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 effectively impossible, will affect nearly 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 direst forecasts so far of the potential effects of rising temperatures around the world - yet it may be an underestimate, 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 huge migration off the land at levels we have not seen before, and at levels poor countries cannot cope with. It will mean huge conflict."
One of Britain's leading expert on the effects of climate change on the developing countries, Andrew Simms from the New Economics Foundation, said last night: "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. I think that 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, long feared, has actually been quantified with a modern supercomputer climate model such as the one operated by the Hadley Centre.
Their impact will be all the greater from the fact they may well be an underestimate, as the study did not include potential effects on drought from global-warming-induced changes to the earth's carbon cycle, such as forests dying back in a warming world.
In one unpublished Met Office study, when the carbon cycle effects are included, future drought is even worse.
The current results are regarded as most valid at the global level and so far there is less confidence in them giving a regional picture, 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 severest effects.
The study, by Dr Eleanor Burke and two Hadley Centre colleagues, models how a widely-used measure of drought known as the Palmer Drought Severity Index (PDSI) is likely to increase globally during the coming century with predicted changes in rainfall and heat around the world because of climate change.
It shows the PDSI figure for moderate drought, currently at 25 per cent of the earth's surface, rising to 50 per cent by 2100, the figure for severe drought, currently at about eight per cent, rising to 40 cent, and the figure for extreme drought, currently three per cent, rising to 30 per cent.
Senior Met Office scientists are sensitive about the study, funded by the Department for Environment, Food and Rural Affairs (Defra), stressing it contains a number of uncertainties: there is only one climate model involved, one future scenario for emissions of greenhouse gases (a moderate-to-high one) and one drought index.
Other studies might give different results.
Nevertheless, the result is "significant", according to Dr Vicky Pope, head of the Hadley Centre's climate programme.
Further work would now be taking place to try and reduce the uncertainties and get a handle on the potential risk of different levels of drought in different places, she said.
The full study - ITALS Modelling the recent evolution of global drought and projections for the 21st century with the Hadley Centre climate model OFFITALS - will be published later this month in ITALS The Journal of Hydrometeorology OFFITALS.
It will be widely publicised by the British Government at the negotiations in Nairobi in November on a successor to the Kyoto climate treaty.
But a preview of it was given by Dr Burke yesterday in a presentation to the Climate Clinic, the roadshow-cum-think-tank formed to press politicians for tougher action on climate change by Britain's major environmental groups, with The Independent as media partner.
The Climate Clinic has been in operation at all three party conferences.
Dr Burke agreed that the predictions in her study were possibly conservative and that an unpublished study she had done which included the global carbon cycle had shown more severe drought still in coming decades, although she declined to give details of it.
While the present study in its entirety will be widely seen as a cause for great concern, it is the figure for the increase in extreme drought that some observers find most frightening.
"It strikes me as overwhelming that we're talking about 30 per cent of the world's land surface becoming essentially uninhabitable in terms of agricultural production in the space of a few decades," said Mark Lynas, the author of ITALS High Tide OFFITALS, the first major account of the visible effects of global warming around the world.
"These are parts of the world where hundreds of millions of people will no longer be able to feed themselves where they live, and will need to migrate to areas of the world that are still inhabitable.
"And it is such a crucial point that this is a conservative prediction and does not include the real rate at which carbon dioxide [the major greenhouse gas] will accumulate in the atmosphere."
Andrew Pendleton of Christian Aid said: "This means you're talking about any form of development going straight out of the window.
The vast majority of poor people in the developing world are small-scale farmers who have no technology and rely on rain for food and livelihood and survival; they're in a position where if you push them only slightly, they're going to fall off.
If we fail to take action to stop this we are committing what in my mind is effectively genocide."