Global warming is the biggest environmental issue facing humanity. According to a 2001 National Research Council report cited by EPA itself, "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise." Impacts of this warming include inundation of coastal lands, increased incidence of severe weather conditions (including storms as well as drought), spread of infectious diseases, and disruption of ecosystems.

Despite mounting evidence of global warming and its impacts, the EPA under the Bush administration has consistently refused to set any limits on the pollutants responsible, including carbon dioxide, methane, nitrous oxide and hydroflourocarbons from motor vehicles. In particular, EPA in 2003 denied a petition by several public interest groups to limit these greenhouse gases. The agency claimed that the Clean Air Act did not authorize such limits, and even if it did, the agency would refuse to set them.

The Supreme Court case in which today's brief is being filed is Commonwealth of Massachusetts v. USEPA, S. Ct. No. 05-1120. Petitioners include Massachusetts; California; Connecticut; Illinois; Maine; New Jersey; New Mexico; New York; Oregon; Rhode Island; Vermont; Washington; the District of Columbia; American Samoa, New York City; Baltimore; the Center for Biological Diversity; Center for Food Safety; Conservation Law Foundation; Environmental Advocates of New York; Environmental Defense; Friends of the Earth; Greenpeace; International Center for Technology Assessment; national Environmental Trust; Natural Resources Defense Council; Sierra Club; Union of Concerned Scientists; U.S. Public Interest Research Group.