Devastating changes to sea levels, rainfall, water supplies, weather systems and crop yields are increasingly likely before the end of the century, scientists will warn tomorrow.
A special report, to be released at the start of climate negotiations in Cancún, Mexico, will reveal that up to a billion people face losing their homes in the next 90 years because of failures to agree curbs on carbon emissions.
Up to three billion people could lose access to clean water supplies because global temperatures cannot now be stopped from rising by 4C.
"The main message is that the closer we get to a four-degree rise, the harder it will be to deal with the consequences," said Dr Mark New, a climate expert at Oxford University, who organised a recent conference entitled "Four Degrees and Beyond" on behalf of the Tyndall Centre for Climate Change Research. Tomorrow the papers from the meeting will be published to coincide with the start of the Cancún climate talks.
A key feature of these papers is that they assume that even if global carbon emission curbs were to be agreed in the future, these would be insufficient to limit global temperature rises to 2C this century – the maximum temperature rise agreed by politicians as acceptable. "To have a realistic chance of doing that, the world would have to get carbon emissions to peak within 15 years and then follow this up with a massive decarbonisation of society," said Dr Chris Huntingford, of the Centre for Ecology and Hydrology in Oxfordshire.
Few experts believe this is a remotely practical proposition, particularly in the wake of the failure of the Copenhagen climate talks last December – a point stressed by Bob Watson, former head of the Intergovernmental Panel on Climate Change and now chief scientist at the Department for Environment, Food and Rural Affairs. As he put it: "Two degrees is now a wishful dream."
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Researchers such as Richard Betts, head of climate impacts at the Met Office, calculate that a 4C rise could occur in less than 50 years, with melting of ice sheets and rising sea levels.
According to François Gemenne, of the Institute for Sustainable Development and International Relations in Paris, this could lead to the creation of "ghost states" whose governments-in-exile would rule over scattered citizens and land lost to rising seas.
Small island states such as Tuvalu and the Maldives are already threatened by inundation. "What would happen if a state was to physically disappear but people want to keep their nationalities?" he asked. "It could continue as a virtual state even though it is a rock under the ocean."
Peter Stott of the Met Office said the most severe effect of all these changes is likely to involve changes to the planet's ability to soak up carbon dioxide. At present, around 50% of man-made carbon emissions are absorbed by the sea and by plants on land.
"However, the amount of carbon dioxide that can be absorbed decreases as temperatures rise. We will reach a tipping point from which temperatures will go up even faster. The world will then start to look very different."