JOHANNESBURG -- The world's poorest countries may lose up to a fifth of their grain-growing capacity by 2080 because of water scarcity inflicted by global warming, according to a scientific study released at the Earth Summit here.

"Some 40 least developing countries, with a projected population of some three billion in the 2080s, may lose on average 10-20 percent of their cereal capacity due to climate change," said one of the report's authors, Mahendra Shah, AFP reported.

The biggest source of concern is sub-Saharan Africa, where 13 countries which currently account for 45 percent of the continent's undernourished would lose cereal-production potential. Eight other countries, presently home to 40 percent of Africa's underfed, would gain potential though.

Rich countries -- whose uncontrollable burning of fossil fuels sparked the global warming phenomenon -- would be agricultural beneficiaries of climate change, according to the study.

High levels of carbon dioxide in the atmosphere and warmer temperatures "will result in improved agricultural potential" in some developed northern-hemisphere countries, it suggested.

"Developed countries substantially gain production, while many developing countries lose," said Shah.

China, the world's largest cereal producer, gains substantially.

Production could rise by up to 25 percent.

The study, climate change and agricultural vulnerability, was conducted by the International Institute for Applied Systems Analysis, located in Laxenburg, Austria.

It is a computer model that crunches a country's key statistics, such as its ecology, geography and agriculture patterns, to produce predictions based on three kinds of scenarios, depending on population growth and economic growth.

Warmer climate will make vast areas of tropical subsistence land hostile to farming, but will also open up big areas in Russia and Canada that today are permafrost.

"Whilst strong gains in potential agricultural land occur in North America (up to 30 percent) and the Russian federation (up to 55 percent), significant losses are projected for Africa, particularly the north (up to 75 percent) and Southern Africa," said coauthor Guenther Fischer.

The intergovernmental Panel on Climate Change (IPCC), the top UN scientific panel on global warming, declared last year that there was now inconvertible evidence that man had caused the earth's atmosphere to warm.

Oil, gas and coal release carbon dioxide when they are burned.

The gas acts like an invisible shroud, trapping the sun's heat instead of letting it radiate safely back into space.

The IPCC predicts the earth's mean surface temperature will rise by between 1.4 and 5.8 C (2.5 to 10.4 F) by 2100 compared with 1990 levels, causing sea levels to surge from eight to 88 centimeters (3.6 to 35 inches).