The Trump administration has said it will rescind the long-standing finding that greenhouse gas emissions endanger human health, removing the legal foundation for all US greenhouse gas regulations.
If finalised, the repeal would end current limits on greenhouse gas pollution from vehicle tailpipes, power plant smokestacks and other sources, and hamper future US efforts to combat global warming.
Environmental Protection Agency Administrator Lee Zeldin announced the agency's plan to rescind the "endangerment finding" at an event at a car dealership in Indiana, alongside Energy Secretary Chris Wright, and called it the largest deregulatory action in US history.
He argued the move reflected the will of US voters for the regulator to protect the economy.
"We at the Trump EPA, under the leadership of President Trump, choose to both protect the environment and grow the economy," he said.
Republican President Donald Trump has cast doubt on climate change, even as a consensus of scientists warns that global temperature rise driven by human activity is triggering more frequent devastating storms, wildfires, and floods.
The proposal, which needs to undergo a public comment period, would cut $54bn in costs annually through the repeal of all greenhouse gas standards, including the vehicle tailpipe standard, he said.
Environmental groups blasted the move, saying it spells the end of the road for US action against climate change, even as the impacts of global warming become more severe.
"With today's announcement, the EPA is telling us in no uncertain terms that US efforts to address climate change are over. For the industries that contribute most to climate change, the message is 'pollute more'.
"For everyone feeling the pain of climate disasters, the message is 'you’re on our own,'" said Abigail Dillen, president of Earthjustice.
The move is expected to trigger legal challenges, according to several environmental groups, states and lawyers.
Mr Zeldin said that a 2024 Supreme Court decision that reduced the power of federal agencies to interpret the laws they administer, known as the Chevron deference, means that the EPA does not have the ability to regulate greenhouse gases.
"We do not have that power on our own to decide as an agency that we are going to combat global climate change because we give ourselves that power," Mr Zeldin said.
He added that if Congress decides it wants to amend the federal Clean Air Act to explicitly state the US should regulate carbon dioxide, methane and other planet-warming gases, the EPA would follow its lead.
The endangerment finding's roots date back to 2009, when the EPA under former democratic president Barack Obama issued a finding that emissions from new motor vehicles contribute to pollution and endanger public health and welfare.
That assessment followed a 2007 US Supreme Court decision in its landmark Massachusetts v. EPA case that said the EPA has the authority under the Clean Air Act to regulate greenhouse gas emissions and required the agency to make a scientific finding on whether those emissions endanger public health.
The endangerment finding was upheld in several legal challenges and underpinned subsequent greenhouse gas regulations, ranging from tailpipe standards for vehicles, carbon dioxide standards for aircraft, and methane standards for oil and gas operations.
