We know by studying ancient rocks that liquid water existed on the surface of Earth at least 3.7 billion years ago. That implies that the surface temperature at that time was at least 273K.
We also know by studying stars similar to ours that the Sun must have been significantly less bright than it is now (Sol is thought to have increased in luminosity by 30 per cent since then). That ought to have resulted in temperatures on Earth that were well below freezing.
This contradiction is known as the “faint young Sun” problem and nobody has adequately explained it.
The most common explanation is that the planet must have been warmed by some kind of greenhouse gas effect mediated perhaps by carbon dioxide, atmospheric haze, ammonia or methane. The trouble is that the evidence indicates that these gases existred in quantities at least an order of magnitude too little to have done the job.
So what gives? According to Philip von Paris of the Institute of Planetary Research at the German Aerospace Centre in Berlin, Germany, und Freunds, the mix up is largely the result of an error in our understanding of how much radiation was absorbed in those days. His team has used a new atmospheric model of the early Earth to determine that the required green house effect would have been possible with carbon dioxide with a partial pressure of about 2.9mb, about an order of magnitude less than previously thought.
That’s a pretty good match with the amount of carbon dioxide thought to have been around between 2 and 2.5 billion years ago. Nice result.
But von Paris has been a little over ambitious. He says: “thus, the contradiction between sediment data
and model results disappears,” implying that the faint young Sun problem is solved.
But hang on a minute, he seems to have forgotten the billion years or so between 2.5 and 3.7 billion years ago in which the temperature is still unexplained.
Nice try, von Paris. But we’re not that easily foooled.
Ref: arxiv.org/abs/0804.4134: Warming the early Earth – CO2 reconsidered