Surveys Uncover Substantial Melting of Greenland Ice Sheet
By WILLIAM K. STEVENS
Published: March 5, 1999
The southern half of the Greenland ice sheet, the second largest expanse of land-bound ice on earth, after Antarctica, has shrunk substantially in the last five years, scientists have found in airborne surveys using new and more precise techniques.
Experts have said for some time that a warming atmosphere has caused many mountain glaciers around the world to shrink. But until now, they have not known what was happening to the Greenland ice cap. While five years is too short a period to determine a trend, the new findings, reported in today's issue of the journal Science, provide the first precise evidence that it, too, is diminishing.
If the big ice sheets melt even partly, sea levels will rise around the world. Melting might also disrupt the ocean currents that modulate the earth's climate by distributing heat around the globe.
Though the observed shrinkage in Greenland has evidently not yet had a major impact, it is not trivial. Each year from 1993 to 1998, on average, southern Greenland lost about two cubic miles of ice, enough to cover Maryland with a sheet one foot thick, said William B. Krabill, the expert in remote sensing who led the National Aeronautics and Space Administration team that surveyed the ice.
In an aircraft equipped with laser altimeters, the team in 1993 and 1994 measured the thickness of the ice across all of Greenland. Last year, guided by satellite-based positioning that enabled them to retrace their original path exactly, they resurveyed the southern half of the island. This year they plan to resurvey the northern half. The measurements are intended to provide a base line for continuing studies.
The laser-altimeter method for the first time allowed scientists to measure the ice well enough to gauge whether it was growing or shrinking. Remote sensing by satellite proved not sufficiently sensitive to detail changes on the fringes of the ice sheet, where most of the shrinkage has taken place.
The magnitude of the shrinkage ''is a little surprising,'' said Dr. Richard Alley of Pennsylvania State University, an expert on glaciation and the behavior of ice sheets.
Dr. Alley also said the shrinkage might be related to a general warming of the earth. But it is unclear at this point whether the shrinkage resulted from natural or human causes or both.
The earth's average surface temperature has increased by about 1 degree or a little more since the late 19th century, and the 1990's have been the warmest decade on record. The temperature rise compares with a warming of 5 to 9 degrees since the depths of the last ice age some 20,000 years ago.
The dominant view among experts is that the increase is at least partly attributable to emissions of heat-trapping waste industrial gases like carbon dioxide, which are emitted into the atmosphere by the burning of fossil fuels like coal and oil.
Mainstream scientists believe the surface temperature will increase by about another 3.5 degrees in the 21st century if emissions of the gases are not reduced. They expect a rise in sea level of about 20 inches to accompany the warming over the next century, inundating many low-lying areas of the world. Melting of glacial ice would contribute to that rise, along with expansion of the ocean's waters as they warm.
Generally speaking, scientists say, warming has two opposing effects on ice sheets. First, a warmer atmosphere holds more moisture, which means that heavier snows fall. This snow, when compacted, thickens the ice. At the same time, warmer temperatures partly melt the ice sheets at their edges and at lower altitudes.
The net effect of these competing influences varies from one expanse of ice to another and determines whether an ice sheet gets thinner or thicker on balance.
Mr. Krabill's team found that in the western part of southern Greenland, thickening and thinning of the ice essentially canceled each other out.
But on the eastern side, thinning outstripped thickening. On previously unsurveyed areas of the ice sheet's periphery, the scientists found that large expanses had thinned by nearly eight inches a year. Nearer the eastern coast, the thinning increased to more than three feet a year.
The thinning could be accounted for by decreased snowfall or excessive summertime melting. But Mr. Krabill and his team believe it is more likely that Greenland's glaciers are flowing faster to the sea, where they break up into icebergs that eventually melt.
The researchers postulated in the Science article that the glaciers' path might be greased, in effect, as more water melted on the surface and penetrated to the glacier's bed, lubricating it. That, they wrote, would mean that Greenland's ice could be transferred to the ocean faster than it is by melting alone.
''If that turns out to be true,'' Dr. Alley said, ''that's something we're going to have to worry about in the future.''
Mr. Krabill said, ''What we're seeing is something that will not have an appreciable effect during our lifetime.'' On the other hand, he said, ''if the trend were to continue, it would be significant in a long-term sense.''
But if the glaciers are flowing more rapidly to the sea, said Dr. Gerard Bond, a glaciation expert at Columbia University's Lamont-Doherty Earth Observatory, it would probably mean that an increased number of icebergs would be breaking off from Greenland and floating free in the Atlantic, there to melt. This infusion of fresh water, Dr. Bond said, could disrupt the strength and flow of dense, salty ocean currents in the North Atlantic by pouring more fresh water into the ocean.
These currents transport heat into the North Atlantic. If they weaken, it could set off atmospheric changes that, for example, would subject Europe to much colder winters.
In the long run, Dr. Bond said, a continuing influx of fresh water could even touch off a chain of events that might lead at some point to a new ice age, although he said that was ''a stretch.''
Dr. Alley said there was a potential that ''putting more fresh water into the Atlantic will cause things to change in a hurry.'' It was likely, he said, that the climate system responded to such changes as if a light switch were being thrown: A little pressure may not cause the system to change, but when the pressure reaches a certain point, it flips suddenly.
Putting more ice into the Atlantic, Dr. Alley said, ''would be pushing it in the direction of doing something.'' But he said scientists ''have no clue'' as to whether the latest findings from Greenland represented a significant push on the switch and whether the switch was close to flipping.