US President Donald Trump's administration is set to finalise on Thursday its repeal of a foundational scientific determination that underpins the US government's authority to regulate greenhouse gas emissions.

The Environmental Protection Agency last summer proposed reversing the so-called Endangerment Finding of 2009, in what was seen as a major blow to climate action in the world's biggest historic contributor of planet-warming pollution.

"On Thursday, President Trump will be joined by Administrator Lee Zeldin to formalise the rescission of the 2009 Obama-era endangerment finding," White House Press Secretary Karoline Leavitt told a news briefing on Tuesday.

"This will be the largest deregulatory action in American history, and it will save the American people US$1.3 trillion in crushing regulation."

The finding under then-president Barack Obama concluded that six greenhouse gases – including carbon dioxide and methane – endanger public health and welfare by driving climate change.

That determination flowed from a 2007 Supreme Court decision, Massachusetts vs 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 the finding initially applied only to a section of the Clean Air Act governing vehicle emissions, it was later incorporated into other regulations.

As a result, repealing the determination would immediately be accompanied by revoking the requirement for federal greenhouse gas emissions standards for vehicles.

And it would place a broader suite of climate regulations in legal jeopardy, including limits on carbon dioxide from power plants and methane from oil and gas operations – a major boon to the fossil fuel sector.

The administration's draft proposal rests on both legal and scientific arguments. Procedurally, it asserts 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.

On the scientific front, the administration has sought to downplay the scale and impacts of human-caused climate change.

It commissioned an Energy Department working group filled with sceptics of human-caused climate change to produce a report challenging the scientific consensus.

That report was widely criticised for misattribution and misstating the conclusions of the studies it cited. Environmental groups sued the Energy Department, alleging the panel was convened behind closed doors in violation of federal rules. Energy Secretary Chris Wright later disbanded the group.

The Trump administration has claimed that repealing the Endangerment Finding would lead to reduced new car costs, which have spiralled since the pandemic.

But the calculations have been challenged – an analysis by the EPA under the administration of Joe Biden concluded that the standards would save the average American driver an estimated US$6,000 in reduced fuel and maintenance over the life of a vehicle.

Moreover, while the administration says the rules imposed more than US$1 trillion in cumulative regulatory costs that were passed on as "hidden taxes," it does not weigh those costs against the monetised benefits from climate protection, public health and fuel savings.

"The Trump EPA is cynically pretending climate change isn't a risk to Americans' health and welfare," said Meredith Hankins, federal climate legal director at the Natural Resources Defence Council.

"This is the biggest attack ever on federal authority to tackle the climate crisis, and a devastating blow to millions of Americans facing growing risks of unnatural disasters."

She added the legal arguments the administration has presented were "slapdash" and "should be laughed out of court," vowing to challenge the decision. (AFP)