Early life on Earth may have survived some 2.8 billion years ago through an atmosphere equipped with reasonable amounts of carbon dioxide and just a dash of methane, a new study suggests.
The period, known as the Archean eon, was characterized in part by a Sun approximately 20 percent dimmer than today's.
Conducted by researchers at the University of Colorado, Boulder, the scientists point to the use of sophisticated three-dimensional climate models running for thousands of hours on the school's supercomputer as a key to their success in coming up with viable hypotheses to a question that has long plagued researchers.
"It's not really that hard in a three-dimensional climate model to get average surface temperatures during the Archean that are in fact moderate," lead study author Eric Wolf, a doctoral student at the university's atmospheric and oceanic sciences department, said in a press release. "Our models indicate the Archean climate may have been similar to our present climate, perhaps a little cooler."
Even if the planet was moving in and out of glacial periods at the time, Wolf says there still would have been ample liquid water in the equatorial regions.
This is important because evolutionary biologists believe life arose on Earth as simple cells 3.5 billion years ago, approximately one billion years after the planet is estimated to have formed, possibly in shallow tide pools, freshwater ponds or deep-sea hydrothermal vents.
The fact that life apparently progressed despite a significantly weaker Sun is called the "faint young Sun paradox" and it has confounded scientists since 1972 when two Cornell University scientists first broached the subject. Since then, many studies using one-dimensional climate models have come up with possible solutions ranging from a hot, tropical planet to one typified by runaway glaciation.
"In our opinion, the one-dimensional models of early Earth created by scientists to solve this paradox are too simple - they are essentially taking the early Earth and reducing it to a single column atmospheric profile," said Brian Toon, a professor at the University of Colorado and Wolf's research team member. "One-dimensional models are simply too crude to give an accurate picture."
To overcome these limitations, Wolf and Toon used a general circulation model known as the Community Atmosopheric Model version 3.0 developed by the National Center of Atmospheric Research in Boulder, which contains three-dimensional atmosphere, ocean, land and cloud components. The researchers also "tuned up" the model with a sophisticated radiative transfer aspect that allowed for the absorption, emission and scattering of solar energy and an accurate calculation of the greenhouse effect for the unusual atmosphere of early Earth.
The simplest solution the researchers found involves roughly 20,000 parts per million of the greenhouse gas carbon dioxide and 1,000 ppm of methane.
"Even if half of the Earth's surface was below freezing back in the Archean and half was above freezing, it still would have constituted a habitable planet since at least 50 percent of the ocean would have remained open," Wolf said. "Most scientists have not considered that there might have been a middle ground for the climate of the Archean."