The Antarctic ice sheet, which contains 90 percent of the planet's ice, has lost significant mass in the past three years, according to satellite data analyzed in the first-ever gravity survey of the entire Antarctic ice sheet.
Isabella Velicogna and John Wahr, both from the University of Colorado, Boulder, conducted the study. They used measurements taken by twin satellites that are part of the Gravity Recovery and Climate Experiment, or GRACE, to conclude the Antarctic ice sheet is losing up to 36 cubic miles of ice, or 152 cubic kilometers, annually.
By comparison, that is about how much water the United States consumes in three months.
"This is the first study to indicate the total mass balance of the Antarctic ice sheet is in significant decline," said Velicogna of CU-Boulder's Cooperative Institute for Research in Environmental Sciences, chief author of the new study that appears in the March 2 online issue of "Science Express."
The estimated mass loss was enough to raise global sea level about 1.2 millimeters (0.05 inches) during the survey period, or about 13 percent of the overall observed sea level rise for the same period.
A floating iceberg off the Antarctic Peninsula. (Photo courtesy CU-Boulder National Snow and Ice Data Center)
The most recent Intergovernmental Panel on Climate Change assessment, completed in 2001, predicted the Antarctic ice sheet would gain mass in the 21st century due to increased precipitation in a warming climate.
But the new study found a reduction in the continent's total ice mass, with most of the loss occurring in the West Antarctic ice sheet.
"Antarctica is Earth's largest reservoir of fresh water," Velicogna said. "The GRACE mission is unique in its ability to measure mass changes directly for entire ice sheets and can determine how Earth's mass distribution changes over time."
"Because ice sheets are a large source of uncertainties in projections of sea level change, this represents a very important step toward more accurate prediction, and has important societal and economic impacts," she said. "As more GRACE data become available, it will become feasible to search for longer-term changes in the rate of Antarctic mass loss."
Launched in 2002, the two GRACE satellites whip around 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 identical twin satellites fly in formation measuring 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.
The Antarctic mass loss findings were enabled by the ability of the identical twin Grace satellites to track minute changes in Earth's gravity field resulting from regional changes in planet mass distribution.
One of the two GRACE satellites is shown orbiting the Earth. (Photo courtesy GRACE) GRACE is a joint partnership between the National Aeronautics and Space Administration (NASA) in the United States and Deutsche Forschungsanstalt für Luft und Raumfahrt in Germany. Dr. Byron Tapley of the University of Texas Center for Space Research is the principal investigator, and Dr. Christoph Reigber of the GeoForschungsZentrum Potsdam is the co-principal investigator. Project management and systems engineering activities are carried out by the NASA Jet Propulsion Laboratory.
"The strength of GRACE is that we were able to assess the entire Antarctic region in one fell swoop to determine whether it was gaining or losing mass," said co-author CU-Boulder physics Professor Wahr of CIRES, a joint campus institute of CU-Boulder and the National Oceanic and Atmospheric Administration.
A study led by CIRES 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.
The peninsula has warmed by an average of 4.5 degrees Fahrenheit in the past 60 years, and its ice shelves have decreased by more than 5,200 square miles in the past three decades, the scientists said.
As Earth's fifth largest continent, Antarctica is twice as large as Australia and contains 70 percent of Earth's fresh water resources.
The ice sheet, which covers about 98 percent of the continent, has an average thickness of about 6,500 feet. Floating ice shelves constitute about 11 percent 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.
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 NASA's Jet Propulsion Laboratory in Pasadena.
"The overall balance of the Antarctic ice is dependent on regional changes in the interior and those in the coastal areas," said Velicogna. "The changes we are seeing are probably a good indicator of the changing climatic conditions there."