A warming climate is melting more than 50 billion tons of water a year from the Greenland ice sheet, adding to a nine-inch global rise in sea level over the last century and increasing the risk of coastal flooding around the world, a study shows.
A NASA high-tech aerial survey shows that more than 11 cubic miles of ice is disappearing from the Greenland ice sheet annually, says the study appearing Friday in the journal Science. That’s equal to about 1.25 trillion gallons and is enough to raise sea level by a .13 millimeters a year.
“We see a significant trend [in loss of ice mass],” said William B. Krabill, an author of the study. “When we can go back after five years and see 10 meters of glacier gone, there is something happening.”
Krabill, a NASA scientist and head of the ice sheet measurement project, said the melting of Greenland ice and the calving of icebergs from Greenland glaciers is responsible for about 7 percent of the annual rise in global sea level.
Over the last century, measurements suggest sea level has risen about nine inches, enough to cause some low-lying areas that were once high and dry to be awash at high tide or during storms. The trend could get worse, said Krabill, if the Greenland ice sheet continues its meltdown.
Rivers of Ice
“The margins of the Greenland ice sheet are undergoing significant thinning, in some places in excess of two meters a year,” said Krabill.
Just how much ice is disappearing from Greenland and Antarctica, the world’s largest reservoirs of ice, has long been uncertain. The NASA survey is the first to give a comprehensive measurement of recent changes in the Greenland ice sheet. Krabill said there is no similar data for Antarctica.
Krabill said the study shows glaciers on Greenland are moving more rapidly to the sea, caused, perhaps, by melt water that coats the base of the glaciers and helps lubricate the downhill slide of the ice rivers.
“We are seeing widespread indications that something like that is going on, causing the glaciers to move faster toward the margins,” said Krabill.
Krabill led a NASA team that used an aerial laser survey technique to measure the level of ice on Greenland and compare that with data from a similar survey five years ago. The technique involves a laser that fires 5,000 bursts of light a second toward the surface from an airplane flying over the ice. The light bounces back to a receiver on the airplane, giving a measure of altitude. The airplane’s location and path is measured using the Global Positioning Satellite system. The result is a precise measurement of altitude of the ice covering Greenland. A survey five years ago used a similar technique.
Rising Sea Level
A comparison of the two surveys found there had been severe thinning along the southern and southeastern margins of the Atlantic Ocean island, while there was a slight thickening of ice in the western highlands. But the net effect, said Krabill, was a significant loss of ice from the island.
“What we see happening in Greenland may be an indication of the bigger picture,” said Waleed Abdalati, another NASA scientist and a co-author of the study. The Greenland ice sheet, he said, is more sensitive to climate change than the ice in Antarctica because Greenland has a more temperate climate.
The process of losing ice can itself speed up the melting, said Abdalati.
“As ice melts, more solar energy is absorbed, causing the surface to get warmer,” accelerating the melt, he said. “This is called a positive feed back.”
Temperature measurements along Greenland’s east coast show a half degree rise over five years, said Abdalati. Measurements elsewhere on the island show no change or even a slight cooling, he said.
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 that would take thousands of years — could raise the world’s sea level by about 21 feet, said Abdalati. But long before that happens, a creeping rise in sea level could displace millions of people who now live along the world’s coasts.
“You would have major effects if the sea level goes up” only a few 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 satellite will orbit nearly directly over the poles, taking almost constant measurements of the ice and determine how it is changing with time.