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 ...