In 2009, when carbon dioxide (CO2) levels were at 387.43 parts per million, the Center for Biological Diversity and 350.org submitted a citizen petition calling on the U.S. Environmental Protection Agency to take steps necessary to institute a national ambient air quality standard (NAAQS) for greenhouse gas (GHG) emissions under §§108-110 of the Clean Air Act (CAA). For 12 years, the petition was simply ignored. Then, the day President Donald Trump left office, outgoing EPA Administrator Andrew Wheeler issued a letter denying the petition. Six weeks later, Acting Administrator Jane Nishida reversed the denial, and the petition remains pending. This Comment addresses the flaws in the denial’s legal and factual reasoning; outlines the technological, scientific, and policy advancements made in the 12 years since the petition was submitted; demonstrates why a GHG NAAQS, particularly one focused on CO2, meets the statute’s purpose and programmatic specifics; and presents a 2021 take on climate change policy that explains why a CO2 NAAQS is an essential and mandatory tool in the CAA toolbox for accomplishing the Biden Administration’s climate agenda.