"Climate change will have a tremendous impact on food security, especially in Africa and also in some parts of Asia and Latin America," said Wulf Killmann who chairs a working group on global warming at the Rome-based food agency.

A large body of scientific evidence has indicated that emissions of certain gases from human activities -- especially carbon dioxide (CO2) from burning fossil fuels -- is contributing to the greenhouse effect that blocks heat in the atmosphere.

The UN Intergovernmental Panel on Climate Change has predicted global temperatures could rise by up to 5.8 degrees Celsius this century.

In the developing world, global warming is likely to reduce the amount of rain-fed land where crops could be grown by 11 percent by 2080, according to a study by the FAO and the International Institute of Applied Systems Analysis (IIASA).

Killmann stressed there were many uncertainties in the scenarios mapped out in the research, but the general conclusion was clear. "Those who are already suffering from hunger will be suffering even more."

Sixty-five developing countries will lose about 280 million tons of potential cereal production, equivalent to about 16 percent of agricultural output.

The area of land in Africa where there is a crop growing season of less than 120 days will increase by 5-8 percent above the current 1.1 billion hectares, the study found.

Crop yields in the northern hemisphere -- including Canada, Russia and the Nordic region -- could be boosted by climate change with more land becoming suitable for cultivation and the effect of additional carbon in the air speeding plant growth.

But there is already evidence that climate change is hitting Africa. "There has been a steep decline in rain fall during the first part of the growing period in Ethiopia in the last 10 years," he said.

"We will have to help countries to adapt their agricultural systems to these new conditions. That will be a big task for international development."

The United States, which produces about one quarter of the world's CO2, is the only G8 country that remains outside the Kyoto Protocol, the treaty aimed at reducing greenhouse gases.