Some three to four billion years ago, Earth was about as warm as it is today, but the sun was much dimmer.

This so-called faint young sun paradox, first identified by astronomer Carl Sagan in the early 1970s, has flummoxed many a scientist—how could Earth be so temperate without a strong sun, when it should have been frozen over?

Now a new computer simulation published in the July issue of the journal Astrobiology has lent strong support to an old solution to the paradox. According to the authors of the study, Eric Wolf and Brian Toon of the University of Colorado at Boulder, the ancient Earth could have been kept warm by high atmospheric concentrations of carbon dioxide and methane. Those greenhouse gases compensated for the dimmer sun by trapping more of its heat in the atmosphere.

"There's been a lot of back and forth about this since Sagan," Wolf said. "But there's no one idea that the community grabs on to and says, 'This is correct.'"

Now, with the more advanced computer model, "I'm more confident of what the planet looked like."

Pink Skies of the Archean

There's no question that Earth was a very different place during the Archean eon, between 4 and 2.5 billion years ago.

The continents had not yet grown to their full size, so there was less land. On the other hand, there were many more active volcanoes spewing carbon dioxide into the atmosphere. The atmosphere contained little or no oxygen, because plants had not yet evolved and photosynthesizing microbes had not really taken hold.

But life had already begun. The oldest fossils, remains of single-celled microbes that lived in liquid water, are found in 3.5-billion-year-old rocks from western Australia. The oldest sedimentary rocks, which had to have been laid down in liquid water, are from Greenland and are around 3.8 billion years old.

So geologists are confident that Earth's average temperature was at least above freezing during the Archean. Some think the planet might even have been entirely ice-free, and thus much hotter than it is today—because so far, at least, none of the distinctive marks left by glaciers have been found in Archean rock formations.

And yet astronomers understand enough about how stars evolve to say with confidence that the Archean sun was only 70 to 80 percent as bright as it is today. That's the faint young sun paradox.

The most likely explanation has long been a higher concentration of greenhouse gases in the Archean atmosphere, with carbon dioxide the most obvious candidate. But when scientists tried to model how much carbon dioxide would have been needed to overcome the effects of a faint young sun, they got huge numbers. Just to nudge the planet above freezing, they found, would take around 30,000 parts per million (ppm), or 75 times as much as there is in the air today. Geochemical analyses of ancient rocks suggest there couldn't have been that much CO2.

Adding methane to the mix helps: It's a more potent greenhouse gas than CO2, and the ancestors of the bacteria that still spew it into the air today, from swamps and rice paddies, might have been present during the Archean.

But there was another limitation to the earlier models, Wolf said. They were one-dimensional, meaning they calculated temperature and radiation in only a single column of Earth's atmosphere. That saved on expensive computer time, but it made the results unreliable.

For a more realistic plunge into the deep past, Wolf and Toon used a supercomputer to run a climate model developed at the National Center for Atmospheric Research in Boulder—the same model that is used to forecast the future of our warming world. That model is fully three-dimensional: It includes clouds, winds, ice, and other geographically varying factors that have a big impact on climate.

With their more sophisticated model, the Colorado researchers found they could keep the Archean Earth from freezing over with a mere 15,000 ppm of carbon dioxide—still an extremely high number, but within the realm of geologic possibility. When they added 1,000 ppm of methane, they got a climate about as warm as today's.

"In a 3-D model, the problem goes away," Wolf said. He and Toon had to assume, though, that Earth had as much ice in the Archean as it does today, and that the evidence has been erased or just hasn't been found yet. Producing an ice-free hothouse with a faint sun would still require unreasonable levels of CO2.

Clouds, or the lack thereof, were one big reason the researchers' simulated Archean Earth didn't freeze entirely. Clouds are formed when sunlight evaporates water. With a dimmer sun, there was less sunlight reaching Earth's surface—but also fewer clouds to reflect sunlight back to space and cool the planet.

With more than 500 times as much methane in the atmosphere as there is today, however, the Archean sky might have contained a thin photochemical haze. That would have given it a pinkish tinge.

By the end of the Archean, the Earth would have lost that ruddy glow. Around 2.4 billion years ago, photosynthesizing microbes began pumping large amounts of oxygen into the atmosphere. If there was indeed a lot of methane in the atmosphere then, it would have been oxidized.

According to James Kasting of Penn State University, that may have produced a radical shift in Earth's climate. The geologic record suggests that, having endured the faint young sun without freezing, the planet entered its first global ice age around 2.3 billion years ago.

Habitable Exoplanets?

Climate scientists such as Wolf, Toon, and Kasting are probing Earth's ancient history for what it can teach them about our planet—but also about other planets. In recent decades astronomers have discovered hundreds of extrasolar planets, or exoplanets, orbiting other stars in our Milky Way galaxy. Just this spring they discovered two new ones that seem to be orbiting at the right distance from their star to have liquid water on their surfaces.

Understanding just what it takes to make a planet habitable has become an urgent and practical scientific question. (See "Think Outside the Box to Find Extraterrestrial Life.")