The Supreme Court’s decision in Massachusetts v. EPA 549 U.S. 497  (2007) found that greenhouse gas emissions meet the definition of “air pollutants” under the Clean Air Act. It required that EPA determine whether or not emissions of greenhouse gases (from new motor vehicles) cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare, or whether the science is too uncertain to make a reasoned decision. On April 17, 2009, EPA issued its proposed endangerment finding . This proposal laid out the basis for the proposed determination that six key greenhouse gases -- carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)—were responsible for contributing to climate change which results in a threat to the public health and welfare of current and future generations. In addition, under the Clean Air Act, EPA must also determine that emissions from particular sources “cause or contribute” to emissions which threaten public health and welfare. As part of its proposal, EPA also proposed to find that the combined emissions of these well-mixed greenhouse gases from new motor vehicles and new motor vehicle engines contribute to the greenhouse gas pollution which threatens public health and welfare.
On December 7, 2009, EPA issued its final endangerment finding and its final cause or contribute finding  for light-duty vehicles. This action does not, by itself, impose any restrictions on any entities. It is however, a required step in the process of leading to specific regulations of greenhouse gas emissions. EPA has proposed to regulate greenhouse gas emissions from light-duty vehicles and engines in a joint proposal  in September 2009 with the National Highway Traffic and Safety Administration. These regulations are expected to become final in March 2010 and would be the first actions to limit greenhouse gases consistent with the Supreme Court’s decision and under authority of the Clean Air Act.