A well-documented rise in global temperatures has coincided with a significant increase in the concentration of carbon dioxide in the atmosphere. Many scientists believe that the two trends are related, because when carbon dioxide is released into the atmosphere, it produces a greenhouse effect, trapping solar heat. Under the Clean Air Act (CAA), the Environmental Protection Agency (EPA) is authorized to regulate “any” air pollutants “emitted into . . . the ambient air” that in its “judgment cause, or contribute to, air pollution.” A group of private organizations asked the EPA to regulate carbon dioxide and other “greenhouse gas” emissions from new motor vehicles. The EPA refused, stating that Congress last amended the CAA in 1990 without authorizing new, binding limits on auto emissions. Nineteen states, including Massachusetts, asked a district court to review the EPA’s denial. Did the EPA have the authority to regulate greenhouse gas emissions from new motor vehicles? If so, was its stated reason for refusing to do so consistent with that authority?

A well-documented rise in global temperatures has coincided with a significant increase in the concentration of carbon dioxide in the atmosphere. Many scientists believe that the two trends are related, because when carbon dioxide is released into the atmosphere, it produces a greenhouse effect, trapping solar heat. Under the Clean Air Act (CAA), the Environmental Protection Agency (EPA) is authorized to regulate “any” air pollutants “emitted into . . . the ambient air” that in its “judgment cause, or contribute to, air pollution.” A group of private organizations asked the EPA to regulate carbon dioxide and other “greenhouse gas” emissions from new motor vehicles. The EPA refused, stating that Congress last amended the CAA in 1990 without authorizing new, binding limits on auto emissions. Nineteen states, including Massachusetts, asked a district court to review the EPA’s denial. Did the EPA have the authority to regulate greenhouse gas emissions from new motor vehicles? If so, was its stated reason for refusing to do so consistent with that authority?