The CAA has a long history, taking its modern form in 1970 under President Richard
Nixon. It has been substantially amended only twice since then, in 1977 and again in
1990. Throughout most of the last decade, the Bush administration argued that greenhouse
gases (GHGs) were not ‘‘pollutants’’ as intended under the Act. However, in 2007, in
Massachusetts v. EPA, the Supreme Court confirmed the authority of the Environmental Protection
Agency (EPA) to regulate GHGs under the CAA.1 Subsequently, the agency made
a formal science-based determination that GHGs were dangerous to human health and
the environment.2 This ‘‘endangerment finding’’ compelled the agency to mitigate that harm
and formed the basis for the agencys first regulation of carbon emissions under the
CAA—the new corporate average fuel efficiency (CAFE) standards for cars and trucks.
Although these regulations focus on transportation sector emissions, they commit the