On June 26, a federal appeals court upheld the U.S. Environmental Protection Agency’s (EPA) authority to regulate greenhouse gases under the Clean Air Act (CAA). The decision reaffirms the EPA's ability to protect our health and the environment from air pollution and allows it to continue combating climate change.

The U.S. Court of Appeals for the District of Columbia Circuit agreed with EPA’s interpretation of the CAA and denied industry and state challenges to its finding that carbon dioxide and other greenhouse gases endanger public health. Environmental groups and leaders in Congress applauded the decision.

In 2007, the U.S. Supreme Court, in a landmark decision (Massachusetts v. EPA), held that EPA could regulate greenhouse gases as air pollutants under the CAA. The Court directed EPA, which had previously refused to regulate greenhouse gases, to determine whether such emissions endanger public health, a finding that triggers regulation under the CAA. In 2009, EPA issued a finding that motor vehicle emissions of greenhouse gases contribute to climate change, and climate change can be reasonably anticipated to endanger public health and welfare.

EPA then issued greenhouse gas emission standards for cars and light trucks as part of a joint rulemaking with the National Highway Traffic Safety Administration (NHTSA). New permitting requirements and standards for facilities emitting pollutants regulated under the CAA followed.

The D.C. Circuit rejected industry challenges to EPA’s endangerment finding and greenhouse gas emission standards, allowing the rules to stand. Affirming the "precautionary and preventive orientation" of the CAA, the court found that "EPA’s interpretation of the governing CAA provisions is unambiguously correct" and refused to substitute its judgment for that of agency experts when interpreting scientific evidence. Public interest and environmental groups rejoiced in the court’s stern rejection of arguments claiming the scientific evidence showing greenhouse gases cause climate change is too uncertain to support regulation.

Other air pollution control standards recently survived an attack on Capitol Hill. The Senate on June 20 voted 46-53 against a resolution introduced by Sen. James Inhofe (R-OK) that would have nullified EPA’s mercury and air toxics standards (MATS) for power plants. The MATS are expected to carry enormous public health benefits but have been targeted by industry groups.

The day after the Senate voted to retain the MATS, a coalition of clean air groups announced that they had collected an unprecedented 2.1 million public comments in support of the new carbon pollution standard that EPA proposed in March. The proposed rule would set national limits on the amount of carbon pollution newly built power plants can emit. According to the clean air groups, this is the largest number of comments ever submitted to the EPA during a public comment period.

Despite the popularity of clean air protections, we can expect more challenges to EPA rules in both Congress and the courts. Although the recent Senate vote and D.C. Circuit decision protected the rules under attack, CAA standards are far from safe. Attempts to weaken EPA air regulations have succeeded in the House, and industry groups have filed lawsuits challenging the MATS. At the same time, EPA faces the possibility of significant funding cuts and rulemaking constraints. The House Appropriations Committee recently approved a bill that would slash EPA’s funding and prevent the agency from implementing certain greenhouse gas standards.