A new model of the effects of global warming predicts that wheat crop production could drop by a third in drought-prone countries and those of low latitude. But crop production could surge elsewhere around the globe.

The study, soon to be published in Science, predicts that Brazil will lose 35 per cent and Egypt about 30 per cent of their potential to produce wheat over the next 50 years. But China will be able to grow 15 per cent more wheat and potential production in the Canadian prairies and the southern Soviet Union will also increase.

Martin Parry, of the University College London geography department, one author of the study, said that the latest predictions show that global temperature would change at twice the rate that ecosystems can adapt.

He was speaking at a London meeting at which scientists agreed that there is no evidence to challenge the central idea of the greenhouse effect - that human activity is pumping gases into the atmosphere causing it to heat up.

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David Carson, director of the Hadley Centre for climate change, said: "The most recent modelling results strengthen my concern about the prospect of climate change in response to man's influence and give me more confidence in models' ability to simulate it."

The centre published a report this week on its efforts to include the effect of sulphur dioxide emissions in its global climate computer models. Sulphur dioxide emissions have masked the greenhouse effect because they turn into sulphate aerosols which cool the atmosphere by reflecting some of the sun's rays away from earth.

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By including sulphur dioxide emissions, the model now predicts an increase in temperature of 0.2C, rather than 0.3C, per decade - more in line with observations. But the effect of sulphur dioxide is transient: "If we stopped producing it today then within a few weeks we would expect it to have lost its influence," said Dr Carson.

Chris Folland, also of the Hadley Centre, said that evidence for global warming includes a new report on glaciers, showing that most are continuing to recede.