Higher levels of atmospheric nitrogen ― almost double today's levels ― were the key to warming the early Earth, suggests a paper published online this week in Nature Geoscience. The paper could help shed light on why the Earth was not covered in glaciers 2.5 billion years ago even though the Sun was much weaker than it is today, a problem known as the "faint young Sun paradox."

Colin Goldblatt, Tim Lenton, and colleagues used numerical modelling to show that, although atmospheric nitrogen itself is not a greenhouse gas, higher levels of atmospheric nitrogen gas would increase the potency of greenhouse gases such as carbon dioxide in the early atmosphere. A doubling of present-day atmospheric nitrogen levels would lead to a warming of 4.4℃.

The authors suggest that the excess nitrogen has since been removed from the atmosphere by biological activity and chemical reactions in the deep ocean, and that it is now stored in the Earth's crust and mantle.