[co-author: Emily Guillaume] On April 5, the U.S. Court of Appeals for the D.C. Circuit vacated a Trump-era rule that would have prevented the Environmental Protection Agency (EPA) from setting greenhouse gas (GHG) emissions standards for almost any class of stationary sources, except for fossil fuel-fired electric generating units. The court’s decision, issued at the request of the new Biden EPA, clears the way for new sector-by-sector GHG regulations should the new administration seek to set new GHG standards under Section 111 of the Clean Air Act (CAA). Finalized in January 2021, the rule was one of the last promulgated by the previous administration and sets forth a new framework for determining what level of GHG pollution constitutes a “significant contribution” under Section 111(b) under the CAA — a prerequisite for setting a “New Source Performance Standard,” as well as any related standard for existing sources. Under the new test, specific industry sectors would have been subject to regulation of GHG emissions under Section 111(b) only if “the amount of those emissions exceeds 3% of total U.S. GHG emissions.” However, only electric utilities would meet that test — EPA indicated utilities emit 25% of total GHG emissions, whereas the next highest emitting category (oil and gas production) emits just under 3%.