Scientists have compiled one of the first comprehensive pictures of what the world might be like when climate change begins to trigger a dramatic increase in epidemics, disease and death.
Teams of specialists have assessed the scale of the dangers to human health when changes in the climate lead to higher incidences of weather extremes, such as high temperatures, floods and drought.
The findings - published today in the journal Nature - come weeks before world leaders meet in Montreal to discuss climate change at the first Conference of Parties to the Kyoto Protocol.
Global warming is likely to lead to an increase in the number of infectious diseases and respiratory illnesses. It will also raise the risk and severity of flooding, and reduce the availability of clean drinking water to millions of people. The studies also found that the countries most likely to be affected by global warming were those least able to combat its effects. Meanwhile, the nations who contribute most to climate change are those that will suffer the least.
Professor Jonathan Patz of the University of Wisconsin in Madison, the lead author of one of the studies, said that it was incumbent on those countries bearing the greatest responsibility for climate change to show moral leadership.
He said: "Those least able to cope and least responsible for the greenhouse gases that cause global warming are most affected. Herein likes an enormous global ethical challenge."
The World Health Organisation (WHO) estimated that changes to the earth's climate was already causing about five million extra cases of severe illness a year and more than 150,000 extra deaths.
By 2030, however, the number of climate-related diseases was likely to more than double, with a dramatic increase in heat-related deaths caused by heart failure, respiratory disorders, the spread of infectious diseases and malnutrition from crop failures.
Countries with coastlines along the Indian and Pacific Oceans and sub-Saharan Africa would suffer a disproportionate share of the extra health burden, said Diarmid Campbell-Lendrum, of the WHO, who took part in the latest study.
"Many of the most important diseases in poor countries, such as diarrhoea and malnutrition, are highly sensitive to climate," Dr Campbell-Lendrum said. "The health sector is already struggling to control these diseases and climate change threatens to undermine these efforts," he said.
Scientists estimate that man-made emissions of greenhouse gases are likely to lead to increases in global average temperatures of between 1.4C and 5.8C by the end of the century.
The number of people at risk of flooding by coastal storm surges is projected to increase from the current 75 million to 200 million by 2080, when sea levels may have risen by 40 centimetres.
A separate study of how rising temperatures will affect water supplies found that severe shortages were likely to affect up to a sixth of the world's population who currently rely on melting snow and glacial "fossil" ice.
Parts of China and India, where vast population centres rely on melting ice from the Himalayas for their supply of drinking water, are highly vulnerable to global warming, the study found.
People living west of the Andes are also likely to suffer from a dwindling water supply once the glaciers have disappeared, the study found. Peru had already suffered a 25 per cent reduction in water supplies over the past 30 years. "Climate warming is a certainty and the bottom line in this analysis is that the impact of warming and the long-term prognosis is clear and very dire," said Tim Barnett of the Scripps Institution of Oceanography in La Jolla, California.
"It's especially clear that regions in Asia and South America are headed for a water-supply crisis because once that fossil water has gone, it's gone."Reuse content