Reining in a Runaway Theory
Monastersky, Richard, Science News
Reining in a runaway theory
Geoscientists have often wondered what kept the early earth from freezing over. Even though the sun was 25 to 30 percent dimmer during the earth's distant past, "there's no evidence for glaciation prior to 2.5 billion years ago, and there is positive evidence for liquid water at 3.8 billion years ago,' says James F. Kasting, a NASA Ames researcher in Moffett Field, Calif.
Many scientists believe the greenhouse properties of carbon dioxide gas saved the early earth from a deep freeze. Atmospheric carbon dioxide traps heat radiated by the earth, thereby raising surface temperatures--a phenomenon, given today's rising carbon dioxide levels, that has many people worried about a future greenhouse effect. In order to counteract the cooler sun, says Kasting, the carbon dioxide concentration in the early atmosphere would have to have been at least 100 to 1,000 times today's level. In recent years, James C. G. Walker at the University of Michigan in Ann Arbor has suggested that even more carbon dioxide was present in the atmosphere.
According to Walker, some researchers have attacked this theory by contending that such high carbon dioxide levels would have caused a "runaway greenhouse' --a condition in which the earth's surface gets hot enough to boil away the oceans. However, in the Dec. 12 SCIENCE, Kasting and Thomas P. Ackerman say the runaway greenhouse scenario is unlikely. Their climate model indicates the early atmosphere was stable, even with carbon dioxide concentrations of 10(5) times that they are today.
Although carbon dioxide tends to raise temperatures through its greenhouse properties, it also provides part of the force that keeps temperatures in check. …