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In 2011, EPA issued the Deferral Rule, excusing generators of biogenic b-CO2—emitted from the combustion of biological materials—from Clean Air Act (CAA) regulations for three years. Citing the need to study b-CO2, EPA invoked three legal doctrines to justify the rule: the de minimus, one-step-at-a-time, and administrative necessity doctrines. This Comment addresses Center for Biological Diversity v. Environmental Protection Agency, where the Center for Biological Diversity challenged the Deferral Rule in the U.S. Court of Appeals for the D.C. Circuit. The D.C. Circuit vacated the rule. Although the court did not decide the issue of statutory authority, it dismissed EPA’s legal justifications as incompetently invoked. Focusing on the one-step-at-a-time doctrine, the decision clarifies that although incremental rulemaking might be justified, it must be in furtherance of congressional intent, which, in this case, was to regulate all CO2—biogenic or otherwise—under the CAA. The D.C. Circuit’s decision is a decisive victory for the environment.