Washington DC - President Donald Trump on Thursday revoked a landmark scientific finding that underpins US regulations aimed at curbing planet-warming pollution, marking the administration's most far-reaching rollback of climate policy to date.

"This determination had no basis in fact, had none whatsoever, and no basis in law," Trump claimed during a White House event where he was joined by Environmental Protection Agency Administrator Lee Zeldin.

As expected, the administration also formally scrapped greenhouse emissions standards on cars. The repeal is expected to be swiftly challenged in court.

The 2009 "endangerment finding" was a determination under then-President Barack Obama that six greenhouse gases threaten public health and welfare by fueling climate change.

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It came about as a result of a prolonged legal battle ending in a 2007 Supreme Court decision, Massachusetts v. EPA, which ruled that greenhouse gases qualify as pollutants under the Clean Air Act and directed the EPA to determine whether they pose a danger to public health and welfare.

While it initially applied only to vehicle emissions, it later became the legal foundation for a broader suite of climate regulations.

Thursday's repeal was thus accompanied by the repeal of the Greenhouse Gas Vehicle Standards.

But the consequences could ripple further, placing a host of climate rules in jeopardy – including limits on carbon dioxide from power plants and methane from oil and gas operations.

The final text will be closely scrutinized to see how it is framed, with the administration advancing procedural, scientific, and cost-based arguments.

Procedurally, the draft proposal asserted that greenhouse gases should not be treated as pollutants in the traditional sense because their effects on human health are indirect and global rather than local.

Regulating them within US borders, it contends, cannot meaningfully resolve a worldwide problem.

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The Supreme Court has re-affirmed the endangerment finding multiple times – most recently in 2022, when the court's composition was much the same as today.

The scientific arguments are just as shaky, critics say. The draft repeal sought to downplay the scale and impacts of human-caused climate change, citing a study commissioned by an Energy Department working group of skeptics to produce a report challenging the scientific consensus.

That report was widely panned by researchers, who said it was riddled with errors and misrepresented the studies it cited. The working group itself was disbanded following a lawsuit by nonprofits that argued it was improperly convened.

And in September, the National Academies of Sciences, Engineering, and Medicine issued its own report saying the evidence for current and future harm to human health and welfare created by human-caused greenhouse gases is beyond scientific dispute.

The administration has also leaned heavily on putative cost savings, claiming repealing the endangerment finding would generate more than $1 trillion in regulatory savings, without detailing how the figure was calculated. It has also said it would lower new car costs.

Environmental advocates say that the administration is ignoring the other side of the ledger, including lives saved from reduced pollution and fuel savings from more efficient vehicles.

They also warn the rollback would further skew the market toward more gas-guzzling cars, undermining the American auto industry's ability to compete in the global race toward electric vehicles.