COPENHAGEN - The Amazonian rainforest is likely to suffer catastrophic damage even with the lowest temperature rises forecast under climate change, researchers have found.

In this photo released by Spectral Agency, more than a thousand indigenous from around the world create a human banner that reads in Portuguese 'Save the Amazon' and a silhouette of an indigenous warrior during a demonstration marking the beginning of the World Social Forum, in Belem, Brazil, Tuesday, Jan. 27, 2009. (AP Photo/ Spectral Agency, Lou Dematteis) Damage will be so severe that it will cause irreversible changes to the world's weather patterns, which would be expected to bring more storms, floods and heatwaves to Britain.

Up to 40 per cent of the rainforest will be lost if temperature rises are restricted to 2C, which most climatologists regard as the least that can be expected by 2050.

Researchers issued the assessment of the forest's fragility after discovering a deadly time lag in the effects of temperature rises on the forest.

It had previously been thought that the trees and other vegetation, and the vast range of animals living among them, would be safe if temperatures rose no more than 2C.

Researchers have now found that even 2C will destroy large tracts of the forest but that the die-back is slow and will take up to a century to reap its full effect.

A 3C rise is likely to result in 75 per cent of the forest disappearing while a 4C rise, regarded as the most likely increase this century unless greenhouse gas emissions are slashed, will kill off 85 per cent of the forest.

Dr Chris Jones, of the Met Office Hadley Centre in Exeter, told a scientific conference in Copenhagen, that the time delay had masked the full impact of temperatures.

He led a team of researchers which calculated that 20 to 40 per cent of the forest will be killed off by 2050 under a 2C rise.

Dr Vicky Pope, the head of climate change advice at the Met Office Hadley Centre, said the findings showed the threat to the forest is much higher than expected.

"Impacts could be much worse than previously thought," she said in Copenhagen as scientists met to discuss the latest research into climate change and its effects.

"Even if temperature rises are limited to 2C above pre-industrial levels, as much as 20 to 40 per cent of the Amazonian rainforest could be lost if this temperature is sustained for 100 years or more."

Other research unveiled at the conference showed that there is a significant chance that, even if greenhouse gas emissions are reduced, temperatures may not start falling for at least a century.

Professor Peter Cox, of the University of Exeter, said of the finding that at least a fifth of the Amazonian rainforest is almost certainly doomed: "Ecologically it would be a catastrophe and it would be taking a huge chance with our own climate.

"The Tropics are drivers of the world's weather systems and killing the Amazon is likely to change them forever. We don't know exactly what would happen but we could expect more extreme weather. Destroying the Amazon would also turn what is a significant carbon sink into a significant source.

"It would amplify global warming significantly. Just as an example at the moment deforestation adds about a fifth of the world's carbon to the atmosphere."