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Last Updated: Monday, 2 February, 2004, 15:22 GMT
Earth 'shook off' ancient warming
Rockface near Whitby in Yorkshire     Cohen
The researchers studied mudrock near Whitby in Yorkshire
UK scientists claim they now know how Earth recovered on its own from a sudden episode of severe global warming at the time of the dinosaurs.

Understanding what happened could help experts plan for the future impact of man-made global warming, experts say.

Rock erosion may have leached chemicals into the sea, where they combined with carbon dioxide, causing levels of the greenhouse gas to fall worldwide.

UK scientists report the details of their research in the journal Geology.

What we have learned from these rocks is how the Earth can, over a long time, combat global warming
Dr Anthony Cohen, Open University
About 180 million years ago, temperatures on Earth rapidly shot up by about 5 Celsius.

The cause is thought to have been a sudden release of huge amounts of methane from the sea bed. Methane is itself a greenhouse gas but it is short-lived.

However, it is easily oxidised to carbon dioxide (CO2) which lingers in the atmosphere for long periods of time.

Mass extinction

Plants and animals were affected by the sudden rise in atmospheric CO2. Scientists have found evidence of a marine mass extinction during this period that killed off 84% of bivalve shellfish.

Gas hydrate     USGS
Vast reserves of methane still remain in ocean sediments
Over a period of about 150,000 years, the Earth returned to normal and life continued flourishing. How this happened was a mystery, but now scientists from the Open University in Milton Keynes claim to have a possible answer.

"Our new evidence has shown that this warming caused the weathering of rocks on the Earth's surface to rapidly increase by at least 400%," said Dr Anthony Cohen, who led the research.

"This intense rock-weathering effectively put a brake on global warming through chemical reactions that consumed the atmosphere's extra carbon dioxide."

They discovered that intense rock weathering coincided with warm conditions and high atmospheric CO2.

'Methane burp'

Weathering occurs through the action of rain. Although the researchers did not uncover direct evidence for increased precipitation, they believe there were no limitations on water during the period.

The warm conditions caused by the "methane burp" would have sped up the rate at which weathering occurred. This led to minerals such as calcium and magnesium eroding from rocks and pouring into the sea.

Calcium combined with CO2, for instance, would have caused the precipitation of calcium carbonate. This process of CO2 consumption would have lowered levels of the greenhouse gas on a global scale.

As CO2 levels fell, so did global temperatures.

"Global warming is affecting the climate today, but it's very difficult to predict what's going to happen," Dr Cohen told BBC News Online.

"The reason for doing these studies is that you get the whole history. If you learn what happened then, that can inform how you deal with [the same problem] in future."

Dr Cohen added that there are still vast reserves of carbon - possibly as much as 14,000 gigatons - locked up as methane ice in ocean sediments.

If global temperatures reach a critical point, it is possible they might suddenly be released into the atmosphere causing a similar event to the one that occurred during the Jurassic.

"What we have learned from these rocks is how the Earth can, over a long time, combat global warming. What we need to discover now is why and at what point it goes into combat mode, and precisely how long the conflict takes to resolve," he explained.

Dr Cohen and his colleagues based their results on studies of mudrock rich in organic material and collected near Whitby in North Yorkshire.

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