May 28, 2011 — andyextance

While Greenland’s glaciers have shrunk faster on average over the past decade, the full detail of the threat the ice lost creates remains unclear. But now, thanks to satellite radar and photographic mapping, US and Netherlands researchers have been able to provide one of the most detailed breakdowns of ice loss yet. “Of Greenland’s three largest glaciers draining ice from the interior into the ocean, the two largest lost a combined 450 billion tonnes of ice mass over the decade, equivalent to the water volume of Lake Erie,” said Ohio State University‘s Ian Howat. “A third glacier, however, actually gained a small amount of mass despite big retreat. Differences in how these glaciers have changed over time shows that not all glaciers behave the same and that we need these careful, high resolution measurements to understand what’s going on.”

Previous satellite-based efforts to study changes in Greenland’s glaciers have had two significant flaws. One typical method measures variations in the Earth’s gravity caused by changes in the weight of ice on its surface. However, this cannot be used to study changes in small areas. Other methods look at the speed that ice sheets are moving, for example by tracking the height of features in the ice moving as the sheets flow. This approach provides limited information on when the changes happened. “In either case, the lack of resolution results in high uncertainties in the total amount of mass these glaciers have lost over the decade, and the variability in that loss,” Howat said.

In a Geophysical Research Letters paper published online ahead of print Howat and his colleagues used new techniques called “speckle tracking” and “feature tracking” to gain a more detailed insight. Each still uses satellite data, with “speckle tracking” exploiting radar measurements and “feature tracking” based on satellite photographs, Howat explained. “Both methods simply use image statistics to track the motion of surface features from one image to the next,” he said. Using this approach, Howat and colleagues from Ohio State, the University of Washington and Utrecht University looked at how ice loss had changed from 2000 and 2010. They showed that although the outermost part of each of the three largest glaciers draining from Greenland had retreated significantly, each had otherwise fared very differently.

Glacier individualism

“All three glaciers did undergo retreat and acceleration in mass loss to the ocean this decade, but Helheim started the decade gaining mass,” Howat said. Consequently, last decade appeared to be a relatively brief interruption in Helheim’s long‐term mass gain. “It went from gaining lots of mass to losing a little bit, and back to gaining,” Howat added. By contrast, the mass lost by Kangerdlugssuaq and Jakobshavn Isbrae would only be regained if they stopped flowing to the sea completely and accumulated snow for 7 and 11 years respectively. Jakobshavn Isbrae had been shrinking for over 100 years, and that process is accelerating further, Howat said, while Kangerdlugssuaq has now slowed how fast it has been losing ice back to nearer previous rates. “Kangerdlugssuaq has been losing mass for at least a decade before 2000, so it went from losing mass, to losing a lot of mass and then back to losing mass like before,” he explained. Overall the three glaciers felt a large net ice reduction, with Helheim gaining just one-fifteenth the amount Jakobshavn Isbrae lost.

The variation between different glaciers undermines the accuracy of predictions built on short-term measurements, Howat underlined. “This means that we can’t assess the long-term stability of these systems from short periods of observations,” he said “We can’t simply extrapolate recent changes into the future. It also means that we need to be complete in our observations, rather than just obtaining snapshots of a few glaciers at a time. Howat and his colleagues are now using their methods to examine what proportion of Greenland’s ice loss these glaciers are responsible for, and what their ice loss might have contributed to sea rise over the study period. He says, however, that comparing their results to other, ice sheet wide estimates, these glaciers should be responsible for around a third of Greenland’s ice loss. “This means that the majority of loss is occurring from many, smaller glaciers, which is a challenge for observations,” Howat said.