Reining in a runaway theory.

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. In this process, known as Rayleigh scattering, carbon dioxide molecules scatter solar radiation. When Kasting ran high concentrations of carbon dioxide through the model, he found that Rayleigh scattering would prevent significant amounts of solar radiation from reaching earth, ensuring that temperatures would not rise enough to cause a runaway greenhouse.

Kasting also applied his model of the early atmosphere to a second question-- whether a geochemical process could have produced oxygen. The early atmosphere lacked free oxygen, but somewhere between 3.5 billion and 2.5 billion years ago, oxygen began to appear in the atmosphere. Most scientists believe that certain types of photosynthesis were the sources of this oxygen, says Kasting, but some have wondered whether sunlight could break apart atmospheric water (H2O), liberating oxygen. His model suggests this process could not have produced significant amounts of oxygen.

Ruling out geochemical oxygen production makes it easier to pinpoint when oxygen-producing photosynthesis began, says Heinrich D. Holland of Harvard University. "If we find evidence of oxygen at 3.5 billion years ago, it means that green-plant photosynthesis was already active at that time, and that's what we seem to be seeing although the evidence is still equivocal,' Holland says.

According to Walker, Kasting has made the idea of high carbon dioxide levels in the early atmosphere "more respectable' by precluding the possibility of a runaway greenhouse. "That,' says Walker "is going to stimulate new thinking, new research on geochemical processes and on chemical evolution.'