The White House buried a report prepared by US government scientists which detailed a rising death toll from heat waves, fires, disease and smog they predicted would be caused by global warming.
Environmental advocates accused President George W Bush's administration of delaying the release of the 149-page report so that it could avoid regulating greenhouse gases.
It was prepared as part of a response to a 2007 Supreme Court ruling under the Clean Air Act, which found the Environmental Protection Agency must regulate greenhouse gases unless there was a scientific reason not to, but was not made public until Monday.
The report lays out for the first time the scientific case for the grave risks that global warming poses to people, and to the food, energy and water on which society depends.
"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. Global warming, they wrote, is "unequivocal," and humans are to blame.
It suggests that extreme weather events and diseases carried by ticks and other organisms could kill more people as temperatures rise and allergies could worsen because climate change could produce more pollen. Smog, a leading cause of respiratory illness and lung disease, could become more severe in many parts of the country. At the same time, global warming could mean fewer illnesses and deaths due to cold.
"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," said Vickie Patton, deputy general counsel for the Environmental Defence Fund. "What this document demonstrates is that the imperative for action is now."
While scientists have pointed to a link between public health and climate change, the Bush administration has worked to discourage such a connection, fearing that doing so would compel the government to regulate greenhouse gases.
On Friday, the White House dismissed the scientists' findings, when it said the Clean Air Act was the wrong tool to control global warming pollution and a new law which dealt solely with global warming was needed.
Stephen Johnson, the EPA chief, said through a spokesman that although he knew "the science is clear, and that climate change is a significant issue," he did not want to make a "rash decision under the wrong law".