President Donald Trump's administration has revoked a scientific finding that has been the central basis for US action to fight climate change.
The Obama-era "endangerment finding" served as the legal basis for federal climate regulations, setting limits on greenhouse gas emissions.
It determined that carbon dioxide and other greenhouse gases threatened public health and welfare.
Curbs on vehicle emissions have also been axed.
The moves come after a year of implementing a string of regulatory cuts and other actions intended to unfetter fossil fuel development and stymie the rollout of clean energy.
"Under the process just completed by the EPA, we are officially terminating the so-called endangerment finding, a disastrous Obama-era policy that severely damaged the American auto industry and drove up prices for American consumers," Mr Trump said.
The president announced the repeal alongside EPA Administrator Lee Zeldin and White House Budget Director Russ Vought, who was a key architect of the conservative policy blueprint Project 2025.
Mr Trump has said he believes climate change is a "con job", and has withdrawn the US from the Paris Agreement, leaving the world's largest historic contributor to global warming out of international efforts.
Former President Barack Obama blasted the move on X, saying without the endangerment finding, "we'll be less safe, less healthy and less able to fight climate change — all so the fossil fuel industry can make even more money".
'The holy grail'
Mr Zeldin said the Trump administration pursued the most consequential climate policy of the past 15 years, something the agency had avoided during his first term.
"Referred to by some as the holy grail of federal regulatory overreach, the 2009 Obama EPA endangerment finding is now eliminated," he said.
The endangerment finding was first adopted by the US in 2009, and led the EPA to take action under the Clean Air Act of 1963 to curb emissions of carbon dioxide, methane, and four other heat-trapping air pollutants from vehicles.
It followed the Supreme Court's 2007 decision in Massachusetts v. EPA, which held that the agency has authority to regulate carbon dioxide and other greenhouse gas emissions under the Clean Air Act.
Its repeal would remove the regulatory requirements to measure, report, certify, and comply with federal greenhouse gas emission standards for cars.
The transportation and power sectors are each responsible for about a quarter of US greenhouse gas output, according to EPA figures.
The EPA said the repeal of vehicle emission standards would save US taxpayers $US1.3 trillion ($1.8 trillion), while the prior administration said the rules would have net benefits for consumers through lower fuel costs and other savings.
The Alliance for Automotive Innovation did not endorse the action but said "automotive emissions regulations finalised in the previous administration are extremely challenging for automakers to achieve given the current marketplace demand for EVs".
The Environmental Defense Fund said that the repeal would end up costing Americans more, despite EPA's statement that climate regulations have driven up costs for consumers.
"Administrator Lee Zeldin has directed EPA to stop protecting the American people from the pollution that's causing worse storms, floods, and skyrocketing insurance costs," EDF President Fred Krupp said.
"This action will only lead to more of this pollution, and that will lead to higher costs and real harms for American families."
Under former presidentJoe Biden, the EPA aimed to cut passenger-vehicle fleet-wide tailpipe emissions by nearly 50 per cent by 2032.
This compares with 2027 projected levels and forecasts that between 35 per cent and 56 per cent of new vehicles sold between 2030 and 2032 will need to be electric.
The agency estimated that the rules would deliver net annual benefits of $99 billion through 2055.
Consumers were expected to save an average of $6,000 over the lifetime of new vehicles from reduced fuel and maintenance costs.
The coal industry celebrated the announcement, saying it would help stave off retirements of aging coal-fired power plants.
"Utilities have announced plans to retire more than 55,000 megawatts of coal-fired generation over the next five years," America's Power President and CEO Michelle Bloodworth said.
"Reversing these retirement decisions could help offset the need to build new, more expensive electricity sources and prevent the loss of reliability attributes, such as fuel security, that the coal fleet provides."
Uncertainty unbound
Legal experts said the policy reversal could lead to a surge in lawsuits known as "public nuisance" actions.
"This may be another classic case where overreach by the Trump administration comes back to bite it," said Robert Percival, a University of Maryland environmental law professor.
Environmental groups have slammed the proposed repeal as a danger to the climate.
Future US administrations seeking to regulate greenhouse gas emissions likely would need to reinstate the endangerment finding.
Several environmental groups, including the Natural Resources Defense Council and Earthjustice, have said they will challenge the reversal in court.
"There'll be a lawsuit brought almost immediately, and we'll see in them in court. And we will win," said David Doniger, senior attorney at the NRDC.
ABC/Wires
