By Richard Black
Environment correspondent, BBC News website
The EPA's first action could cut emissions from cars
The US government is to regulate carbon dioxide emissions, having decided that it and five other greenhouse gases may endanger human health and well-being.
The Environmental Protection Agency (EPA) announced the move following a review of the scientific evidence.
The decision marks a major change from the Bush presidency, when the EPA argued it could not regulate CO2 because the gas was not a pollutant.
Developing countries have asked for the US to show leadership on climate.
Many are not prepared to curtail their own emissions without firm indications that the US is willing to make significant reductions.
Carbon-cutting legislation is being proposed in Congress, but the EPA decision - known as an "endangerment finding" - will allow the agency to mandate some cuts without waiting for the draft bills to become law.
"This finding confirms that greenhouse gas pollution is a serious problem now and for future generations," said EPA administrator Lisa Jackson.
"Fortunately, it follows President Obama's call for a low-carbon economy and strong leadership in Congress on clean energy and climate legislation; and... the solution is one that will create millions of green jobs and end our country's dependence on foreign oil."
In 2007, the Supreme Court instructed the agency to review the evidence on climate change with a view to reconsidering its stance on the possible threats of climate change.
In the endangerment finding, the EPA now cites a number of impacts that it believes may impact significantly on US citizens, including:
- an increased risk of droughts and floods
- sea level rise
- more intense storms and heatwaves
- harm to water supplies, agriculture and wildlife
Ms Jackson concluded that these impacts would fall disproportionately on people who were poor or in ill health, and on indigenous groups.
The EPA quoted a 2007 report by a group of retired generals and admirals who said that climate change presented "national security challenges" for the US.
Lisa Jackson said Mr Obama would prefer new legislation on climate
Environmental groups said this was the latest sign that the Obama administration is taking a very different line on climate change from its predecessor.
"This reclaims the US role on the international stage as a leader," said Phyllis Cuttino, director of the US global warming programme at the Pew Environment Group in Washington DC.
"The EPA's next step will probably be to grant the 'California waiver', which would allow states to restrict tailpipe emissions, and that we expect to happen very soon," she told BBC News.
Under the Bush administration, the agency denied California the right to set targets for vehicle efficiency, for which it needed a waiver from federal law.
At least 15 states plan to follow California's lead.
The EPA's decision now goes out to public consultation. The agency says it will "conduct an appropriate process and consider stakeholder input" before imposing restrictions on any part of the economy.
The endangerment finding also empowers the EPA to regulate the other five greenhouse gases included in the Kyoto Protocol - methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride.