Shrinking freshwater supplies aggravated by population growth and rising standards of living will "very likely" -- a 90 percent certainty -- "adversely affect more than a billion people in Asia by the 2050s," the UN's Intergovernmental Panel on Climate Change (IPCC) warns.

The report, which deals with the impacts of global warming, is the second volume in a three-volume review of the evidence for climate change, the first since 2001.

A draft of the 1,400-page document acquired by AFP says that even if dramatic measures are taken to reduce output of the carbon dioxide (CO2) that drives warming, temperatures worldwide will continue to climb for decades to come, unleashing unwelcome changes across the planet.

And if nothing at all is done to mitigate climate change, the impact in some regions could be catastrophic by century's end.

By 2080, the report says, it is likely that 1.1 to 3.2 billion people worldwide will experience water scarcity, 200 to 600 million will suffer from hunger, and each year an additional two to seven million people will be victims of coastal flooding. The predictions vary according to the temperature increases forecast and the capacity to adapt, but many parts of Asia, the world's most populous region, will be among the hardest hit under any scenario.

In South Asia, cereal yields could drop in some areas by up to 30 percent by 2050 due to drought.

Even modest rises in sea levels, caused by warming ocean surface temperatures and melting ice caps, will cause flooding and economic disruption is many of the region's densely-populated mega-deltas.

Among the most vulnerable are three -- the Yangtze, the Yellow River, and the Zhujiang -- in China, the Red River delta in northern Vietnam, and especially the low-lying Ganges-Brahmaputra delta in Bangladesh.

Nearly 300 million people live in these delicately-balanced flood plains.

The report notes that "climate-induced diseases" are already a reality, and predicts increases in the frequency and toxicity of cholera outbreaks due to the rising temperatures of coastal waters, especially in South Asia.

The malarial footprint is also very likely to expand as the mosquitoes that carry the disease move into areas once too cold for them to survive.

In the Himalayas, which feed rivers flowing into southern China as well as South and Southeast Asia, glaciers less than four km long are projected to disappear entirely if average global temperatures rise by 3 C (5.4 F)

If current warming trends continue, the area covered by central Asia's glaciers is likely to shrink by four-fifths from 500,000 square kilometres.

The impact will be felt locally and downstream: increased flooding and mudslides, and eventually a decrease in river flows, exacerbated by reduced rainfall.

In the first volume of its report, issued in February, the IPCC predicted that world temperatures would go up 1.8 C to 4.0 C (3.2-7.2 F) by 2100, depending on how much greenhouse gas is emitted into the atmosphere. A third volume, issued at the end of April, will look at ways of reducing those emissions.

In the Brussels meeting, the document will be issued Friday after delegates hammer out an all-important "summary for policy makers," distilling in a couple of dozen pages their most crucial findings.

The process is often contentious, with sharp disagreements on some scientific questions, and on which conclusions should drive policy decisions.

Some participants complained that the summary, as currently written, does not adequately highlight the fact that those least able to adapt -- poor people -- will be the hardest hit.

"The message that is missing is the vulnerability of certain developing nations," said a Western delegate from a non-European country.

The draft summary concludes that global warming is "unequivocal," that human activity is the main driver, and that "changes in climate are now affecting physical and biological systems on every continent."

Besides the human impact, numerous ecosystems and other living organisms are threatened by global warming.

Across the planet, 20 to 30 percent of animal and plant species face extinction if temperatures rise 1.5 C to 2.5 C (2.7-4.5 F).

Increases greater than 4 C (7.2 F) above 1990-2000 levels would lead to "major increases in vulnerability" that would exceed "the adaptive capacity of many systems," the report says.