Melting of Greenland Raises Sea Level

W A S H I N G T O N, July 20, 2000 -- A warming climate is melting more than 50billion tons of water a year from the Greenland ice sheet, addingto a nine-inch global rise in sea level over the last century andincreasing the risk of coastal flooding around the world, a studyshows.

A NASA high-tech aerial survey shows that more than 11 cubicmiles of ice is disappearing from the Greenland ice sheet annually,says the study appearing Friday in the journal Science. That’sequal to about 1.25 trillion gallons and is enough to raise sealevel by a .13 millimeters a year.

“We see a significant trend [in loss of ice mass],” saidWilliam B. Krabill, an author of the study. “When we can goback after five years and see 10 meters of glacier gone, there issomething happening.”

Krabill, a NASA scientist and head of the ice sheet measurementproject, said the melting of Greenland ice and the calving oficebergs from Greenland glaciers is responsible for about 7 percentof the annual rise in global sea level.

Over the last century, measurements suggest sea level has risenabout nine inches, enough to cause some low-lying areas that wereonce high and dry to be awash at high tide or during storms. Thetrend could get worse, said Krabill, if the Greenland ice sheetcontinues its meltdown.

Rivers of Ice

“The margins of the Greenland ice sheet are undergoingsignificant thinning, in some places in excess of two meters ayear,” said Krabill.

Just how much ice is disappearing from Greenland and Antarctica,the world’s largest reservoirs of ice, has long been uncertain. TheNASA survey is the first to give a comprehensive measurement ofrecent changes in the Greenland ice sheet. Krabill said there is nosimilar data for Antarctica.

Krabill said the study shows glaciers on Greenland are movingmore rapidly to the sea, caused, perhaps, by melt water that coatsthe base of the glaciers and helps lubricate the downhill slide ofthe ice rivers.

“We are seeing widespread indications that something like thatis going on, causing the glaciers to move faster toward themargins,” said Krabill.

Krabill led a NASA team that used an aerial laser surveytechnique to measure the level of ice on Greenland and compare thatwith data from a similar survey five years ago. The techniqueinvolves a laser that fires 5,000 bursts of light a second towardthe surface from an airplane flying over the ice. The light bouncesback to a receiver on the airplane, giving a measure of altitude.The airplane’s location and path is measured using the GlobalPositioning Satellite system. The result is a precise measurementof altitude of the ice covering Greenland. A survey five years agoused a similar technique.

Rising Sea Level

A comparison of the two surveys found there had been severethinning along the southern and southeastern margins of theAtlantic Ocean island, while there was a slight thickening of icein the western highlands. But the net effect, said Krabill, was asignificant loss of ice from the island.

“What we see happening in Greenland may be an indication of thebigger picture,” said Waleed Abdalati, another NASA scientist anda co-author of the study. The Greenland ice sheet, he said, is moresensitive to climate change than the ice in Antarctica becauseGreenland has a more temperate climate.

The process of losing ice can itself speed up the melting, saidAbdalati.

“As ice melts, more solar energy is absorbed, causing thesurface to get warmer,” accelerating the melt, he said. “This iscalled a positive feed back.”

Temperature measurements along Greenland’s east coast show ahalf degree rise over five years, said Abdalati. Measurementselsewhere on the island show no change or even a slight cooling, hesaid.

Greenland contains about 8 percent of the Earth’s grounded ice.Antarctica, at the South Pole, holds about 91 percent.

Melting all the ice on Greenland — a very unlikely event thatwould take thousands of years — could raise the world’s sea levelby about 21 feet, said Abdalati. But long before that happens, acreeping rise in sea level could displace millions of people whonow live along the world’s coasts.

“You would have major effects if the sea level goes up” only afew inches, Abdalati said.

NASA is planning in 2001 to launch a satellite, called IceSat,that will survey the major ice sheets in the world. The satellitewill orbit nearly directly over the poles, taking almost constantmeasurements of the ice and determine how it is changing with time.