An artist's conception of the Earth about 2.8 billion years ago. (Charlie Meeks )
The "faint young sun paradox" has been stumping scientists for decades, but in a new study two researchers say maybe the paradox isn't so paradoxical after all.
The faint young sun paradox, or problem, was first brought to the attention of the scientific community in 1972 by Carl Sagan and George Muller. At issue were two facts that were difficult to reconcile.
Fact one: The first microbial life appears in the fossil record about 3.8 billion years ago, suggesting that liquid water existed on the planet at the time.
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Fact two: The sun, which has grown steadily brighter throughout its life, was 30% less bright 3.8 billion years ago than it is today.
So how was the early Earth warm enough to support life - and liquid water - at the same time that the sun was so dim?
That's the faint young sun paradox.
Over the years different solutions to the puzzling problem have been proposed. Some have suggested that while the sun may have been dimmer, the solar wind was stronger. Others have suggested that high levels of carbon dioxide and methane gas in the atmosphere created a greenhouse effect, trapping heat on the planet.
But no one has been able to create a successful model that conclusively solves the problem until, possibly, now.
In a paper published in the July issue of Astrobiology, University of Colorado doctoral student Eric Wolf found that if you make a complex 3-D climate model of the Earth 2.8 billion years ago, when the sun was 20% less bright than it is today, the faint young sun paradox is not that difficult to solve.
Previous climate models of early Earth were one dimensional and didn't include important climate factors such as clouds, wind and sea ice, Wolf said in an interview with the Los Angeles Times.
"In the paradigm of the one-dimensional models, it is either the whole world is frozen or the whole world is nice and warm," he said.
But in Wolf's 3-D model of the late Archean time period, it doesn't have to be one or the other. It might be freezing at the ice caps and at the same time warm enough to support liquid water at the equator.
"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% of the ocean would have remained open," Wolf said in a statement.
To create the model, Wolf plugged what is known about the Earth 2.8 billion years ago into a computer model that is usually used to forecast climate change in the present day.
2.8 billion years ago there was no oxygen and no ozone, but there was a lot more carbon dioxide in the atmosphere, as well as much more methane gas, he said. There was also less land and fewer clouds.
Wolf said he couldn't tell exactly what the climate was like on early Earth - there are still too many variables he cannot be sure of like the exact amount of CO2 in the atmosphere and how much methane there was - but he can say that the faint young sun paradox is much easier to solve if you use a complex 3-D model.