"We expect climate change to aggravate current problems of the number of millions of people at risk of hunger, probably to the tune of 50 million," said Professor Martin Parry of the Hadley Centre of the UK Meteorological Office.

"The greatest proportion, about three-quarters of that number, will be in Africa."

Parry told the British Association science conference that it would take huge reductions in emissions of greenhouse gases -- about 20 times those required by the Kyoto Protocol -- to avoid the additional risk of hunger.

The 1997 protocol demands cuts in greenhouse emissions by 5.2 percent below 1990 levels by 2008-12.

The United States, the world's biggest polluter, has refused to back the protocol, saying it would hurt its economy. It also believes the pact is flawed because it omits rapidly industrialising emerging economies such as India and China.

In a separate presentation at the meeting Professor Steve Long noted that although it is widely recognised that climate change will decrease the yield of crops, a rise in CO2, which is a major driver of global warming, will increase plant growth.

"So as you increase carbon dioxide, plant growth is actually boosted," said Long, from the University of Illinois in the United States.

But in field experiments Long and his colleagues found that the yield increase due to carbon dioxide is only about half of what is predicted and for corn there was no increase at all.

He added that a second change in the atmosphere -- rising levels of ozone in the northern hemisphere -- further complicates the picture because there have been experiments that have suggested this would decrease crop yields.

"We've conducted, in Illinois, the first of these experiments in the open field situation. In fact, we find that with our soya bean crops that the elevation to the levels expected by 2050 decrease the yield by about 15 percent," he explained.