A runaway greenhouse Armageddon in which the oceans boil dry could theoretically happen on Earth, researchers claim.

The good news is that human activity on its own will probably not be enough to trigger such an end-of-the-world scenario in the near future.

Previously it was thought more energy from the sun would be needed for global warming on Earth truly to spin out of control.

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But new calculations from Canadian and US scientists show that catastrophic warming can occur more easily than had been assumed.

For a planet receiving the same amount of solar radiation as the Earth, a runaway greenhouse effect is a realistic possibility.

The team, led by Colin Goldblatt from the University of Victoria in Canada, wrote in the journal Nature Geoscience: "The runaway greenhouse may be much easier to initiate than previously thought.

"A renewed modelling effort is needed, addressing both Earth and planetary science applications."

To see what might happen to the Earth if it was ever caught in the grip of runaway global warming, it is only necessary to look next door.

Venus, our closest neighbour in space, is believed to have experienced a runaway greenhouse effect in the past.

Shrouded in a dense carbon dioxide atmosphere, it has an average surface temperature of around 460C - hot enough to melt lead.

The new study used a simplified model which did not take into account the effect of clouds. But it still suggests that under certain atmospheric conditions a stable Earth could switch to a runaway greenhouse state.

Looking back in the Earth's history reveals past episodes of global warming, but none involving a runaway greenhouse effect. There is no clue to the size of any "safety margin", said the scientists.

A carbon dioxide greenhouse effect caused the "hothouse" climate of the Eocene period 55 million years ago, when the Earth underwent more warming than at any previous time in its history. Average global temperature rose by up to 4C, and palm trees grew in the Arctic.

Eocene atmospheric carbon dioxide levels and temperature were both higher than what is expected in the foreseeable future from man-made greenhouse gas emissions, without triggering a runaway effect.

"This implies that an anthropogenic (human-caused) runaway greenhouse is unlikely," the scientists wrote.

PA