On December 7, 2009, the U.S. Environmental Protection Agency issued a final determination that officially declared six greenhouse gases are a threat to public health and welfare. The EPA went on to find that emissions of those GHGs from on-road vehicles contribute to the threat. The EPA's December 7th determination is the latest in a string of actions regarding GHGs taken by the agency this year that could ultimately lead to the direct regulation of GHG emissions from power plants and other stationary sources.

The EPA was required to make a determination on this issue by the U.S. Supreme Court's ruling in Massachusetts v. EPA, which held that GHGs are an air pollutant as defined under the Clean Air Act. While the EPA's final determination does not impose any new regulations regarding emissions, it is viewed as a very public signal by the EPA that it is moving closer to regulating GHGs under the Clean Air Act. Having made this final determination, the EPA is now required by law to issue regulations for GHG emissions from new motor vehicles. However, the EPA still has substantial latitude with respect to the implementation and scope of any such regulations.

Significantly for operators of stationary sources such as power plants, the determination is limited in scope to only section 202(a) of the Clean Air Act regulating new motor vehicles, meaning that the finding does not apply to stationary sources regulated under the Clean Air Act, such as power plants. While important differences exist between section 202(a) and those sections of the Clean Air Act regulating power plants and other stationary sources, it is thought that a finding on GHG emissions from stationary sources could follow from, and could be substantially similar to, the EPA's final determination for on-road motor vehicles.

The EPA could act on GHG emissions from stationary sources under the existing regulatory structure, and this final determination makes that seem more likely. For the time being, though, it remains unclear whether the EPA will pursue this route despite the ongoing developments with respect to GHG regulation at the national and international levels, or if the EPA will wait for Congress to pass more targeted legislation. Barring a legislative reversal of the EPA's final determination, it is no longer a question of "if"—GHGs will be regulated at the federal level, at least for on-road motor vehicles. However, the power sector continues to await word on which stationary sources will be regulated and how extensively they will be regulated, leaving planning for a carbon-constrained world still filled with uncertainty.