Climatologists poring over Greenland's ancient past say global cooling, unleashed by a fall in atmospheric greenhouse gases, caused the vast island to ice over around three million years ago.

In a study released Wednesday, the British research team say that for aeons, Greenland was mostly ice-free and may have hosted grasslands and forests before it became smothered in a thick, glacial crust in a relatively short time.

The ice sheet can only be explained by a decrease in naturally-occurring, heat-trapping carbon gases in the atmosphere, they say.

Over a period of around 300,000 years, concentrations of greenhouses gases fell by more than a third, to around the same level as before the start of the Industrial Revolution in the late 18th century, they calculate.

Their conclusions are based on a powerful computer model that crunched data about the circulation of the atmosphere and ocean and recreated the growth of the Greenland icesheet, starting from a small white spot on the eastern highlands to a deep slab covering virtually all of the island.

The scientists, who published their findings in the London-based science journal Nature, looked at three other competing theories for explaining the coming of the ice.

These include a change in ocean circulation that blocked the supply of warm sea currents to Greenland; the uplifting of the Rocky Mountains, which deflected the cold jetstream of air over Greenland; and changes in Earth's orbit, which influenced the amount of solar heat reaching our planet.

Such factors did affect the amount of ice cover, but not enough to contribute to the massive, long-term growth of an ice sheet, the researchers contend.

Lead author Dan Lunt, from the University of Bristol in western England, said a reverse greenhouse effect clearly played the major role in Greenland's glaciation, but how it happened remained unclear.

Among the ideas being kicked around is geological weathering, in which certain kinds of rock, undergoing chemical change, absorb more carbon dioxide (CO2), he told AFP.

Understanding why would be a vital insight into the mechanisms of climate change, he said.

"There is a huge amount of uncertainty as to why there are big, natural swings in CO2 levels," he said.

Lunt said there were also implications for today's problems of man-made global warming, stoked by the burning of fossil fuels.

He noted that greenhouse-gas levels before the Industrial Revolution were 280 parts per million (ppm), and now stand at around 385 ppm -- just shy of the 400 ppm that prevailed in Greenland's pre-ice era.

"It's clear from our work that the Greenland ice sheet is very sensitive to carbon dioxide levels, but it's wrong to draw inferences about what will happen," he said.

"You can't say that if you get to the level of 400 ppm, the ice sheet will melt in the next hundred years, because we don't know enough about the process. It might be easier to create an icesheet than to melt. If you reverse what you did before, you don't necessarily get the same answer back."

Next to Antarctica, Greenland is the biggest source of land ice in the world.

On August 21, US researchers, adding to concern about the state of Greenland's ice cover in the face of higher temperatures, said the island's two largest glaciers had lost at least 40 square kilometers (14 square miles) of ice since the last melt season.