Global Warming Impact on Polar Ice Sheets Confirmed

GREENBELT, Maryland, March 13, 2006 (ENS) - Climate warming is changing how much water remains in Earth's greatest treasuries of ice and snow, NASA scientists have confirmed. The most comprehensive survey ever undertaken of the enormous ice sheets covering both Greenland and Antarctica shows a net loss of ice to the sea.

The 10 year survey shows there was a net loss of ice from the combined polar ice sheets between 1992 and 2002 and a corresponding rise in sea level.

The 20 billion net tons of water melted into the oceans each of those 10 years is equivalent to the amount of fresh water annually used in homes, businesses, and farming in New York, New Jersey, and Virginia.

The survey documented extensive thinning of the West Antarctic ice shelves, an increase in snowfall in the interior of Greenland, and thinning of ice at the edges.

Other recent studies have shown increasing losses of ice in parts of both these sheets. This NASA survey is the first to inventory the losses of ice and the addition of new snow on both in a consistent and comprehensive way throughout an entire decade.

"If the trends we're seeing continue and climate warming continues as predicted, the polar ice sheets could change dramatically," said survey lead author Dr. H. Jay Zwally of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"The Greenland ice sheet could be facing an irreversible decline by the end of the century," Zwally warned.

Dr. Zwally has been involved in glaciology and polar research since 1972. (Photo courtesy NASA)

The record heat of 2005 is part of a longer term warming trend due to the rise of heat-trapping gases in Earth's atmosphere that is due to human activities - burning fossil fuels and clearing forests. Nineteen of the hottest 20 years on record have occurred since 1980.

The Zwally survey, published in the March 8 issue of the "Journal of Glaciology," combines new satellite mapping of the height of the ice sheets from two European Space Agency satellites. It also used previous NASA airborne mapping of the edges of the Greenland ice sheets to determine how fast the thickness is changing.

Researchers used nine years of elevation mapping over much of Antarctica and 10.5 years of data over Greenland from the European Remote Sensing Satellites 1 and 2.

The survey shows where the ice sheets were thinning and where they were growing.

Satellites are used to map the extent and duration of snowmelt on the Greenland ice sheet. The dark red area represents the extent of snowmelt in 2005, three years beyond Dr. Zwally's survey. It is the most extensive in the 27 year history of data collection. (Figure courtesy of NOAA and CIRES)

This study suggests there was a slight gain in the total mass of frozen water in the ice sheet over the decade studied, contrary to previous assessments. According to Zwally, this situation may have changed in just the past few years.

"The melting of ice at the edges of the ice sheet is also increasing, which causes the ice to flow faster," he said. "A race is going on in Greenland between these competing forces of snow build-up in the interior and ice loss on the edges. But we don't know how long they will be approximately in balance with each other or if that balance has already tipped in favor of the recently accelerating outflow from glaciers."

H5> In Antarctica, the ice sheets had a major net loss of ice due to outflow from West Antarctica. These losses, which may have been going on for decades, outweighed the gains in snow and ice seen in the East Antarctic ice sheet and parts of West Antarctica.

Also thinning were the ice shelves around West Antarctica, where temperatures have been increasing.

Earlier this month, scientists with the University of Colorado, Boulder published research that showed the Antarctic ice sheet is losing up to 36 cubic miles of ice, or 152 cubic kilometers, annually.

Antarctica lost more ice to the sea than it gained from snowfall, resulting in sea level rise. (Photo courtesy NASA/SVS)

Last month NASA scientists at the Jet Propulsion Laboratory, Pasadena, California, reported a speed up of ice flow into the sea from several Greenland glaciers. That study included observations through 2005; Zwally's survey concluded with 2002 data.

When the scientists added up the gains and losses of ice from the Greenland and Antarctic sheets, more ice was lost to the sea than was gained.

The Greenland ice sheet annually gained approximately 11 billion tons of water, while Antarctica lost about 31 billion tons per year.

"The study indicates that the contribution of the ice sheets to sea-level rise during the decade studied was much smaller than expected, just two percent of the recent increase of nearly three millimeters (0.12 inches) a year," Zwally said."

"Current estimates of the other major sources of sea-level rise - expansion of the ocean by warming temperatures and runoff from low-latitude glaciers - do not make up the difference, so we have a mystery on our hands as to where the water is coming from," Zwally said.

Meanwhile, a study of a massive ice sheet that existed in northern Europe at the end of the last Ice Age has outlined its behavior for the first time, and a team of researchers from Oregon State University believes it may provide a preview of how major ice sheets in Greenland and Antarctica will act in global warming conditions.

The study, which will be published Friday in the journal "Science," shows that ice sheets can react differently depending on the climatic conditions at the time global warming occurs - sometimes growing larger and sometimes rapidly disappearing, depending on whether increased snow offsets melting effects, or not.

This analysis examined the Scandinavian Ice Sheet that existed 10,000 years ago. It once covered much of Northern Europe, including Poland, Estonia, Latvia, Lithuania, Belarus and Finland to a meximum thickness of 6,000 feet, and was the second largest ice sheet of its era, next to one covering North America.

Ice melting into the Arctic Ocean (Photo by Peter West courtesy National Science Foundation)

Oregon State University scientists say the same forces are operating today, and probably mean that as global warming continues, the ice in large parts of Antarctica may increase, while the massive Greenland ice sheet, which exists in a slightly warmer environment, will almost certainly disappear.

"This study supports what we've been learning about the Greenland ice sheet, which is that it will completely melt within 500 to 1,000 years," said Peter Clark, a professor of geosciences at Oregon State University and an international expert in the study of ancient ice sheets.

In their study, the Oregon State researchers used a new, sophisticated technique, which measures isotopes of the element beryllium, to date the time that rocks have been exposed at the surface, after they had been dumped into the open by an ice sheet.

"Our new analysis of the ancient Scandinavian Ice Sheet, like other studies, is showing how these events unfolded in the past, which will help us better understand what the future will hold," Clark said.

By itself and without any offsetting mechanisms, a collapse of the Greenland ice sheet would raise sea levels by about 20-25 feet, Clark and his team predict.

One prediction is that sea levels should increase by a foot or two by 2100, and up to 25 feet within 500 years. Some of that sea level rise is based on melting of glaciers and major ice sheets, and some is based on thermal expansion of water in the oceans, which increases in size as it gets warmer.