US government scientists have warned of a rising death toll from heat waves, wildfires, disease and smog caused by global warming, in a study the White House repeatedly tried to bury to avoid regulating greenhouse emissions.

In a 149-page analysis released last night, experts for the first time laid out the grave risks that climate change poses to human health, and to the supplies of food, water and energy on which populations depend.

“Risk to human health, society and the environment increases with increases in both the rate and magnitude of climate change,” scientists at the Environmental Protection Agency said. In a more absolute pronouncement on the science of climate change than the White House has so far been prepared to accept, they said that global warming was “unequivocal,” and that humans were to blame.

The document warns that extreme weather events, such as hurricanes, and diseases carried by ticks and other organisms could kill more people as temperatures rise. Intensifying heat waves will result in increased mortality, particularly amongst the young and old, while flooding could impact water quality.

Allergies could become more severe because climate change could produce more pollen, it says, while worsening smog will increase the risk of respiratory illness, asthma and even premature death, it says. However at the same time, global warming could mean fewer illnesses and deaths due to cold.

Food supplies are likely to diminish as crops fail, while water is expected to become more scarce as a result of greater evaporation and reduced snowpack, leading to increased competition for resources.

The scientists also raised the nightmare prospect of repeats of the catastrophe suffered by New Orleans during Hurricane Katrina. Not only is the United States set to experience increasingly severe and frequent hurricanes, rising sea levels mean coastal communities face worsening flooding in the event of storms.

"This is a long-awaited EPA analysis that has been kept under wraps by the White House,” Vickie Patton, deputy general counsel of the Environmental Defense Fund, said. “This document inescapably, unmistakably shows that global warming pollution not only threatens human health and welfare, but it is adversely impacting human health and welfare today.

“What this document demonstrates is that the imperative for action is now.”

The Bush administration has long worked to discourage a connection between climate change and public health. Acknowledging such a link would compel it to regulate greenhouse gases, a move the President has resisted throughout his tenure in office.

The analysis was produced as part of a response to a 2007 Supreme Court ruling that under the Clean Air Act, the EPA must regulate greenhouse gases unless it could provide a scientific reason not to do so. The ruling came after number of states and environmental groups brought a landmark case against the EPA for refusing to restrict exhaust gases from cars.

But EPA chief Stephen Johnson, a cabinet level member of the Administration who has frequently courted controversy with his apparent refusal to act on the findings of his scientists, signalled last night that he would not bow to the court’s demands.

After the White House on Friday dismissed the scientists’ findings, insisting that the 1970 law was the wrong tool to control global warming pollution, Mr Johnson fell in line, saying greenhouse gases should be considered as a separate issue by Congress.

He declined to take immediate steps, instead declaring a 120-day period for public comment on a nearly 1,000-page response to the Supreme Court ruling. Any action during the Bush administration is therefore unlikely.

It is not the first time that the administration, including Mr Johnson, has set itself against EPA scientists.

In December, the White House refused to open an e-mail from the EPA that included findings similar to those released yesterday on the links between climate change and public health.

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