This is the first study to indicate the total mass balance of the Antarctic ice sheet is in significant decline.

—Isabella Velicogna, chief author

The findings counter the prediction of the most recent Intergovernmental Panel on Climate Change assessment, completed in 2001, that projected that the Antarctic ice sheet would gain mass in the 21^st century due to increased precipitation in a warming climate.

Researchers used GRACE data to calculate the total ice mass in Antarctica between April 2002 and August 2005 for the study, said Velicogna, who also is affiliated with the Jet Propulsion Laboratory in Pasadena.

Launched in 2002 by NASA and Germany, the two GRACE satellites orbit Earth 16 times a day at an altitude of 310 miles, sensing subtle variations in Earth’s mass and gravitational pull. Separated by 137 miles at all times, the satellites measure changes in Earth’s gravity field caused by regional changes in the planet’s mass, including such things as ice sheets, oceans and water stored in the soil and in underground aquifers.

A change in gravity due to a pass over a portion of the Antarctic ice sheet, for example, imperceptibly tugs the lead satellite away from the trailing satellite, according to Velicogna. A sensitive ranging system allows researchers to measure the distance of the two satellites down to as small as 1 micron and to then calculate the ice mass in particular regions of the continent.

A study spearheaded by researchers at CU-Boulder and published in September 2004 concluded that glaciers on the Antarctic Peninsula—which juts north from the West Antarctic ice sheet toward South America—sped up dramatically following the collapse of Larsen B ice shelf in 2002. Ice shelves on the peninsula—which has warmed by an average of 4.5 degrees Fahrenheit in the past 60 years—have decreased by more than 5,200 square miles in the past three decades.

The fifth largest continent, Antarctica is the Earth’s highest, windiest, coldest, and driest land mass. Its surface is 99.7% covered by a vast ice sheet with an average thickness of ~2 km and a total volume of ~25M km³.  Floating ice shelves constitute about 11% of the continent.

The melting of the West Antarctic ice sheet alone—which is about eight times smaller in volume than the East Antarctic ice sheet—would raise global sea levels by more than 20 feet, according to researchers from the British Antarctic Survey. The possible deglaciation of that polar ice sheet and that of Greenland are two topics of considerable concern in the British report on Avoiding Dangerous Climate Change. (Earlier post.)

Resources:

• Animation of GRACE and Ice

• “Measurements of Time-Variable Gravity Show Mass Loss in Antarctica”; Isabella Velicogna and John Wahr; Science online, 2 March 2006; DOI:10.1126/science.1123785

• “The Antarctic Ice Sheet and Sea Level Rise”; Chris Rapley, British Antarctic Survey; Avoiding Dangerous Climate Change

• Antarctic Glaciological Data Center