Scientists have long talked of the dangers of the buildup of atmospheric carbon dioxide (CO2). Now it appears that the critical point may be reached sooner than expected.
CO2 absorbs infrared (heat) radiation emitted by the planet and returns some of it to Earth's surface. Thus CO2 -- released as fossil fuels are burned and accumulating in the atmosphere -- could gradually warm the planet in a process often referred to as the greenhouse effect.
Hermann Flohn of the University of Bonn now puts the risk level for serious climatic effects at 560 to 680 parts per million (p.p.m.) for concentrations of the heat-trapping gas.
The 560-680 p.p.m. figure represents a 5-degree C. average temperature rise a year. This seemingly modest figure is enough to melt the West Antarctic icecap, which might raise sea level enough to flood some of the world's major cities.
A 5-degree temperature rise could also eliminate the permanent Arctic sea ice. Flohn says he believes the Arctic sea ice would go rather quickly, before the West Antarctic ice were affected. Even without a simultaneous melting of Antarctic glaciers, he projects this could substantially change climate. It might shift rainfall zones, bringing semiarid conditions to parts of Europe and reducing rainfall in such climatically sensitive areas as California, the Mediterranean, and the Near East.
Flohn's figures (560-680 p.p.m.) are substantially below former estimates of 750 p.p.m. As he explained during a session of the annual meeting of the American Association for the Advancement of Science, other trace gases such as methane, nitrous oxide, and fluorocarbons have also been found to have a significant greenhouse effect. Thus less CO2 is needed for a 5-degree warming than had earlier been estimated. Perhaps the most sinister figure of all is that the present concentration of 340 p.p.m. is growing at 1.22 p.p.m. per year.
James Hansen of the Goddard Institute for Space Studies, speaking at the same session, said that, during the 1970s, these other gases increased the greenhouse effect by anywhere from 50 to 100 percent. He predicted that early signs of a greenhouse warming - an average temperature rise of a few tenths of a degree - may show up during the 1980s.
While all such projections are uncertain, this does add to the urgency of research to pin down the CO2 impact on climate. Most of the new CO2 expected to be added over the next century would come from burning coal. Since this is the ''alternative fuel'' on which China, the Soviet Union, and the United States in particular expect to rely, it is important to find out to what extent it can be used without triggering a climatic disaster.
As Roger Revelle of the University of California at San Diego pointed out, if coal burning does have to be curbed, at least this won't require wide international agreement. ''We have only to convince China, the Soviet Union, and ourselves,'' he said. That's nice, but it still sounds complicated.