One third of the planet will be desert by the year 2100, say climate experts in the most dire warning yet of the effects of global warming writes Michael McCarthy, Environmental Editor in The Independent.

Drought threatening the lives of millions will spread across half the land surface of the Earth in the coming century because of global warming, according to new predictions from Britain's leading climate scientists.

Extreme drought, in which agriculture is in effect impossible, will affect about a third of the planet, according to the study from the Met Office's Hadley Centre for Climate Prediction and Research. It is one of the most dire forecasts so far of the potential effects of rising temperatures around the world - yet it may be an underestimation, the scientists involved said yesterday.

The findings, released at the Climate Clinic at the Conservative Party conference in Bournemouth, drew astonished and dismayed reactions from aid agencies and development specialists, who fear that the poor of developing countries will be worst hit.

"This is genuinely terrifying," said Andrew Pendleton of Christian Aid. "It is a death sentence for many millions of people. It will mean migration off the land at levels we have not seen before, and at levels poor countries cannot cope with." One of Britain's leading experts on the effects of climate change on the developing countries, Andrew Simms from the New Economics Foundation, said: "There's almost no aspect of life in the developing countries that these predictions don't undermine - the ability to grow food, the ability to have a safe sanitation system, the availability of water. For hundreds of millions of people for whom getting through the day is already a struggle, this is going to push them over the precipice."

The findings represent the first time that the threat of increased drought from climate change has been quantified with a supercomputer climate model such as the one operated by the Hadley Centre. Their impact is likely to even greater because the findings may be an underestimate. The study did not include potential effects on drought from global-warming-induced changes to the Earth's carbon cycle. In one unpublished Met Office study, when the carbon cycle effects are included, future drought is even worse. The results are regarded as most valid at the global level, but the clear implication is that the parts of the world already stricken by drought, such as Africa, will be the places where the projected increase will have the most severe effects.