In 2003, the EPA rejected a petition that called on the agency to regulate greenhouse gases (GHGs) under the Clean Air Act. The EPA, under the George W. Bush administration, said, contrary to the earlier EPA legal opinion (the “Cannon Memo,”) that it did not have the authority to regulate GHGs – and even if it did, the agency wasn't required to do so. In response, several states and organizations sued the EPA. In 2007, the Supreme Court decided, in a tight 5-4 decision, that the EPA could regulate GHGs under the Clean Air Act. And, as that act stipulated, if the agency found that GHGs endanger public health, the agency had to regulate them. Eventually, under the Obama administration, the EPA determined that GHGs were a danger to public health and the agency subsequently issued regulations to curb GHGs.
Massachusetts v. EPA, 549 U.S. 497 (2007)