The Supreme Court has affirmed on multiple occasions that the EPA not only has the authority but also a legal obligation to regulate CO2 emissions as an air pollutant through the Clean Air Act. Following is a summary of the most relevant case law.

Some background on the Clean Air Act and Air Pollutants.

The Clean Air Act (CAA) was passed in 1970 in large part to protect the public’s health and welfare by setting air pollutant and reduction standards. The CAA authorizes the US Environmental Protection Agency (EPA) to regulate emissions of mercury, nitrogen oxide, sulfur dioxide, and hundreds of other types of air pollutants from stationary sources, such as existing and new power plants, and from mobile sources, such as automobiles and trucks.  Until recently, however, CO₂ was not regulated as an air pollutant.

Two Supreme Court cases authorized EPA to regulate carbon dioxide as an air pollutant. The first case involved a disagreement with the argument being made at the time by the EPA (Massachusetts v. EPA, 127 S. Ct. 1438),  that the CAA did not authorize the agency to regulate CO₂ emissions as an air pollutant, and, even if it did, the EPA asserted it didn’t have to exercise its authority over greenhouse gases under the CAA if they chose not to. The Supreme Court disagreed and held that CO₂ emissions “fit within the Clean Air Act’s capacious definition of ‘air pollutant.’” Under the CAA, if something falls under the definition of "air pollutant" the EPA must determine whether that pollutant endangers public health or welfare. If the EPA makes an "endangerment finding," the agency must regulate the producers of that air pollutant.

A second Supreme Court case further solidified the EPA's regulatory authority over CO₂ emissions. American Electric Power Co. v. Connecticut, 582 F. 3d 309 (2011). This case was filed before Massachusetts v. EPA but took longer to wend its way through the court system and was not heard by the Supreme Court until four years later.. In American Electric Power Co., eight states, the City of New York, and three nonprofit land trusts filed suit against the five electric utilities that were the largest emitters of CO₂ in the U.S. At the time of the filing, the plaintiffs were dissatisfied with federal efforts on climate change mitigation, so they asked the Court to implement an annually decreasing emissions cap on the utilities' operations. As already noted, however, by the time the Court heard the case, Massachusetts v. EPA had already been decided, and in its opinion the Court underscored that it had already delegated the authority to regulate CO₂ emissions to the EPA.

In future posts we’ll look at more nuanced legal issues, such as whether the EPA has the authority to regulate CO₂ under section 111(d) of the proposed rule, whether the EPA can regulate CO₂ sources that are already subject to regulation under section 112, and the critical question of whether the EPA’s Building Blocks approach does or does not step outside of its authority, and, if a court were to overturn only a portion of the rule, including Building Blocks Three and Four, what that might mean for Arizona’s ability to comply with our 2030 goal.

Written by Maren Mahoney, EPIC