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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 re- 2 consider its decision not to regulate 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 U.S. policy on regulating greenhouse gas (GHG) emissions to combat global warming. This newsletter examines the U.S. role in global warming, the Massachusetts decision, and the decision’s impact on U.S. GHG regulation policies.