The U.S. Environmental Protection Agency (EPA) announced Monday that greenhouse gases (GHGs) threaten the public health and welfare of the American people as a major UN climate change conference opens in Copenhagen.

GHGs are the primary driver of climate change, which can lead to hotter, longer heat waves that threaten the health of the sick, poor or elderly, increases in ground-level ozone pollution linked to asthma and other respiratory illnesses as well as other threats to the health and welfare of Americans, the EPA said.

EPA's endangerment finding covers emissions of six key greenhouse gases -- carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride -- that have been the subject of scrutiny and intense analysis for decades by scientists in the United States and around the world.

"These long-overdue findings cement 2009's place in history as the year when the United States government began addressing the challenge of greenhouse-gas pollution and seizing the opportunity of clean-energy reform," said EPA Administrator Lisa Jackson in a statement.

"Business leaders, security experts, government officials, concerned citizens and the United States Supreme Court have called for enduring, pragmatic solutions to reduce the greenhouse gas pollution that is causing climate change. This continues our work towards clean energy reform that will cut GHGs and reduce the dependence on foreign oil that threatens our national security and our economy," said the statement.

EPA's final finding responds to the 2007 U.S. Supreme Court decision that GHGs fit within the Clean Air Act definition of air pollutants. Under the Supreme Court ruling, the so-called endangerment finding is needed before the EPA can regulate carbon dioxide and five other GHGs released from automobiles, power plants, and factories under the federal Clean Air Act.

The announcement came on the first day of the Copenhagen conference, which is scheduled to be held from Dec. 7 to 18. At the conference, about 190 countries are expected to renew GHGs emissions reduction targets set by the Kyoto Protocol, the first stage of which is to expire in 2012. The conference is also expected to outline the post-2012 negotiation path.

EPA's move will allow the agency to regulate planet-warming gases even without legislation in the U.S. Congress. Experts say the finding is timed to boost the administration's arguments at Copenhagen that the United States is aggressively taking actions to combat global warming, even though Congress has yet to act on climate legislation.

Without a climate legislation, the U.S. was heading into Copenhagen hard-pressed to explain exactly how it would reach the targets President Barack Obama is set to offer.

On Nov. 25, the White House said the United States will offer a17-percent reduction target of its greenhouse gas emissions below 2005 levels by 2020 at the Copenhagen conference, which is less than four percent emissions cut below 1990 levels. Developing countries demand developed countries slash their emissions by at least 40 percent below 1990 levels by 2020.

However, a White House spokesman says President Barack Obama still prefers legislation on climate change and the timing of the EPA announcement and the opening of the UN climate change conference in Copenhagen was coincidental.

Obama "still believes the best way to move forward is through the legislative process," Robert Gibbs told reporters at his daily briefing on Monday.

Scientific consensus shows that as a result of human activities, GHG concentrations in the atmosphere are at record high levels and data shows that the Earth has been warming over the past 100 years, with the steepest increase in warming in recent decades. The evidence of human-induced climate change goes beyond observed increases in average surface temperatures; it includes melting ice in the Arctic, melting glaciers around the world, increasing ocean temperatures, rising sea levels, acidification of the oceans due to excess carbon dioxide, changing precipitation patterns, and changing patterns of ecosystems and wildlife.

EPA issued the proposed findings in April 2009 and held a 60-day public comment period. The agency received more than 380,000 comments, which were carefully reviewed and considered during the development of the final findings.