Climate change has already extended the range of mosquitoes and ticks, helped spread diarrhoeal disease, boosted the length and location of pollen seasons and pumped up the intensity of dangerous heatwaves, says the report.

In the coming decades, such problems are likely to amplify and for many people, hunger and poor nutrition will be added to the mix, it says.

"Adverse health impacts will be greatest in low-income countries," says the report. "Those at greater risk include, in all countries, the urban poor, the elderly and children, traditional societies, subsistence farmers and coastal population."

The 1,400-page document is due to be issued on Friday by the Intergovernmental Panel on Climate Change (IPCC), the top UN scientific authority on global warming and its impacts.

It is part of the fourth assessment report by the IPCC since it was founded in 1988 to inform policymakers about man-made climate change. The previous report was in 2001.

A final draft of the report points out that human health can be affected in subtle and sometimes poorly perceptible ways by climate change.

For instance, if food production is hit by drought or flooding, that in turn affects nutrition -- or may prompt a rural exodus that boosts the population of shanty towns, where overcrowding and communicable disease can be rife.

A study of the heatwave that struck Western Europe in 2003 found that nearly a third of Switzerland's excess mortality came from ground-level ozone -- the pollution caused by a reaction between sunlight and traffic exhausts, which can be dangerous for people with respiratory or cardiac problems.

In some cases, says the report, climate change may be positive. In northerly latitudes, winters will become shorter and milder, easing the risk to poor and elderly people of cold waves.

But overall, the outcome will be "overwhelmingly negative", hitting most of all poor tropical countries with water stress, poor sanitation and shaky medical infrastructure.

One study cited in the report projects that the number of people at risk of hunger in the Sahel country of Mali will roughly double, from 34 percent today to between 64-72 percent in the 2050s, if nothing is done to help the population adapt to the threat.

The report puts the spotlight on populations in these areas:

-- COASTAL AND LOW-LYING AREAS: A quarter of the world's six billion people lives within 100 kilometres (62 miles) distance and 100 metres (325 feet) elevation of the coastline.

Depending on the levels of carbon pollution in the atmosphere, these areas are at risk from rising sea levels, more powerful storms, coastal flooding, damage to fisheries and saltwater intrusion into freshwater resources.

According to one estimate, nearly five percent of the population of Bangladesh could face inundation if temperatures rise 2 C (3.8 F), the sea level increases 30 centimetres (12 inches) and monsoon rainfall rises 18 percent, which are middle-of-the-range estimates.

This could increase to 57 percent of the population in a computer model of a high-range 4 C (7.2 F) temperature increase, a 100-cm (40-inch) rise in sea level and a 33 percent increase in monsoon precipitation.

-- MOUNTAIN REGIONS: Glaciers are in rapid retreat in the Himalayas, Greenland, the European Alps, the Andes and East Africa, causing for some populations the future risk of water insecurity. Nearly a quarter of China's population, for instance, lives in western regions where glacial melt provides the main water source in the dry season.

A warmer climate will enable mosquitoes to live at altitudes that previously were too cold, while more extreme rainfall will boost the number of floods and landslides.

-- POLAR REGIONS: Indigenous peoples who comprise roughly 10 percent of the circumpolar population are "particularly vulnerable" to climate change, both in threats to their habitat and their lifestyle. Warmer temperatures will increase the range of disease-bearing wildlife and badly affect traditional nutrition because of changes in animal migration and distribution.

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