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By Steve Connor, Science Editor

Global warming over the past quarter century has led to a fall in the yield of some of the most important food crops in the world, according to one of the first scientific studies of how climate change has affected cereal crops.

Rising temperatures between 1981 and 2002 caused a loss in production of wheat, corn and barley that amounted in effect to some 40 million tons a year - equivalent to annual losses of some £2.6bn ($US 5,051,748,000).

Although these numbers are not large compared to the world-wide production of cereal crops, scientists warned that the findings demonstrated how climate change was already having an impact on the global production of staple foods. "Most people tend to think of climate change as something that will impact the future, but this study shows that warming over the past two decades has already had real effects on global food supply," said Christopher Field of the Carnegie Institution in Stanford, California.

The study, published in the journal Environmental Research Letters, analysed yields of cereals from around the world during a period when average temperatures rose by about 0.7C between 1980 and 2002 - although the rise was even higher in certain crop-growing regions of the world.

There was a clear trend, showing the cereal crops were suffering from lower yields during a time when agricultural technology, including the use of chemical fertilisers and pesticides, became more intensive. The study's co-author, David Lobell of America's Lawrence Livermore National Laboratory in California, said that the observed fall in cereal yields could be clearly linked with increased temperatures during the period covered by the study.

"Though the impacts are relatively small compared to the technological yield gains over the same period, the results demonstrate that negative impacts of climate trends on crop yields at the global scale are already occurring," Dr Lobell said.

The two scientists analysed six of the most widely grown crops in the world - wheat, rice, maize, soybeans, barley and sorghum. Production of these crops accounts for more than 40 per cent of the land in the world used for crops, 55 per cent of the non-meat calories in food and more than 70 per cent of animal feed.

They also analysed rainfall and average temperatures for the major growing regions and compared them against the crop yield figures of the Food and Agriculture Organisation for the period 1961 to 2002.

"To do this, we assumed that farmers have not yet adapted to climate change, for example by selecting new crop varieties to deal with climate change," Dr Lobell said.

"If they have been adapting, something that is very difficult to measure, then the effects of warming may have been lower," he said.

The study revealed a simple relationship between temperature and crop yields, with a fall of between 3 and 5 per cent for every 0.5C increase in average temperatures, the scientists said.