President Donald Trump said Thursday that the Environmental Protection Agency is withdrawing the key legal determination it has used for nearly 20 years to control climate-warming pollution from vehicles, oil refineries and industrial facilities.

The move reverses a pivotal ruling known as the endangerment finding that has served as the backbone of federal climate policy. Eliminating it is expected to dismantle much of the United States’ framework for reducing greenhouse gas emissions.

Issued in 2009, the finding concluded that greenhouse gases such as carbon dioxide and methane contribute to global warming in ways that threaten the health and welfare of both current and future generations.

“We are officially terminating the so called endangerment finding, a disastrous Obama era policy,” Trump said in a news conference Thursday. “This determination had no basis in fact none whatsoever. And it had no basis in law. On the contrary, over the generations, fossil fuels have saved millions of lives and lifted billions of people out of poverty all over the world.”

Prominent environmental organizations have rejected the administration’s reasoning and are preparing legal action to challenge the repeal.

For years, the endangerment finding formed the legal foundation for the EPA’s authority to regulate greenhouse gas emissions from cars and power plants and to require companies to disclose their emissions. It also obligated the federal government to address climate change under the Clean Air Act.

In 2007, the Supreme Court of the United States determined that the EPA had the authority to regulate greenhouse gases and recognized that the dangers linked to climate change are “serious and well recognized.” That ruling paved the way for the agency’s endangerment finding two years later.

The White House and the EPA have described rescinding the finding as “the largest deregulatory action in American history.”

The decision marks the administration’s most sweeping effort yet to scale back federal climate initiatives. The U.S. formally exited the 2015 Paris Agreement again last month and is expected to withdraw from the United Nations Framework Convention on Climate Change, moves that would significantly reduce America’s role in international climate negotiations.

Trump, who has repeatedly labeled climate change a “con job,” canceled nearly $8 billion in clean energy funding in October, though a judge later ruled that some of those cancellations were unlawful. Meanwhile, the U.S. Department of Energy announced Wednesday it will allocate $175 million to prolong operations at six coal-fired power plants, continuing a broader push to support the coal industry.

According to the Copernicus Climate Change Service, last year ranked as the third warmest on record. The past 11 years collectively represent the hottest period ever documented.

Trump and EPA Administrator Lee Zeldin also said Thursday that the agency will eliminate all greenhouse gas emission standards for vehicles.

“We are repealing the ridiculous endangerment finding and terminating all additional green emissions standards imposed unnecessarily on vehicle models and engines between 2012 and 2027 and beyond,” Trump said.

However, the EPA will continue regulating other harmful tailpipe pollutants that degrade air quality, including carbon monoxide, lead and ozone.

Former President Barack Obama, whose administration put the endangerment finding in place, said in a statement Thursday that dismantling it would leave Americans “less safe, less healthy and less able to fight climate change, all so the fossil fuel industry can make even more money.”

The U.S. Climate Alliance, co-chaired by California Gov. Gavin Newsom, and Wisconsin Gov. Tony Evers called the repeal “unlawful, ignores basic science, and denies reality.”

A number of public health and environmental groups, including the American Lung Association and the American Public Health Association, have already signaled plans to sue.

“As organizations committed to protecting public health, we will challenge this unlawful repeal,” they said in a statement.

At a briefing last month, Manish Bapna, president and CEO of the Natural Resources Defense Council, said his group was also preparing a legal fight, describing the anticipated rollback as “a gift-wrapped package for the fossil fuel industry.”

“It is unscientific, it is bad economics and it is illegal, so we’re going to fight it. We will see them in court,” he said.

The expected legal challenges are likely to stretch on for years, as courts weigh the administration’s rationale against extensive scientific research documenting the impacts of climate change.

In a draft rule outlining its repeal, the EPA argued that earlier assessments overstated the dangers of extreme heat, projected more warming than has occurred and minimized potential benefits of higher carbon dioxide levels, such as enhanced plant growth. Independent scientific bodies have rejected many of those claims and criticized an Energy Department report cited in the proposal.

“The climate is changing faster than ever before, driven by human activities, and the resulting impacts on people and the world we depend on are becoming ever more dire,” the American Geophysical Union said in a statement.

“The changing climate is directly causing or exacerbating global average temperature increases and heat waves, sea level rise and storm surge, and ocean acidification, and is causing extreme weather events such as hurricanes, floods, wildfires, and drought to occur with greater frequency, intensity, or both.”

The administration has also indicated it is reviewing additional regulations tied to the endangerment finding, including rules governing methane emissions.

Interior Secretary Doug Burgum said Wednesday on Fox Business that repealing the determination would help revive coal production.

“CO₂ [carbon dioxide] was never a pollutant,” he said. “The whole endangerment thing opens up the opportunity for the revival of clean, beautiful American coal.”