July 1, 2009 -- Vegetation helped save Earth from runaway cooling that would have encased the planet in ice, according to a study published on Wednesday.

The paper sheds light on the big natural mechanisms that over hundreds of millions of years have swung the globe like a pendulum between deep chill and intense heat.

Around 50 million years ago, the planet's poles were ice-free and crocodiles roamed the Arctic.

But that was followed by a long period of cooling, in which levels of carbon dioxide (CO2), the principal "greenhouse" gas that traps solar heat, progressively declined.

Belching volcanoes provided the main source of this CO2 -- in contrast to today, when the gas comes overwhelmingly from burning fossil fuels and is driving dangerous climate change.

But there was also a force which removed CO2: a chemical reaction that occurs when silica rocks are weathered.