|Large numbers of bergs are calved each year from the fast-flowing terminus of Kangerdlussuaq Glacier, East Greenland. Iceberg production is a major form of mass loss from ice sheets. Image: J.A. Dowdeswell|
The amount of ice that Greenland’s glaciers dump into the Atlantic Ocean has almost doubled in the last five years because glaciers are moving faster, according to a new study published in the current issue of Science. This is the first study to incorporate recent changes in glacier velocity into estimates of the overall mass of ice being lost for nearly all of Greenland.
Rising surface air temperatures appear to be triggering the increases in glacier speed in the southern half of Greenland, according to the authors of the study. As a result, many estimates of Greenland’s future contributions to sea-level rise could be too low.
The behavior of the glaciers that dump ice into the sea is the most important aspect of understanding how an ice sheet will evolve in a changing climate. It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes.—Eric Rignot, Jet Propulsion Laboratory, study author
Rignot expressed concern that the models now used to predict Greenland's ice loss and contribution to sea level rise are inadequate because they do not account for changes in the speed of outlet glaciers that flow into the sea.
Taking higher glacier speeds into account, the authors calculate that Greenland contributes about 0.5 millimeters per year to global sea level rise which currently stands at 3 millimeters per year. Recent increases in glacier speed on Greenland are responsible for more than two-thirds of Greenland's contribution to sea level rise, the authors say.
Since 1996, Southeast Greenland’s outlet glaciers have been largely responsible for increases in overall glacier flow. After 2000, glaciers further north have also rapidly increased in speed, and the northward spread of warmer temperatures may be responsible, according to Rignot.
Over the last 20 years, the air temperature in southeast Greenland has risen by 3º Celsius. The warmer temperatures increase the amount of melt water reaching the glacier-rock interface where it serves as a lubricant that eases glaciers’ march to the ocean, the authors say.
Using satellite data, the researchers generated a glacier velocity map for nearly all of Greenland for 2000. By incorporating satellite measurements from 1996 and 2005, the researchers analyzed how glacier velocity has changed over the last 10 years. They combined this glacier velocity information with ice thickness estimates to calculate changes in Greenland’s total annual ice loss and mass balance over the same period.
The Greenland Ice Sheet gains mass through snowfall and loses mass when ice melts, erodes or vaporizes off the surface, when ice breaks off and forms icebergs due to glacier flow, and when ice melts from the base of floating ice connected to glaciers.
Due to the recent speed up, more ice is being dumped into the sea. The component of ice loss due to glacier flow has increased from 50 cubic kilometers of ice loss per year in 1996 to 150 cubic kilometers of ice loss per year in 2005.
When the researchers included findings from other groups on ice loss from glacier melting and ice accumulation from snowfall, they found that the Greenland Ice Sheet’s overall mass loss has increased from 90 cubic kilometers of ice loss per year in 1996 to 224 cubic kilometers of ice loss per year in 2005.
The Greenland Ice Sheet is 1.7 million square kilometers, up to 3 kilometers thick and a little smaller than Mexico. If the Greenland Ice Sheet completely melted, it would raise global sea level by about 7 meters.
The mass balance of the great ice sheets of Greenland and Antarctica represents the largest unknown in predictions of global sea-level rise over the coming decades, writes the author of a related Perspective article in Science who notes that the mass balance of large ice sheets can depend on the behavior of a small number of outlet glaciers.
Separately, Lonnie Thompson, a professor of geological sciences at Ohio State University, whose work was highlighted in the book Thin Ice by Mark Bowen, says the ice loss on Mount Kilimanjaro in Kenya may be accelerating.
In 2002, Thompson and his colleagues predicted that the ice fields capping the mountain would disappear between 2015 and 2020, the victims, at least in part, of global warming.
“Changes in the Velocity Structure of the Greenland Ice Sheet”; Eric Rignot and Pannir Kanagaratnam; Science 17 February 2006: Vol. 311. no. 5763, pp. 986–990 DOI:10.1126/science.1121381
“The Greenland Ice Sheet and Global Sea-Level Rise”; Julian A. Dowdeswell; Science 17 February 2006: Vol. 311. no. 5763, pp. 963–964 DOI: 10.1126/science.1124190