Denver Challenges EPA's Illegal Rollback of Greenhouse Gas Rule

Denver Challenges EPA's Illegal Rollback of Greenhouse Gas Rule

Published on March 19, 2026

The City and County of Denver today joined a coalition of 24 states and 11 other counties and cities to challenge the U.S. Environmental Protection Agency's unlawful attempt to rescind its 2009 Endangerment Finding - the agency's seminal determination that greenhouse gas pollution from motor vehicles drives climate change and endangers public health and welfare.

"Every Denverite deserves clean air, a healthy environment, and a safe future - and by rescinding the Endangerment Finding, EPA is undermining all three," said Denver Mayor Mike Johnston. "This roll back from the Trump Administration ignores decades of science and settled law. It leaves cities like ours to bear the costs of hotter summers, dirtier air, and extreme weather. Denver will continue to lead with partners nationwide to defend the protections our community depends on."

The 2009 Endangerment Finding was the direct result of the landmark 2007 Supreme Court decision in Massachusetts v. EPA, which confirmed that the Clean Air Act authorizes EPA to regulate greenhouse gas emissions that endanger public health and welfare. Based on years of rigorous scientific analysis and review, in 2009 EPA determined that emissions from motor vehicles contribute to air pollution that harms public health and the environment. EPA then set federal standards to limit those emissions, which have led to significant reductions in greenhouse gas emissions from motor vehicles.

Now, almost two decades later, EPA has rushed a rulemaking process to rescind the Endangerment Finding and repeal all motor vehicles greenhouse gas standards, blatantly disregarding the law and science. EPA's rescission is based on flawed interpretations of the law --- previously rejected by the Supreme Court - that the agency lacks authority to regulate greenhouse gas emissions. The rescission also ignores decades of peer-reviewed scientific evidence confirming the reality and severity of climate change. By eliminating all existing and future federal vehicle greenhouse gas emission standards, the rule violates EPA's legal obligations, fundamental principles of administrative law, and its mission to protect public health and welfare.

Denver is already experiencing accelerating climate impacts, with four of its five hottest summers on record occurring within the past five years. Record-setting heat waves are becoming at least five times more likely due to human-caused climate change. Climate-driven drought is also straining Denver's water supply, with statewide streamflows projected to decline 5-30% by 2050. Extreme weather, including billion-dollar hailstorms, flooding, and wildfire smoke, continues to impose growing economic and infrastructure costs on the city.

To better understand where these impacts fall hardest, Denver maintains a Climate Vulnerability Index, which maps neighborhoods facing the greatest exposure to extreme heat, poor air quality, and climate-related inequities.

Today's lawsuit is the latest action taken by the City and County of Denver and the coalition in their ongoing effort to fight back against EPA's unlawful rescission of the 2009 Endangerment Finding. In the fall of 2025, 23 attorneys general and seven counties and cities submitted two comment letters urging EPA to abandon the proposal, arguing that it would violate settled law, clear Supreme Court precedent, and scientific consensus, endanger hundreds of millions of Americans-particularly communities disproportionately burdened by environmental harms-and cause unprecedented disruption to the regulatory landscape with catastrophic consequences for residents, industries, natural resources, and public investments.

Co-led by the attorneys general of California, Connecticut, New York and Massachusetts, the City and County of Denver is joined in filing this challenge by the attorneys general of: Arizona, Colorado, Delaware, Hawai'i, Illinois, Maine, Maryland, Michigan, Minnesota, Nevada, New Jersey, New Mexico, North Carolina, Oregon, Rhode Island, Vermont, the Commonwealth of Virginia, Washington, Wisconsin, the District of Columbia, and the United States Virgin Islands, as well as Josh Shapiro, in his official capacity as Governor of the Commonwealth of Pennsylvania; the Cities of Albuquerque, New Mexico; Boston, Massachusetts; Chicago, Illinois; Cleveland, Ohio; Columbus, Ohio; Los Angeles, California; and New York, New York; the Counties of Harris, Texas; Martin Luther King, Jr., Washington; and Santa Clara, California; and the City and County of San Francisco, California.

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