Scientists have ruled out a theory as to why Earth was warm enough to sustain the planet's earliest life forms when the Sun's energy was roughly three-quarters the strength it is today.
Life evolved on Earth during the Archean, between 3.8 and 2.4 billion years ago, but the weak Sun should have meant the planet was too cold for life to take hold at this time; scientists have therefore been trying to find an explanation for this conundrum, what is dubbed the 'faint, young Sun paradox.'
Study author, Dr Ray Burgess, from Manchester's School of Earth, Atmospheric and Environmental Sciences, said that during the Archean the solar energy received at the surface of the Earth was about 20 to 25 percent lower than present.
Burgess said that if the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread.
Lead author Professor Bernard Marty, from the CRPG-CNRS University of Lorraine, said that to counter the effect of the weaker Sun, carbon dioxide concentrations in the Earth's atmosphere would need to have been 1,000 times higher than present.
Marty asserted that however, ancient fossil soils - the best indicators of ancient carbon dioxide levels in the atmosphere - suggest only modest levels during the Archean .
He said that other atmospheric greenhouse gases were also present, in particular ammonia and methane, but these gases are fragile and easily destroyed by ultraviolet solar radiation, so are unlikely to have had any effect.
But another climate-warming theory - one the team wanted to test - is that the amount of nitrogen could have been higher in the ancient atmosphere, which would amplify the greenhouse effect of carbon dioxide and allow the Earth to remain ice-free.
The new study has been published in the journal Science.