Antarctica Sheds Ice and Scientists Wonder Why
Published: August 14, 1990
(Page 2 of 2)
The accumulation that he estimates is already occurring would lower worldwide sea levels by one-tenth to four-tenths of an inch per year. But sea levels seem to be rising. This means the melting ice must be coming from elsewhere, possibly Greenland or mountain glaciers, he said in a telephone interview.
Melting Greenland Ice
Dr. Roy M. Koerner of the Geological Survey of Canada has argued that the rise in sea level at the end of the next-to-last ice age to 20 feet above the present level was almost all caused by shedding of ice from Greenland. Samples from the bottoms of two holes drilled through the Greenland ice cap, he argues, show that the ice formed on bare land at the start of the most recent ice age, after all ice from the previous ice age had already vanished.
He noted that so far, samples from the sea floor off Antarctica have not shown evidence that West Antarctica totally shed its ice after the next-to-last ice age.
Evidence that polar ice disintegrated after the last ice age far more rapidly than had been supposed has been derived from corals on the island of Barbados by Dr. Richard G. Fairbanks of the Columbia University Lamont-Doherty Geological Observatory. He has reconstructed a record of sea levels over the past 18,000 years. This, he said in an interview, shows that 14,200 years ago, sea levels were rising at a rate of 14 feet per century.
In a subsequent surge, 11,300 years ago, seas rose 12 feet per century. Whether the melting came from Antarctica or elsewhere is uncertain, he said. The rapid filling of widely separated lake beds in the African desert was reported in a recent issue of the journal Nature by French researchers who studied lake sediment.
Participants in the NASA meeting acknowledged that ice is also flowing swiftly off Greenland. For example, Jakobshavn Glacier, the world's fastest, is flowing into the sea at the rate of 75 feet a day. They decided, however, that their first focus should be on the West Antarctic sheet because of its vulnerability. More offshore sampling could help determine its history, as well as drilling through it to see whether any ice from the previous ice age remains.
Participants also noted that shedding of the Antarctic ice would have many little-recognized effects, changing ocean and air circulation probably as far north as the tropics. Such circulation is now strongly influenced by the violent winds that pour off the South Polar ice.
Monitoring by Satellites
NASA officials believe that satellites carrying laser altimeters will be able to spot lowering of the surface, if a collapse is occurring. The program assumes that satellites, as well as surface expeditions, will monitor changing ice volume, including flow down the glaciers and ice streams. Scientists from several countries are building radio-observing stations in Antarctica to provide highly precise reference points for such observations.
By comparing radio signals from distant celestial objects like quasars, scientists at stations on land and on the ice can determine their locations to within a fraction of an inch. The Japanese have recently built such a station at Showa, their base in Enderby Land. The West Germans have started one near the Antarctic Peninsula. A Soviet station is projected, but the site is still uncertain.
Dr. James Garwin of NASA, who is working on plans for satellite mapping of Mars as well as the height of the Antarctic ice, said a challenge would be to determine the satellite's orbit precisely enough to identify where its radar or laser beam is looking.
Other efforts are already under way. Dr. Baerbel K. Lucchitta of the Geological Survey office in Flagstaff, Ariz., told the Fairbanks meeting of efforts to learn the extent to which Antarctica, through its many coastal glaciers, is dumping ice directly into the sea. The 25-nation Scientific Committee on Antarctic Research has begun a program to take duplicate photographs from space of glaciers recorded by satellite in the early 1970's.
So far, she said, such repeated images of 15 glaciers have been recorded, showing rates of annual motion ranging from 328 feet to 1.4 miles. Because the flowing glaciers carry with them crevasse patterns, splits in the ice that are easily recognized in the images, their motions can be followed from year to year. The fastest are the Pine Island and Land Glaciers, both draining Marie Byrd Land.
Robert A. Wharton Jr., of the Desert Research Institute in Reno, told of 10 years of observing ice thickness on Lake Hoare, a permanently ice-covered lake in southern Victoria Land, near McMurdo Sound. From 1979 to 1986, he said, it thinned from 18 feet to 11 feet, then stabilized.
''An increase in mean annual temperature is the most plausible explanation,'' he said. He and his colleagues believe that peak summer temperatures have been above freezing ''for a progressively longer period of time each year since 1973.''
Another temperature measurement was reported by Gary D. Clow and colleagues at the Geological Survey branch in Menlo Park, Calif. They studied the manner in which heat has propagated downward in permanently frozen ground, concluding that during the past century much of northern Alaska has warmed about 4 to 8 degrees Fahrenheit.
Dr. Clow said a drill hole in the Taylor Dry Valley of Antarctica showed a rise of about two degrees in the past 15 years, but its link to warming was uncertain.
The June 28 issue of Nature discussed evidence for Arctic ice thinning obtained by upward-looking sonar on two British submarines cruising under the ice. Earlier expeditions have found average thicknesses to be highly variable, ranging from 10 to 15 feet. The thinning from 1976 to 1987 was found to be substantial, but subject to so many factors that it could not be taken as evidence for climate change.
There seems to be widespread, though not universal, agreement that the world has become warmer in recent decades. Philip D. Jones and Tom M. L. Wigley at the University of East Anglia in Britain have completed a 10-year analysis of available data. In the August issue of Scientific American they say: ''Our work shows conclusively that the world's climate, though highly variable over periods of a decade or less, has become generally warmer during the past century.''
The effect, they say, has been less than that predicted from carbon dioxide and other gases added to the atmosphere by human activity. There is no proof that such an effect is occurring, they say, but they argue that this should not be used as an excuse for inaction.
''The longer the world waits to act, the greater will be the climate change that future generations will have to endure,'' they warned.
Photos: Rifts began to appear in the Filchner ice shelf in Antarctica, left, in 1973. By 1986, giant icebergs had formed on the seaward side of the rifts. (U.S. Geological Survey) (pg. C1); map of East Antarctica (source: United States Geological Survey) (pg. C8)