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Not So Fast: Greenland Ice Melting, But Slower Than Thought

John Roach
for National Geographic News
October 19, 2006
 
The Greenland ice sheet is rapidly wasting away—but not as quickly as some recent studies have found, according to a new analysis.

Several recent studies have suggested that the island has been losing ice at a rate sufficient to push global sea levels up by 0.02 inch (0.5 millimeter) a year.

(Related news: "Greenland's Ice Melt Grew by 250 Percent, Satellites Show" [September 20, 2006].)

The new analysis cuts that rate in half, but the ice-mass loss is still happening fast enough to alarm scientists.

"Greenland is losing each year 20 percent more mass than goes into the ice sheet as snowfall," said Jay Zwally, a glacier expert at the NASA Goddard Space Flight Center in Greenbelt, Maryland.

Zwally is a co-author of the new study, which is reported tomorrow in the journal Science.

The new finding suggests that the Danish-owned island (Greenland map) lost about 110 billion tons (100 billion metric tons) of ice a year between 2003 and 2005.

That's enough water to keep the U.S.'s Colorado River flowing for six years.

Finer Resolution

The new study and several previous studies are based on analyses of data collected by a pair of orbiting satellites that measure tiny variations in gravity caused by changes in Earth's mass.

Because ice melt changes Greenland's mass, the satellites can measure changes in mass over time to determine how much ice is lost or gained.

In the most recent study, Zwally and colleagues applied a new technique to determine ice-mass changes along specific drainage basins on Greenland.

The team showed that ice is melting fastest at coastal edges while the interior and high elevations are gaining mass from snowfall.

They also found a seasonal cycle to the mass changes, with loss during summer melting and gains during winter snowfall.

"The other analyses looked at all of Greenland as one big area," Zwally said.

"When you look at one big area, the regions that are increasing and decreasing all smear together, and signals from the outside—the oceans—also affect the solutions," he added.

The new study "provides results of finer resolution," Anny Cazenave, a climate scientist with the Observatoire Midi-Pyrenees in Toulouse, France, wrote in an accompanying commentary.

Richard Alley is a glacier expert at Pennsylvania State University in University Park who was not involved in the current or previous studies.

While each method is subject to error from influences beyond Greenland, such as the tides and seasons, he says, all the studies reach an indisputable conclusion: Greenland is melting.

(Related: See photos of Greenland's hunters, whose way of life is threatened by the island's ice melt.)

"For now it is hard to imagine that the work is the wrong sign," he said in an email.

"I believe that we can have high confidence—based on these and many other data sets—that over this relatively short interval the ice sheet is losing mass at a rate sufficient to matter to sea-level rise," he continued.

Discussion among the various research groups, he adds, ought to narrow the uncertainties in how fast the island is melting, lead to preferred methods of measurement, and increase confidence in the results.

Sudden Meltdown

According to Zwally, the Greenland meltdown is a sudden and even unexpected trend.

In the 1990s he and colleagues found that Greenland was gaining about as much ice through inland snowfall as it lost to melting and iceberg discharge along the coasts each year.

"Thinning at the edges and growth inland is the scenario predicted by ice and climate models," he said.

But the latest study shows that "we are seeing significant changes in a relatively short period of time," he said.

The process of losing mass along the edges of the ice sheet is not being balanced by inland growth, he says.

"We believe these [changes] are climate induced, and we are only in the early part of the climate warming predicted for this century," he continued.

The rate of change, Zwally adds, closely corresponds with a recently observed increase in the rate at which glaciers are slipping into the sea along southeast and western Greenland.

(Related news: "Greenland Glaciers Losing Ice Much Faster, Study Says" [February 16, 2006].)

And the meltdown is expected to continue accelerating, Zwally says. According to climate models, Earth has only experienced 10 to 20 percent of the warming predicted for this century.

"We could be looking at an effect five to ten times larger than what we've already seen assuming we continue with business as usual," he said.

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