The Supreme Court curbed the Environmental Protection Agency's ability to broadly regulate carbon emissions from existing power plants, a major defeat for the Biden administration's attempts to slash emissions at a moment when scientists are sounding alarms about the accelerating pace of global warming.

In addition, the court cut back agency authority in general invoking the so-called "major questions" doctrine -- a ruling that will impact the federal government's authority to regulate in other areas of climate policy, as well as regulation of the internet and worker safety.

The decision issued Thursday will send shockwaves across the federal government, threatening agency action that comes without clear congressional authorization.

The ruling was 6-3. Chief Justice John Roberts wrote the opinion for the conservative majority, with the three liberal justices dissenting. Roberts said that "our precedent counsels skepticism toward EPA's claim" that the law "empowers it to devise carbon emissions caps based on a generation shifting approach."

"Under our precedents, this is a major questions case," Roberts wrote, adding that "there is little reason to think Congress assigned such decisions to the Agency."

Steve Vladeck, CNN Supreme Court analyst and professor at the University of Texas School of Law, said the ruling "could be cataclysmic for modern administrative law."

"For a century, the federal government has functioned on the assumption that Congress can broadly delegate regulatory power to executive branch agencies. Today's ruling opens the door to endless challenges to those delegations -- on everything from climate change to food safety standards -- on the ground that Congress wasn't specific enough in giving the agency the power to regulate such 'major' issues," Vladeck said.

Regarding the EPA, Roberts wrote that capping carbon dioxide emissions at a level that will force a nationwide transition away from the use of coal may be a "sensible" solution.

"But it is not plausible that Congress gave EPA the authority to adopt on its own such a regulatory scheme" under the law in question.

"A decision of such magnitude and consequence rests with Congress itself, or an agency acting pursuant to a clear delegation from that representative body," he wrote.

Writing separately, Justice Neil Gorsuch emphasized the court's move to limit agency power, which he considers unaccountable to the public.

"While we all agree that administrative agencies have important roles to play in a modern nation, surely none of us wishes to abandon our Republic's promise that the people and their representatives should have a meaningful say in the laws that govern them," Gorsuch wrote.

Justice Elena Kagan, writing for the dissenters, sounded the alarm about global warming and said that the court's decision "strips" the EPA of the "power Congress gave it to respond to 'the most pressing environmental challenge of our time.'"

"The Court appoints itself -- instead of Congress or the expert agency -- the decision-maker on climate policy," she wrote.

"I cannot think of many things more frightening," she concluded.

The White House on Thursday blasted the ruling.

"This is another devastating decision from the Court that aims to take our country backwards," a White House official said in a statement. "While the Court's decision risks damaging our ability to keep our air clean and combat climate change, President Biden will not relent in using the authorities that he has under law to protect public health and tackle the climate change crisis."

Meanwhile, US Department of Health and Human Services Secretary Xavier Becerra said the ruling is "a public health disaster" that will hurt Americans' health.

"A failure to regulate power plant emissions will lead to increases in asthma, lung cancer, and other diseases associated with poor air quality, and in many places, those impacts are likely to fall hardest in already heavily polluted neighborhoods," Becerra said.

Action from Congress is unlikely

The opinion calls into question the future of federal-level climate action in the US, and puts even more pressure on Congress to act to reduce planet-warming emissions.

But broad action from Congress is unlikely. Democrats in Congress have been embroiled in difficult negotiations on a climate and clean energy bill with their main holdout, Sen. Joe Manchin of West Virginia for months, with no clear end in sight.

It's unclear whether those negotiations on a package of clean energy tax credits and other emissions-cutting programs will yield a result. And without both major investments on clean energy and strong regulations cutting emissions by the EPA, President Joe Biden has very little hope of meeting his climate goal, independent analysis shows.

Congressional Democrats also expressed fury at the court's decision and warned of the environmental consequences.

Democratic Rep. Raúl Grijalva of Arizona, who chairs the House Natural Resources Committee, said the court has "sentenced our planet to death."

"Make no mistake, this decision -- and all of the decisions this week -- will be directly responsible for the harm and deaths of countless Americans," he said.

Democratic Sen. Ed Markey, who proposed the Green New Deal, said the decision "lets polluters turn back the clock on fifty years of reduced pollution and improved air quality all across the country."

"The result of this dangerous decision is an EPA that is undermined in its ability to protect the public," he said, adding that Congress should pass "meaningful climate and clean energy funding" and the "climate justice and clean energy package."

Vladeck said the lack of congressional willpower will lead to the erosion of federal power.

"It would be one thing if Congress could be expected to respond to this ruling by updating all of those delegations to make them more specific, but we -- and the Court -- know that it won't, which will almost surely lead to significant deregulation across a wide swath of federal authority."

Climate impacts of power plants

Fossil fuels in the power sector are a huge contributor to the climate crisis. Around 25% of planet-warming greenhouse gas emissions around the globe and in the US come from generating electricity, according to the EPA. And coal, the dirtiest fossil fuel, powers about 20% of US electricity. Emissions from power production rose last year for the first time since 2014, an increase that was mainly driven by coal use.

The surge in fossil fuel use is worrying not only for Biden's climate goals -- the President in his first months in office pledged to slash US emissions in half by 2030 -- but also the planet. Scientists have become increasingly urgent in their warnings that to make headway on the climate crisis, emissions need to not only be reduced going forward, but the world needs to develop ways to also remove the greenhouse gas that's been pumped into the atmosphere in decades past.

In a landmark report last year, scientists reported that the planet is warming faster than they had previously imagined it would. As it does, they said, extreme weather will become more deadly; water crises will develop and worsen; food insecurity will grow and disease will spread. To avoid the worst consequences, the world must limit global warming to 1.5 degrees Celsius (it's already passed 1.1 degrees), and the only way to do that is to keep the vast majority of the Earth's remaining fossil fuel stored in the ground.

Scope of Clean Air Act

Central to the case was the section of the Clean Air Act concerning the scope of the EPA's ability to regulate power plants.

The Obama administration interpreted the provision broadly in a way that would justify regulations on a statewide basis. The Trump administration, however, said the EPA's authority is much more narrow and that it could only target requirements aimed at making individual power plants more efficient.

The DC Circuit vacated the Trump rule in 2021 holding that it rested on a "fundamental misconstruction" of the law. The Biden administration is currently working on a new rule.

Because there is no current EPA rule on power plant emissions on the books, court watchers were surprised when the Supreme Court agreed to take up a challenge to the lower court opinion brought by Republican-led states.

West Virginia Attorney General Patrick Morrisey was the main plaintiff in this case, joined by Republican attorneys general from more than a dozen other states. He was joined by lawyers representing two coal companies: The North American Coal Corporation, and Westmoreland Mining Holdings, LLC.

The plaintiffs challenged the EPA's authority to broadly regulate greenhouse gas emissions coming from power plants and said this power should be taken away from the agency and given to Congress. They said a provision of the Clean Air Act did not give the agency such broad authority, and citing a legal principle called "the major questions doctrine," the challengers said that such decisions with such sweeping implications had to have Congress' clear authorization.

On the other side, Solicitor General Elizabeth Prelogar argued that the petitioners' case should be dismissed because there is no federal regulation for power plants on the books right now -- with both the Obama-era Clean Power Plan and the Trump-era Affordable Clean Energy rule dead. The EPA has not yet issued a new rule.

Republicans 'pleased' with decision

Morrisey on Thursday said the court "made the correct decision to rein in the EPA, an unelected bureaucracy."

"We are pleased this case returned the power to decide one of the major environmental issues of the day to the right place to decide it: the U.S. Congress, comprised of those elected by the people to serve the people," he said. "This is about maintaining the separation of powers, not climate change."

West Virginia Gov. Jim Justice, a Republican, also celebrated the ruling, saying that it "will stop unelected bureaucrats in Washington, D.C. from being able to unilaterally decarbonize our economy just because they feel like it."

- by Ariane de Vogue, Ella Nilsen and Veronica Stracqualursi, CNN