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Climate change is usually considered mainly an environmental calamity, with humans largely immune to its devastating effects.

But a new report warns that people won't escape unscathed from global warming, which will likely turn out to be the biggest threat to human health this century, causing the spread of diseases, increased malnutrition and other severe medical problems.

The assertion that global warming has the potential to become a major health disaster is being made jointly by two influential organizations, the Lancet, a major British-based medical journal, and University College London, whose researchers compiled the report.

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"Climate change is a health issue affecting billions of people, not just an environmental issue about polar bears and deforestation," commented Anthony Costello.

A pediatrician, Dr. Costello is the lead author of the report and co-director of the university's Institute for Global Health.

The report was based on a review of about 175 scientific studies that have investigated how a warmer world will influence such aspects of health as insect-borne disease patterns, water and food insecurity, threats to cities from rising sea levels, and harm from extreme climate events, like killer heat waves and floods. It even raised the possibility that mental health will be impacted by anxiety about the future and that climate change will cause large scale population movements and civil unrest.

Although the report didn't make a projection on the number of illnesses to be caused by global warming, it classified the likely impact as "immense" and expressed concern that the figure could reach tens of millions of premature deaths.

At a news conference in London this week, Dr. Costello said he has had a personal change of view about the health threat posed by global warming, initially viewing it as relatively unimportant before he began thinking about it in more detail.

"Eighteen months ago I would have said that climate change was perhaps an issue but it's warm Sunday afternoons and a bit about polar bears. There are more pressing issues for maternal and child health. I've now changed my view about that," he said, calling for medical doctors "to be in the forefront of arguments about carbon mitigation."

An obvious impact of global warming will be an increase in deadly heat waves. The one in Europe in 2003 claimed up to 70,000 lives, mainly due to heatstroke, and respiratory and cardiovascular ailments. But the report said future warm spells might be even more deadly and frequent, with summer temperatures projected to peak over 50 C in parts of Australia and northeast India and at over 40 C in southern Europe by 2100.

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Climate change will also have a profound impact on the worldwide pattern of diseases. For parasites spread by insects, the frequency of bites, the reproduction of the pest that carries the disease and the development cycle of the parasite all generally speed up in warmer temperatures.

The report concluded that malaria, tick-borne encephalitis and dengue fever will become increasingly widespread. It said mosquitoes that cause the spread of malaria will be able to expand their range to new, higher elevation areas that are currently free of the disease, with anywhere from 260 million to 320 million more people affected by 2080.

The number of people at risk of contracting dengue fever - another tropical, mosquito-spread ailment noted for causing intense joint pain and severe headaches - could reach six billion, compared to only 3.5 billion if the climate didn't change. Another worry is that rising water temperatures will prompt more cholera, a severe intestinal disease caused by a bacteria.

Given the current worry over swine and avian flu, the report said there is "no clear evidence" for a climate link to their spread.

Climate change also risks causing more malnourishment because higher temperatures and droughts will impair crop yields, particularly for such staples as rice and corn. One estimate cited in the report projected that half the world's population could face severe food shortages by the end of the century.

Although the report warned of the health impacts of global warming, it said the use of fossil fuels has had its benefits, particularly in advanced countries, by helping to contribute to a doubling of life expectancy and reduced poverty.

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But reducing reliance on fossil fuels would now have health benefits. These might involve such steps as replacing car use, for instance, with more walking or bike riding. Dr. Costello said such a move, which he dubbed a low-carbon lifestyle, would offer health benefits of reducing obesity, heart disease and stress.

But Dr. Costello also termed climate change a "bad diagnosis" that he said will pose major health threats "for our children and grandchildren."

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