BILLIONS of years ago, a weaker sun should have made the Earth a chilly place 鈥� so why was it balmy instead?
The sun that shone on the early Earth was around 25 per cent dimmer than today, so atmospheric temperatures should have been colder by around 25 掳C. But ancient rocks show that liquid water existed, proving that temperatures must have been above freezing.
This can be explained if greenhouse gases acted as an insulator 鈥� but modelling has showed that carbon dioxide in the atmosphere would need to have had an implausibly high partial pressure of over 50 millibars to trap the heat.
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Now Philip von Paris of the German Aerospace Centre in Berlin and colleagues have built a model that they say better simulates the types, pressures and layering of atmospheric gases at that time. This showed that the atmosphere itself was a better insulator than we thought, and CO2 with a partial pressure of only 2.9 millibars would have kept temperatures above freezing between 2 and 2.5 billion years ago. The work will appear in Planetary and Space Science.
Unfortunately, this still doesn鈥檛 resolve the paradox between 2.5 and 4.6 billion years ago, when the sun was even weaker, and not everyone is convinced the researchers are right. 鈥淚 don鈥檛 trust the tuning of their model,鈥� says Jim Kasting of Pennsylvania State University at University Park.
