Earth may have been warm enough to support life more than 3 billion years ago, even though the Sun was 20 per cent dimmer than today, a new study has found.
Researchers from the University of Colorado Boulder say all that was required to sustain liquid water and primitive life on Earth during the Archean eon 2.8 billion years ago were reasonable atmospheric carbon dioxide amounts believed to be present at the time and perhaps a dash of methane.
The key to the solution was the use of sophisticated three-dimensional climate models that were run for thousands of hours on CU's Janus supercomputer, said doctoral student Eric Wolf, lead study author.
"It's really not that hard in a three-dimensional climate model to get average surface temperatures during the Archean that are in fact moderate," said Wolf.
"Our models indicate the Archean climate may have been similar to our present climate, perhaps a little cooler. Even if Earth was sliding in and out of glacial periods back then, there still would have been a large amount of liquid water in equatorial regions, just like today," he said.
Evolutionary biologists believe life arose on Earth as simple cells roughly 3.5 billion years ago, about a billion years after the planet is thought to have formed.
Scientists have speculated the first life may have evolved in shallow tide pools, freshwater ponds, freshwater or deep-sea hydrothermal vents, or even arrived on objects from space.
Wolf and CU-Boulder Professor Brian Toon used a general circulation model known as the Community Atmospheric Model version 3.0 developed by the National Center for Atmospheric
Research in Boulder and which contains 3-D atmosphere, ocean, land, cloud and sea ice components.
The two researchers also "tuned up" the model with a sophisticated radiative transfer component 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, where there was no oxygen and no ozone, but lots of CO2 and possibly methane.
"Our results indicate that a weak version of the faint young sun paradox, requiring only that some portion of the planet’s surface maintain liquid water, may be resolved with moderate greenhouse gas inventories," researchers wrote in Astrobiology.
"Even if half of 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 per cent of the ocean would have remained open," said Wolf.