The Supreme Court will tackle the answer to this question this fall. At the end of August, a coalition of environmental groups, states, cities, religious organizations, power companies and a ski resort filed briefs before the court seeking a ruling on whether the EPA has the authority and the duty to regulate greenhouse gases such as CO₂ that are emitted from automobiles and power plants.

“What's at stake is whether or not the federal government has the authority to deal with climate change, said David Bookbinder, senior attorney for the Sierra Club in an interview published by Newsweek in early September.

The final decision will influence rules made by states such as California, which recently decided to regulate such emissions and claimed authority under the Clean Air Act. If the court decides the Congress did not grant the EPA such authority, then the state's would be forced to scale back its control over CO₂. Power companies are interested in the outcome because they are in the process of designing new plants and expanding existing ones, and need to know what standards should apply.

“We don't want a patchwork of states setting different standards for power plants,” said Bookbinder. “So we're trying to get EPA to regulate both auto standards and set a standard for power plants and other stationary sources.”

Meanwhile, on Aug. 30, 2006, California announced an agreement had been reached between the state legislature and Gov. Arnold Schwarzenegger that calls for a 25-percent reduction in CO₂ emissions by 2020 and could establish controls on the largest industrial sectors including utilities, oil refineries and cement plants.