Oslo, Mar 14: Global warming could cause severe food and water shortages for millions of people by 2100 and trigger a melt of polar ice that could keep ocean levels rising for centuries, a draft UN report shows.
It said the poor were most at risk, for instance in sub-Saharan Africa and around deltas of major rivers in Asia.
The survey by the world's top climate scientists, due for release in Brussels on April 6, said climate change widely blamed on human activities was already under way with impacts ranging from melting glaciers to earlier than normal plant growth in spring.
''Many natural systems, on all continents and in some oceans, are being affected by regional climate changes, particularly temperature increases,'' according to a copy seen by Reuters. The draft will have a final review by governments and experts.
''Impacts are very likely to increase due to increased frequencies and intensities of extreme weather impacts,'' the draft said. It said there were still chances to prevent the most damaging impacts if governments acted.
It pointed to threats such as a melting of Greenland and west Antarctic ice sheets that could cause sea levels to rise, extinctions of species from the Amazon to the Arctic or more severe heat waves in US cities.
''Climate change increases the number of people at risk of hunger marginally,'' it said, compared to projected declines caused by economic growth. But a large rise in temperatures could put up to 120 million people at risk of hunger.
A melting of glaciers, such as in the Himalayas, could cut summer and autumn flows in regions where more than a billion people live.
Farmers near the equator were likely to suffer falling crop yields even with small temperature rises, while farmers living nearer the poles might see some immediate benefits.
''Global agricultural production potential is likely to increase with increases in global average temperature up to about 3 Celsius, but above this is very likely to decrease,'' the draft said.
Hundreds of millions of people would suffer from water scarcity even with a small rise in temperatures. Between 1.1 to 3.2 billion might suffer if temperatures jumped by more than 4 Celsius, at the higher end of forecasts.
Water scarcity could damage semi-arid regions such as the Mediterranean basin, the western United States, southern Africa, northeastern Brazil, southern and eastern Australia.
The draft, detailing likely impacts and ways to adapt to climate change, is the second of four studies this year based on the work of 2,500 climate experts in the UN's Intergovernmental Panel on Climate Change (IPCC).
The first report concluded there was at least 90 per cent certainty that human activities, led by burning fossil fuels, were stoking warming. The reports will guide governments trying to combat climate change.
The draft report said hundreds of millions of people would be vulnerable to rising sea levels that could swamp Pacific islands, coasts and cities from New York to Shanghai.
It said there was ''medium confidence'' that a rise in temperatures of more than 1 to 2 Celsius might melt parts of Greenland and west Antarctica, ''causing sea level rise of 4 to 6 metres over centuries to millennia.'' And it said that about 20-30 percent of species could be at risk of extinction with a moderate temperature rise.