The U.S. Environmental Protection Agency has declared greenhouse gases are a threat to both public health and the environment.
The agency's announcement Monday paves the way for government regulation of emissions even without congressional approval.
EPA Administrator Lisa Jackson said science overwhelmingly shows greenhouse gas concentrations are at unprecedented levels due to human activity.
The announcement comes as U.S. President Barack Obama prepares to attend the United Nations climate change conference in Copenhagen.
President Obama met Monday with former Vice President Al Gore, who won a Nobel Peace Prize in 2007 for his work on fighting global warming.
Mr. Obama is scheduled to be in Copenhagen on December 18 at the end of the climate talks.
The president had originally planned to attend the start of the talks, but believes U.S. leadership would be most productive at the end of the session.