The repeal of the Environmental Protection Agency's 2009 "endangerment finding" was paired with the elimination of greenhouse gas standards on automobiles.

But it also places a host of other climate rules in jeopardy, including carbon dioxide emissions from power plants and methane leaks for oil and gas producers.

Legal challenges are expected to follow swiftly.

- Climate change 'a scam' -

"This determination had no basis in fact, had none whatsoever, and no basis in law," Trump said.

The president dismissed concerns that the repeal could cost lives by worsening climate change, reiterating his belief that human-caused global warming is a hoax: "I tell them, don't worry about it, because it has nothing to do with public health, this was all a scam, a giant scam."

The administration also framed the measure as a cost-saving move, claiming it would generate more than $1 trillion in regulatory savings and bring down new car costs by thousands of dollars.

The announcement immediately drew condemnation from Democrats and from green groups.

"This decision betrays the American people and cements the Republican Party's status as the pro-pollution party," said California Governor Gavin Newsom, seen as a possible presidential candidate.

Manish Bapna, president of the nonprofit Natural Resources Defense Council, told AFP it was the "single biggest attack in history on the United States federal government's efforts to tackle the climate crisis."

- Key finding -

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.

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 Environmental Protection Agency 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, which could now be vulnerable.

- Legal case -

Procedurally, the draft proposal argued 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.

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.

- Shaky science -

The scientific arguments are just as shaky, critics say. The draft repeal sought to downplay the impact 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.

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.

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.