Secrets of the Icecap

Five hundred years before the arrival of other Europeans in the New World, Vikings settled in Greenland and founded a colony that eventually grew to 3,000 people. During the 12th century, the Norsemen began returning to Europe; by 1410 they had completely abandoned Greenland. For years historians have debated the cause of the mysterious demise. Were the Vikings driven out by hostile natives? Did excessive inbreeding cause genetic deterioration of the tough Norse stock? Now scientists have suggested a simpler explanation: the mild weather that the Vikings originally encountered in Greenland gradually changed and became too harsh even for their hardy tastes.

The evidence comes from U.S. and Danish scientists working above the Arctic Circle at a remote encampment 120 miles east of Thule. For several years, they drilled through the 4,500-ft.-thick Greenland icecap, gathering cores, or cylindrical samples, that provide a remarkably accurate record of Greenland's weather. The cores consist of layers of ice, each representing a year's precipitation. They have remained virtually unchanged for centuries. By analyzing these layers, scientists have been able to reconstruct a climatic history that reaches back for 100,000 years.

Records of ancient temperatures are provided by O18, a heavy isotope of oxygen that has 18 neutrons in its nucleus instead of the 16 found in ordinary oxygen atoms. About one of every 500 atoms of oxygen in water is O18, and water molecules containing the heavy isotope will fall from clouds in the form of rain or snow before those with ordinary oxygen atoms. In colder weather, the isotope falls even more rapidly. Thus, by the time that clouds arrive over the site where the ice cores were taken, the ratio of O18 atoms to ordinary oxygen atoms in the precipitation is lower than usual.

By dating each ice layer like growth rings on a tree, the scientists have been able to use the oxygen-isotope ratio to chart yearly variations in weather to depths of 300 ft. Beyond that level, the annual record becomes blurred. But it is still clear enough to let scientists distinguish broad climatological trends. Analysis of the layers showed, for example, that the earth's last ice age began some 70,000 years ago and did not end until about 10,000 years ago. The investigators also made some long-range forecasts. Projecting the established weather pattern, they predicted that temperatures in the Northern Hemisphere will continue to drop for 25 years before a warming trend sets in.

Nature's Relics. Since they published their findings in Science last month, Chester C. Langway Jr. of the U.S. Army's Cold Regions Research and Engineering Lab at Hanover, N.H., and his three Danish collaborators have been deluged with requests for ice specimens. The interest of other scientists is understandable. The ice now being preserved in deep freezes at Hanover may contain a wide assortment of nature's rare relics, ranging from evidence of past cosmic-ray bombardment to bubbles of ancient trapped air that will tell much about the composition of the earth's atmosphere thousands of years ago.

After years of enduring the rigors of life at the top of the world, Langway will soon go to the bottom to help organize a large new core-drilling operation in eastern Antarctica. There, the drilling will be even more difficult because the temperature of the subsurface ice is -40DEG F. (v. -13DEG F. in Greenland), but Langway's southern trip may well be worth the effort. By comparing his new findings with those in Greenland, he hopes to determine whether the climatic changes in one hemisphere triggered changes in the other. He also thinks that ice cores may hold the answer to an old scientific puzzle: What caused the earth's last ice age?

This file is automatically generated by a robot program, so reader's discretion is required.