In 2009, the Environmental Protection Agency’s (EPA) “endangerment finding” empowered the U.S. to regulate emissions of greenhouse gases. This scientific and legal determination ruled that planet-warming gases such as carbon dioxide are dangerous to human health and welfare. Now, the Trump administration has moved to rescind that finding.

At an auto dealership in Indiana on Tuesday, July 29, EPA Administrator Lee Zeldin unveiled the agency’s proposal to repeal the finding. The EPA claims this move would save Americans $54 billion in costs annually through the elimination of all greenhouse gas standards for motor vehicles and engines, including Biden’s electric vehicle mandate. The rollback marks the most aggressive attempt by President Donald Trump to unravel federal restrictions on fossil fuels. Zeldin called it the “largest deregulatory action in the history of America” on Tuesday, the Associated Press reports.

“With this proposal, the Trump EPA is proposing to end 16 years of uncertainty for automakers and American consumers,” Zeldin said, according to an agency release.

Scientists, climate advocates, environmental policy experts, and former EPA leaders warn the repeal would have severe consequences for American health, well-being, and the climate. “Abandoning all efforts to address climate change is not in the best interest of anyone but the fossil fuel industry, which has made trillions of dollars over the last 50 years and has shown that if unchecked, it will pursue profits at any cost, even if that destroys the American way of life,” Shannon Baker-Branstetter, senior director of domestic climate at the Center for American Progress, said in a statement.

The Clean Air Act requires the EPA to regulate any air pollutant that endangers public health or welfare. In 2007, the Supreme Court ruled that greenhouse gases are subject to this mandate. Two years later, the endangerment finding determined that current and projected atmospheric concentrations of six key greenhouse gases “threaten the public health and welfare of current and future generations.” The simultaneously issued “cause or contribute finding” pointed to motor vehicles and engines as major sources of hazardous greenhouse gas emissions. These determinations serve as the legal basis for the EPA to regulate planet-warming pollution.

In the 16 years since the agency issued the endangerment finding, scientists have found overwhelming evidence to show that greenhouse gases endanger public health and drive global warming. Even under EPA regulation, emissions have led to deadly consequences for countless Americans, driving more frequent and intense extreme weather events and worsening air quality. Despite this, conservatives and some congressional Republicans have argued that the real threat is overregulation and hidden taxes.

In March, Zeldin announced a formal reconsideration of the endangerment finding on the grounds that the EPA failed to consider the regulatory fallout back in 2009. That move was part of a series of environmental rollbacks that aimed to eliminate 31 regulations on clean air, clean water, climate change, and more, according to the AP. Trump set the precedent for these actions with a day-one executive order to drastically scale back environmental regulations, which also demanded the EPA submit a report “on the legality and continuing applicability” of the endangerment finding.

Now, Zeldin aims to “undo the underpinning of $1 trillion in costly regulations” by revoking the finding altogether, according to the EPA release. The proposal must go through a lengthy review process, including public comment, before it’s finalized. Environmental groups have voiced strong opposition to the decision and vowed to fight it, including the National Resources Defense Council (NRDC).

“The EPA wants to shirk its responsibility to protect us from climate pollution, but science and the law say otherwise,” Christy Goldfuss, executive director of the NRDC, said in a statement. “NRDC’s lawyers and scientists are not going to let that happen without a fight. If EPA finalizes this illegal and cynical approach, we will see them in court.”