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Global warming is irreversible, some of the world's top climate scientists have concluded.

Their research shows that carbon dioxide emitted from today's homes, cars and factories will continue heating the planet for hundreds of thousands of years.

Their findings - which contradict a widespread belief that the atmosphere would recover quickly once humanity stopped polluting it - come at the beginning of the most crucial week for the climate this year.

This week, Britain's Climate Change Committee will lay out a plan to put the country on track to slash its greenhouse gas emissions by 80 per cent by 2050.

And the same time, the world's governments will meet at Poznan, in Poland, to try to set the world on the path to agreeing on a new international treaty next year, billed as the last chance to keep global warming to tolerable levels.

The new research will add to the pressure on ministers to take radical steps. And it will add urgency to attempts to find ways of removing excess carbon dioxide already in the atmosphere, as well as trying to prevent further emissions.

It comes as a shock because most Governments, and even many scientists, have assumed that carbon dioxide emissions would work their way out of the atmosphere in about a century, enabling it to clean itself fairly rapidly once the world switched to clean sources of energy.

But one of the main researchers, Professor David Archer of Chicago University, says the effects on the climate of releasing fossil fuel carbon dioxide into the atmosphere will last "longer than Stonehenge, longer than time capsules, far longer than the age of human civilisation".

Ultimate recovery would take hundreds of thousands of years - 'a geologic longevity associated in public perceptions with nuclear waste".

Carbon dioxide leaves the atmosphere mainly through being soaked up by the oceans, but Professor Archer says the belief of climate science researchers and the public that this happens quickly is no longer accurate.

He and other leading scientists spell out why in a paper to be published in the journal Annual Reviews of Earth and Planetary Sciences.

"The ocean is getting fed up with absorbing our CO2," he says.

The surface waters, up to about 100m deep, which used to sop up the gas quite fast, are now getting saturated with it - turning acid in the process.

They need to be replaced with fresh water from deep down, but this overturning circulation can take up to a thousand years. And global warming is expected to slow this down; the hotter the surface layer becomes, the longer the replenishment takes.

Indeed, research shows that even this renewing process will not be enough to remove all the vast amounts of carbon dioxide that humanity is now adding to the atmosphere. Much of it will have to wait hundreds of thousands of years before being removed by another, infinitely slower, process, the natural weathering of rocks, which incorporates the gas into other substances. And the more pollution that is emitted now, the worse this will become.

Research by another of the paper's authors, Professor Ken Caldeira of the Carnegie Institute for Science, in Stanford, California, adds another alarming twist to the story.

He was surprised to find that even after the pollution stops, the Earth's temperature will not fall but will settle at a higher level.

Professor James Hansen, director of Nasa's Goddard Institute of Space Studies, and perhaps the world's leading climate scientist, warns that the "long lifetime of carbon dioxide emitted by fossil fuel burning" means that slowing down emissions is no solution.

Instead, he says, some of the fuels must "be left in the ground" forever, and carbon dioxide must be removed from the air.

He proposes removing the gas by growing trees, which soak it up as they grow, and then burning them to produce electricity, capturing the gas before it is emitted.

He also proposes that no more coal-fired power stations be built.

The report by the British Climate Change Committee - chaired by Lord Turner, who also heads the Financial Services Authority - is expected to discourage the construction of new coal-fired stations, such as one already proposed for Kent.

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