Dec. 9, 2002 In the absence of oxygen, methane may have been the most abundant greenhouse gas in Earth's atmosphere, reaching a stable balance between a greenhouse and anti-greenhouse until oxygen producing single-celled organisms burst upon the scene, according to Penn State researchers.
"Today, methanogenic bacteria grow in anaerobic environments such as the intestines of ruminants and the waterlogged soils underlying rice paddies," says Dr. James F. Kasting, professor of geosciences and meteorology. "In the Archean, 2.5 billion years ago, this type of bacteria could live anywhere because there was little free oxygen in the atmosphere."
Estimates suggest that there were 1,000 parts per million methane in the Archean atmosphere or one tenth of one percent of the atmosphere was methane then. Today, the atmosphere has about 1.6 parts per million of methane because in the presence of oxygen, methane rapidly reacts.
"We have every reason to believe that methanogenic bacteria were around in the Archean," Kasting told attendee at the fall meeting of the American Geophysical Union today (Dec. 6) in San Francisco. "They look evolutionarily ancient."
Some type of greenhouse gas was needed to warm the Earth enough so that water would not freeze. This is especially true since the Sun was much cooler then than it is now. The Faint Young Sun Paradox was originally solved by suggesting that carbon dioxide was the greenhouse gas warming the Earth in the Archaean, but studies of paleosols, ancient soil surfaces and theoretical studies of how carbon dioxide is sequestered in the solid Earth, suggest that carbon dioxide levels were actually low 2.5 billion years ago.
Greenhouse gases can warm the Earth because they let sunlight through the atmosphere, but they retard the flow of outgoing infrared (heat) radiation. By doing so, they warm the globe.
Most methanogenic bacteria are thermophilic, liking a warm environment. When methane first began to build up in the atmosphere and the Earth warmed, these bacteria happily grew and produced more methane. The warmer it got, the happier the hyperthermophilic methanogens became, creating a positive feedback loop where warmer temperatures meant more methane and consequently still warmer temperatures. Warmer temperature also causes carbon dioxide concentrations to decrease because they speed up the rate at which it reacts with surface rocks. Eventually the atmosphere should have contained as much methane as carbon dioxide. When this point was reached, sunlight began to polymerize the methane, forming small hydrocarbon particles.
According to Kasting and Dr. A. A. Pavlov, a recent Penn State Ph.D. graduate currently at the University of Colorado, the Earth would have had a thin layer of hydrocarbon smog surrounding it the way Titan, Saturn's moon, does today. As the smog built up, less sunlight would enter the atmosphere and the Earth would cool slightly under these anti-greenhouse conditions. As the Earth cooled, methane production would drop and the smog layer would thin, allowing more sunlight in and warming the Earth. This negative feedback loop would create an optically thin hydrocarbon haze in the atmosphere, which mediated temperatures and lasted until oxygen produced by cyanobacteria began to accumulate in the atmosphere.
"We are trying to understood how the Earth's climate worked during the first half of the planet's history," says Kasting, a member of Penn State's NASA-sponsored Astrobiology Research Center. "We are also looking for ways to identify planets in other solar systems that might contain life."
In about 15 years, NASA plans to build the Terrestrial Planet Finder, a probe to locate life on planets around other stars. If free oxygen is available in planetary atmospheres, it is likely that life exists, but the presence of methane or a thin hydrocarbon smog might also indicate a planet on which life was present.
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