown in shades of brown while areas above sea level are shown in green. Sea level is shown in yellow.