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On April 2, 2007, the U.S. Supreme Court in 2 Massachusetts v. EPA ruled 5-4 that carbon dioxide (CO ) was an air pollutant under the Federal Clean Air Act (CAA). The court’s decision means that the Federal Environmental Protection Agency (EPA) is required to reconsider its decision not to regulate carbon monoxide (CO) as an air pollutant when establishing auto tailpipe emission requirements. The court’s decision is an important victory for those seeking a more active United States policy on regulating greenhouse gas (GHG) emissions to combat global warming. This newsletter examines the United States role in global warming, the Massachusetts decision, and the decision’s impact on United States GHG regulation policies.