* But major crop growing regions of the U.S., Canada and Mexico have escaped the warming trend - and are just as cool as they were 30 years ago Higher food bills are a result of global warming, a major new study claims.
Scientists at Stanford University said warmer temperatures and changes in rainfall patterns have pushed up the cost of wheat, maize, rice and soya by 5 per cent since the 1980s.
However, the study also found that major crop growing regions of the U.S., Canada and Mexico have escaped the warming trend - and are just as cool as they were 30 years ago.
Dr David Lobell, who reports the findings in the journal Science, said: 'It appears as if farmers in North America got a pass on the first round of global warming.
'That was surprising, given how fast we see weather has been changing in agricultural areas around the world as a whole.'
Dr Lobell and colleagues studied temperature and rainfall records since 1980 for the world's most important crop-growing regions.
The researchers then used computer models to estimate what the harvests would have been like over the last 30 years had the climate remained unchanged.
Wheat production was 5.5 per cent lower than it would have been had the climate remained stable, while production of maize was down by nearly 4 per cent, they found.
However, global warming does not appear to have affected the production of rice and soybean.
The U.S., which grows more soybean and maize than any other country, experienced 'a slight cooling trend' and its output was not affected, the researchers said.
Until it was recently overtaken by China, the U.S. was the world's biggest emitter of carbon dioxide, the main greenhouse gas thought by scientists to be responsible for man-made global warming.
'Yields in most countries are still going up, but not as fast as we estimate they would be without climate trends,' Dr Lobell said.
'Given the relatively small temperature trends in the U.S. corn belt, it shouldn't be surprising if complacency or even scepticism about global warming has set in, but this study suggests that would be misguided,' he added.
Since 1950, the average global temperature has increased at a rate of roughly 0.13 degrees Celsius per decade. he said.
But over the next two to three decades average global temperature is expected to rise approximately 50 per cent faster than that, according to the United Nations Intergovernmental Panel On Climate Change.
Dr Lobell said crop-growing regions of the U.S. were unlikely to escape rising temperatures in the coming decades.
'The climate science is still unclear about why summers in the corn belt haven't been warming. But most explanations suggest that warming in the future is just as likely there as elsewhere in the world,' Dr Lobell said.
Dr Wolfram Schlenker, an economist at Columbia University and a co-author of the paper, said: 'We found that since 1980, the effects of climate change on crop yields have caused an increase of approximately 20 percent in global market prices.'
If the beneficial effects of more carbon dioxide levels in the atmosphere on crop growth are taken into account, the rise in prices is around 5 per cent.
'Five per cent sounds small until you realise that at current prices world production of these four crops are together worth nearly $1trillion per year,' he added.
'So a price increase of 5 per cent implies roughly $50billion per year more spent on food.'
Dr Phil Jones, a climate scientist at the University of East Anglia, said temperatures had risen overall across North America over the last 200 years.
'However, in an area of the mid west where cereal is grown - they have been stable,' he said. 'But if you look at California, then temperatures have gone up.'
