Environmental Protection Agency Administrator Lee Zeldin on Thursday signed an order eliminating the 2009 EPA Endangerment Finding, a determination that classified carbon dioxide, methane, nitrous oxide, and other greenhouse gases as threats to public health and welfare under the Clean Air Act.

That finding, issued during the Obama administration, formed the legal basis for a broad range of federal greenhouse gas emissions standards for vehicles and engines. A source familiar with the decision said all subsequent federal vehicle greenhouse gas rules stemming from the 2009 declaration will be rescinded.

The regulatory structure was reinforced by the Supreme Court’s ruling in Massachusetts v. EPA, which held that greenhouse gases qualify as air pollutants under the Clean Air Act and can be regulated if deemed harmful to public health or welfare.

Zeldin said the move will restore consumer choice and reduce regulatory burdens on automakers and drivers. He specifically criticized the federally incentivized auto start-stop feature, which shuts off engines at idle to reduce emissions.

“As I traveled across all 50 states this past year, I heard from countless Americans who not only dislike the start-stop feature but passionately advocated for this mechanism to be a thing of the past,” Zeldin said. He added that the technology places strain on vehicle batteries while offering minimal environmental benefit.

The EPA created credits for start-stop systems in 2012, allowing automakers to claim greenhouse gas compliance benefits. Critics have described the mechanism as a regulatory workaround that did not produce measurable emissions reductions.

White House Press Secretary Karoline Leavitt said rescinding the 2009 finding will amount to “the largest deregulatory action in American history,” estimating $1.3 trillion in regulatory savings.

Environmental groups signaled they are preparing legal challenges. Earthjustice President Abigail Dillen told PBS the organization intends to contest the decision in court, arguing it conflicts with established science and statutory requirements.

Some state leaders also voiced opposition. Colorado Gov. Jared Polis said he supports the prior federal standards and warned that rolling them back creates uncertainty for businesses and consumers. He added that Colorado will continue pursuing policies aimed at reducing vehicle emissions and protecting air quality.

The EPA’s order sets the stage for a legal and political battle over federal authority to regulate greenhouse gases, with implementation expected to face immediate scrutiny from environmental advocates and Democratic-led states.