The Environmental Protection Agency (EPA) moved on Tuesday to repeal a landmark environmental decision to regulate greenhouse gas emissions and fight climate change.
The rule, which is known as the "endangerment finding," is a 2009 declaration that determined the current and projected concentrations of six greenhouse gases, including carbon dioxide, "endanger both the public health and the public welfare of current and future generations."
The decision was based on a nearly 200-page EPA analysis of the science and more than 380,000 public comments on greenhouse gas pollution.
On Tuesday, EPA Administrator Lee Zeldin, appointed by President Donald Trump, announced plans to repeal the endangerment finding, arguing that the EPA lacked the legal authority to determine that greenhouse gases -- such as carbon dioxide, methane and hydrofluorocarbons -- endanger public health and welfare.
He alleged that the initial rule "made predictions about the science that was not just pessimistic, they turned out not to be true."
At an event in Indiana, and joined by Gov. Mike Braun, R-Ind., and Energy Secretary Chris Wright, Zeldin said if the repeal of the endangerment finding is finalized, it would be "the largest deregulatory action in the history of the United States" and justified the decision by saying, "Agencies of this federal government do not have the ability to just creatively come up with our own law. 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."
The endangerment finding is the result of the 2007 Supreme Court decision Massachusetts v. EPA, which decided that the EPA could regulate greenhouse gases for motor vehicles under the 1970 Clean Air Act because those gases are considered air pollutants.
That ruling became the legal foundation for many of the federal government's greenhouse gas emissions regulations for vehicles, fossil fuel power plants and other pollution sources responsible for climate change.
MORE: EPA proposes rolling back clean air rules for power plants: What to knowZeldin estimated that if finalized, the proposal could save $54 billion in costs annually and added that if Congress wanted to amend the Clean Air Act to include greenhouse gases, the EPA would regulate them.
At the press conference, Zeldin also announced plans to not only repeal the endangerment finding but "all greenhouse gas emissions that followed on light, medium and heavy duty vehicles."
The transportation sector is the most significant contributor of direct greenhouse gas emissions in the U.S.
In 2024, the EPA determined that the vehicle model year 2027 standards currently on the chopping block were "expected to avoid 7.2 billion tons of CO2 emissions through 2055, roughly equal to four times the emissions of the entire transportation sector in 2021" and save the average driver around $6,000 in fuel costs and maintenance over the life of their vehicle.
Critics of the administration's plan argue that the move to repeal the endangerment finding lacks both a scientific basis and a legal foundation and will exacerbate the harmful impacts of climate change.
The Sierra Club told ABC News in a statement that it's exploring all legal options, including litigation.
MORE: Trump's policies could impact the environment long after he leaves office, some experts say"As if any doubt remained, the Trump Administration has formalized climate denial as the official policy of the United States government. If approved, rescinding the endangerment finding would strike a decisive blow to the EPA's authority to limit deadly greenhouse gas emissions and protect our people and our planet from the very worst of the climate crisis," said Loren Blackford, the Sierra Club's acting executive director.
Mike Gerrard, an environmental lawyer and the founder and faculty director of the Sabin Center for Climate Change Law at Columbia University, told ABC News the EPA decision to repeal the endangerment finding is the latest salvo in the Trump administration's ongoing effort to eliminate climate change regulations.
"The short-term consequences are limited because the things that are enabled by the endangerment finding, they're not doing anyway. They're cutting back on the greenhouse gas emissions controls for motor vehicles. They're eliminating the regulation of greenhouse gases from power plants and other things," said Gerrard.
Gerrard said lawsuits are a certainty, and it's likely that the case will end up at the Supreme Court, a court that is very different from the one that issued the 2007 ruling.
"What they hope for is a binding legal outcome that would preclude, either explicitly or effectively, future regulation of greenhouse gasses under the Clean Air Act," said Joe Goffman, the former EPA assistant administrator for its Office of Air and Radiation during the Biden administration at press briefing.
In the decade and a half since that announcement, the science on how greenhouse gases impact human health has only gotten more robust.
A 2021 study published in the journal Nature Climate Change found that between 1991 and 2018, 37% of heat-related deaths could be attributed to human-amplified climate change.
And according to the Fifth National Climate Assessment, a report that the federal government describes as providing "authoritative scientific information about climate change risks, impacts, and responses in the U.S.," found that "Climate changes are making it harder to maintain safe homes and healthy families; reliable public services; a sustainable economy; thriving ecosystems, cultures, and traditions; and strong communities."
The public will have an opportunity to comment on the proposed rule before it becomes final. Lawsuits could also delay its implementation. The courts have uniformly rejected previous attempts to challenge the endangerment finding on legal grounds.