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 emissions.

"This amounts to the largest act of deregulation in the history of the United States," EPA Administrator Lee Zeldin told The Wall Street Journal.

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 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 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 finding would immediately revoke the requirement for federal greenhouse gas emissions standards for automobiles.

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.

"The Obama Administration made one of the most damaging decisions in modern history," the EPA said in a statement to AFP Tuesday.

"The Endangerment Finding is the legal prerequisite used by the Obama and Biden Administrations to justify trillions of dollars of greenhouse gas regulations covering new vehicles and engines," it added.

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

That report was widely criticized 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.

Environmental organizations are expected to move quickly to challenge in court the elimination of the 2009 determination.

"If the EPA follows through and tries to repeal the Endangerment Finding, we will see them in court," Manish Bapna, president of the Natural Resources Defense Council, said recently.