Their vast report will shed light on the costs from heightened water stress, tropical storms, floods, droughts, species loss and human disease this century as a result of global warming.

"(The) vulnerabilities could be considerable," warns the 1,400-page document on the impact of climate change.

The report is due to be issued in Brussels on Friday by the Intergovernmental Panel on Climate Change (IPCC), gathering top specialists in climate science, economics, biology and other disciplines under the UN banner.

They will meet for four days to finalise the document, marking the second out of three volumes in the IPCC's massive update of knowledge about climate change. It is the first such review since 2001.

The bill for climate change will depend on atmospheric levels of greenhouse gas, which is principally caused by burning fossil fuels, and on the effort to adapt to its effects, according to a draft seen by AFP.

Experts disagree widely as to the ultimate dollar figure, as the calculation has many variables.

The equation changes, for instance, if one estimates the impact over the short or the long term, factors in the cost of biodiversity loss or not or attributes damage from extreme weather events to man-made global warming or to a natural phenomenon.

"Depending on the assumptions used... total economic impacts are typically estimated to be in the range of a few percent of global product for a few degrees (Celsius) of warming," it says.

But there are also many unknowns, it adds.

Climate change can have a knock-on effect in many areas and there are also poorly-understood triggers that scientists fear could dramatically accelerate the warming.

To give an indication, estimates range from a benefit of three dollars per tonne of CO2 emitted into the atmosphere -- mainly because warming would open up frozen lands in the northern hemisphere to agriculture -- to a cost of more than 400 dollars per tonne.

In 2005, around 7.9 billion tonnes of CO2, the principal greenhouse gas, were released into the atmosphere, according to the Global Carbon Project, a research organisation.

In the first volume of its report, issued in February, the IPCC predicted a temperature rise in the likely range of 1.8-4.0 C (3.2-7.2 F) by the end of the century.

The upcoming report makes these points:

-- As the temperature rises, so will the "social costs," or the OVERALL ECONOMIC BILL caused by every tonne of CO2.

"It is virtually certain the real social cost of carbon and other greenhouse gases will rise over time; it is very likely that the rate of increase will be two to four percent per year," says the report.

Depending on the scenario of CO2 concentrations, "by 2080, it is likely that 1.1 to 3.2 billion people will be experiencing water scarcity; 200 to 600 million hunger; two to seven million more per year, coastal flooding."

-- A modest rise could open up huge areas of land for AGRICULTURE in North America, Northern Europe and Russia. But sub-Saharan Africa would lose farmland because of less rainfall while yields for wheat in South Asia and for rain-fed rice production in China would also be badly hit.

A very high increase (5.5 C, or 9.9 F) would widely damage crop and livestock production. Global cereal prices would rise by 30 percent, according to one study.

-- The biggest potential costs will come from EXTREME WEATHER EVENTS, such as storms, droughts and floods, which are "very likely" -- a 90 percent certitude -- to become more powerful and possibly more frequent too. Their impact will be amplified by a rise in the world's population, which is projected to reach between 8.7 and 9.3 billion by 2030, of which two billion could be slum dwellers whose homes are typically at risk from inundation and landslide.

"Costs of major [weather] events can range from several percent of annual regional GDP" for large economies to "more than 25 percent in smaller areas that are affected by the events," says the report.

It notes that Hurricane Katrina, which devastated New Orleans in 2005, inflicted total economic costs of more than 100 billion dollars. A British assessment in 2005 suggested that annual weather-related damages to land use and property in Britain could increase by three to nine times by the 2080s.

-- A big rise in SEA LEVELS would be very costly, as it would swamp low-lying coastal regions and deltas and threaten small island states, but is considered a low probability.

A rise of one metre (3.25 feet) would cost 944 billion dollars, almost half of it in Asia, according to the study. In February, the IPCC said the global sea level would rise by between 18 and 59 centimetres (7.2 and 23.2 inches), but could be boosted if icesheets melt faster.

-- REDUCING carbon emissions would help to brake the warming, and PREPARING for the effects will reduce the costs.

For instance, if sea levels rose by 65 cms (26 inches), an exceptional tide would cause damage of 5.2 billion dollars in China's booming Pearl River delta; flood defences and other preparations, though, would cost only 400 million -- a saving of 4.8 billion dollars.

Up to the middle of the century, a mix of mitigation and adaptation will be effective, "but even a combination of aggressive mitigation and significant investment in adaptive capacity could be overwhelmed by the end of the century."

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