French and Soviet scientists, using 160,000-year-old ice samples, have obtained the strongest evidence yet to link an increase in atmospheric carbon dioxide to warming of the earth--the potentially catastrophic "greenhouse effect."
In three reports in today's Nature magazine, the researchers said that carbon dioxide levels in the atmosphere dropped by 40% and global temperatures fell by about 18 degrees Fahrenheit the last two times that glaciers swept south as far as St. Louis and New York City.
Carbon dioxide concentrations and surface temperatures then returned to normal as the glaciers retreated.
Sun's Heat Trapped
The reports appear to confirm fears that excess carbon dioxide released into the atmosphere by the burning of fossil fuels would cause a greater global warming than has ever been observed, with harmful effects on climate and agriculture.
The carbon dioxide allows the sun's light to reach the earth's surface, but absorbs heat that would otherwise be radiated back into space, thus trapping heat--as in a greenhouse.
Climatologists estimate that the earth's surface temperature has already increased between 0.5 and 1.25 degrees Fahrenheit since 1850. This summer, scientists from the National Oceanographic and Atmospheric Administration reported that this temperature increase has been accompanied by a northward shift of rainfall.
If the trend continues, according to a 1983 National Academy of Sciences report, agriculture in the southern United States will require massive irrigation just to maintain present levels of production.
"Scientists have theorized about the effects of carbon dioxide on climate since the 19th Century," said geologist Eric T. Sundquist of the U.S. Geological Survey in Reston, Va., "but there was little hard evidence to back the theory."
In 1979 and 1980, however, studies of Antarctic ice indicated that carbon dioxide levels had increased about 40% to 50% as the last glaciers retreated about 10,000 years ago.
Since 1980, researchers have shown that carbon dioxide levels have increased from about 270 parts per million in the mid-1800s to about 345 parts per million now, while surface temperatures have increased slightly, by comparison. "But the increase in temperature has not been large enough to rule out the possibility that it is simply part of the natural global variability," Sundquist said.
The new results are "fantastically important," said geologist Wallace S. Broeckel of Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.J., "because they extend the record of climate and carbon dioxide concentrations much further back."
Those results were obtained with ice samples retrieved from a 6,874-foot-deep hole that Soviet scientists drilled at their Vostok station in East Antarctica. The oldest samples, buried by century after century of falling snow, date from 160,000 years ago.
As fresh snow in the Antarctic was turned into ice by the increasing weight of snow above it, it trapped small bubbles of air and held it locked in place. Claude Lorius and his colleagues at the Laboratory of Glaciology and Environmental Geophysics in St. Martin d'Heres, France, measured the carbon dioxide concentrations in this air in ice taken at three-foot intervals down the entire length of the hole.
By measuring the proportion of deuterium, a heavy form of hydrogen, in the ice, they were also able to estimate temperatures. The amount of deuterium in rain or snow increases with increasing temperature.
"We found a very strong correlation between carbon dioxide fluctuations and temperature fluctuations over the entire 160,000 years," Lorius said in a telephone interview Wednesday. "The changes in carbon dioxide magnified changes in temperature that resulted from variations in the earth's orbit around the sun," he said.
"These results definitely suggest that carbon dioxide was responsible for the warming."