London, May 12 : Scientists have built a model which suggests that the Earth didn't freeze under a faint young sun between 2 and 2.5 billion years ago because the planet's atmosphere at that time was itself a better insulator than earlier believed, with carbon dioxide (CO2) keeping the temperatures above freezing point.
According to a report in New Scientist, 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 ¸C.
But ancient rocks show that liquid water existed, proving that temperatures must have been above freezing.
Though this can be explained if greenhouse gases acted as an insulator, 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.
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.