Dan Dry

Hard evidence.
Iron compounds in this 3.77-billion-year-old rock suggest that Earth's atmosphere was far richer in CO2 than it is now.

The Good Greenhouse

Carbon dioxide has received quite a bit of bad press recently because of its connection with atmospheric warming. But in another epoch, the gas might have prevented Earth from plunging into a permanent deep freeze, according to a new study.

During its first billion years, Earth was a hellish place. Nearly constant volcanic eruptions pumped out huge amounts of lava, gas, and dust. Despite all this heat, however, Earth eventually cooled significantly and seemed destined for a fate similar to that of Mars, with temperatures falling well below 0 degrees Celsius. This big chill would have locked the planet's water into permanent ice caps and probably precluded the emergence of life. The reason: Back then, the sun was 25% to 30% dimmer than it is now, and less solar energy reached our planet. Yet Earth's average temperature somehow managed to remain above 0 degrees Celsius and allowed oceans to form in a phenomenon called "the faint young sun paradox."

Scientists have long suspected that atmospheric CO2 was the key to that paradox. But now a team of researchers claims to have found conclusive data in ancient sedimentary rocks discovered by Canadian geologists in 2001 in northern Quebec. The rocks are at least 3.77 billion years old, according to chemical dating tests, the researchers found, and contain iron carbonates. "The only way to make those carbonates in a marine environment was in an atmosphere that was loaded with CO2," says geologist Stephen Mojzsis of the University of Colorado in Boulder. In the 28 February issue of Earth and Planetary Science Letters, Mojzsis and colleagues conclude, based on the composition of the rocks, that the CO2 content of early Earth's atmosphere was at least 1000 times higher than it is today--the byproduct of volcanic activity. That much CO2 would have gone a long way toward preserving the planet's heat, the scientists say.

The remaining wild card in the process is how much methane ancient volcanoes expelled, because methane also would have contributed to the warming, Mojzsis says. That answer will require further testing, he says, "to see if there's a methane signal in the rocks."

The iron carbonate found in the Quebec rocks strongly suggests a high CO2 content in early Earth's atmosphere that would have caused substantial warming and confirms the existence of liquid water, enough to produce seas from which the rocks precipitated, says geoscientist Jim Kasting of Pennsylvania State University in State College. "It's hard to see otherwise," he adds. Regarding the role of methane, Kasting says it probably did help to elevate Earth's temperature, but to a lesser extent than CO2. "If there was too much methane (in the atmosphere), it would have created a haze and counteracted the greenhouse effect," he says.

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