1. Where did the endangerment finding come from?

In 2007, the supreme court ruled in the lawsuit Massachusetts v EPA that the Clean Air Act gives the government authority to regulate greenhouse gas emissions from motor vehicles – something advocates and scientists had demanded for years.

Two years later, under Barack Obama, the EPA issued the endangerment finding, which determined that six greenhouse gases threaten the public health and welfare of current and future generations.

The finding empowered the agency to act and barred it from ignoring the climate crisis, becoming the bedrock of federal climate regulation.

2. What will this repeal do?

Trump’s EPA contends that a section of the Clean Air Act which regulates emissions from cars and trucks applies only to “pollution that harms health or the environment through local and regional exposure”. The move effectively voids all federal greenhouse gas regulations on motor vehicles.

It will also prompt a broader unspooling of climate regulations, experts say. Stationary pollution sources such as power plants and oil and gas facilities are regulated under a different section of the Clean Air Act, but the legal theory laid out by EPA on Thursday attacks the endangerment finding in its entirety, said Andres Restrepo, a senior attorney at the green group the Sierra Club.

“What they’re basically doing is announcing a position that the Clean Air Act does not allow EPA to find that greenhouse gases endanger health and welfare,” he said. “That will require them to end emissions standards for power plants, too, because they cannot continue regulating power plants if that’s their legal position.”

Joseph Goffman, who served as EPA air chief under Joe Biden, expects the agency will apply its vehicles-focused arguments to stationary polluters in the coming months, noting it has already laid the groundwork for that move. Over the summer, officials proposed to find that emissions from power plants “do not contribute significantly to dangerous air pollution” and therefore should not be regulated.

“Instead of the entire house of cards of all EPA climate regulation collapsing all at once today, it’s going to be like a row of dominoes falling,” said Goffman, who helped write and implement the Clean Air Act and worked directly on the endangerment finding.

The Guardian has contacted EPA to confirm its plans for power plant regulations.

Adam Zuckerman, senior clean vehicles campaigner on the climate program at consumer advocacy group Public Citizen, said this week’s move could also have “perilous” consequences for US carmakers and the workers they employ.

“As China and Europe race toward building [electric vehicles], the American auto industry will need to make some challenging choices about how it decides to invest in innovation,” he said “The move puts American automakers in a bind, pushing them to make dirty, obsolete vehicles for the US that they can’t sell to most of the world.”

3. Didn’t the Trump administration say this will save the US money?

The administration says their new rule will save the US $1.3tn.

But that claim, experts say, ignores the far greater costs Americans will incur due to extreme weather and other impacts of climate change.

One estimate from environmental nonprofit Environmental Defense Fund found the repeal of the endangerment finding could impose up to $4.7tn in additional expenses tied to climate-warming and toxic pollution over the next two decades.

“Today’s action by President Trump, if unchecked by the courts or Congress, will put more money in the pockets of the CEOs who pollute our air and cut big checks for Republican politicians,” said Jason Walsh, executive director of BlueGreen Alliance, a coalition of labor unions and environmental groups.

“But it will put more regular folks in the hospital or force them to flee climate disasters while the billionaire class is safely protected on their mountaintops and private islands.”

The EPA spokesperson said: “Under President Trump, EPA is returning to common‑sense policies and adhering to the law. That is how we keep dollars in the pockets of American families and protect human health and the environment, refusing to let any ‘climate religion’ trump the laws passed by Congress.”

4. Will this rollback hold up in court?

Green groups, health-focused organizations, and the states of California and Connecticut have pledged to sue the administration over the rollback, noting that federal courts have repeatedly upheld and affirmed the endangerment finding.

“This cynical and devastating action by the Trump EPA will not go forward without a fight,” said Manish Bapna, president of the conservation organization the Natural Resources Defense Council. “We will see them in court – and we will win.”

Groups will begin filing litigation to the DC circuit court “very soon”, said Michael Gerrard, a climate law expert at Columbia University. The challenges could wend their way through the courts for years. But they could also be settled quickly, said Gerrard.

“We have quite a history of the supreme court jumping into these kinds of things quickly,” he said, noting that they high court took unprecedentedly quick action to issue a stay on Obama-era power plant regulations in 2016. “This could all take less than a year, potentially.”