The U.S. Supreme Court on June 26 agreed to review whether the Environmental Protection Agency (EPA) has the authority under the Clean Air Act to regulate greenhouse gas emissions from automobiles. The case in question, Massachusetts vs. EPA, stems from a petition filed by environmental groups calling for EPA to establish automotive emission standards for carbon dioxide and other greenhouse gases. In August 2003, EPA denied the petition and indicated that they would not publish such standards, even if the authority to do so existed under the Clean Air Act. Numerous environmental groups and states appealed the decision and the lawsuits were consolidated for review.

At the heart of the case is Section 202(a)(1) of the Clean Air Act that states, "The Administrator shall by regulation prescribe (and from time to time revise) in accordance with the provisions of this section, standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare." The D.C. Circuit Court's ruling indicated that EPA relied on reasonable scientific uncertainty about the effects of greenhouse gas emissions on the public health, as well as other policy determinations in deciding against regulating carbon dioxide emissions. The plaintiffs, including 12 states and numerous environmental groups, argue that the decision by EPA not to regulate carbon dioxide emissions was arbitrary and violated the requirements of the act. On appeal, the U.S. Court of Appeals for the District of Columbia Circuit held in July 2005 that EPA did not violate section 202(a)(1) of the act, 42 U.S.C. §7521(a)(1) by declining to regulate carbon dioxide emissions from automobiles. Arguments before the Supreme Court will be made late in 2006, with a decision expected by June 2007.