The United States government has declared that greenhouse gases are endangering the health of American people.
The ruling means President Barack Obama can now order cuts in emissions without having to win the backing of the US Senate, where climate change legislation is stalled.
The move coincides with the beginning of the United Nations-organised conference on climate change in Copenhagen.
The announcement had been expected for some time, but is an important signal before Mr Obama travels to the Danish capital for the leaders' summit, the BBC reports.
At present, legislation restricting carbon emissions faces stiff opposition in the Senate.
Some say restrictions will cost the US in jobs and push up energy prices, while others simply do not accept that climate change is real or man-made.
If the legislation is passed, it will not be until the northern hemisphere spring, but this declaration raises the possibility that if Mr Obama is blocked, his administration could simply impose new rules.