In a blow to the fight against climate change, the Supreme Court on Thursday limited how the US's main anti-air pollution law can be used to reduce carbon dioxide emissions from power plants.
US President Joe Biden called the decision "devastating".
By a 6-3 vote, with conservatives in the majority, the court said that the Clean Air Act does not give the Environmental Protection Agency broad authority to regulate greenhouse gas emissions from power plants that contribute to global warming.
The decision, said environmental advocates and dissenting liberal justices, was a major step in the wrong direction - "a gut punch", one prominent meteorologist said - at a time of increasing environmental damage attributable to climate change and dire warnings about the future.
The court's ruling could complicate the administration's plans to combat climate change. Its proposal to regulate power plant emissions is expected by the end of the year. The decision also could have a broader effect on other agencies' regulatory efforts beyond climate change and air pollution.
The decision put an exclamation point on a court term in which a conservative majority, bolstered by three appointees of former President Donald Trump, also overturned the nearly 50-year-old nationwide right to abortion, expanded gun rights and issued major religious rights rulings, all over liberal dissents.
President Joe Biden aims to cut the nation's greenhouse gas emissions in half by the end of the decade and to have an emissions-free power sector by 2035. Power plants account for roughly 30 per cent of carbon dioxide output.
"Capping carbon dioxide emissions at a level that will force a nationwide transition away from the use of coal to generate electricity may be a sensible 'solution to the crisis of the day'," Chief Justice John Roberts wrote in his opinion for the court.
But Roberts wrote that the Clean Air Act doesn't give EPA the authority to do so and that Congress must speak clearly on this subject.
"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.
In a dissent, Justice Elena Kagan wrote that the decision strips the EPA of the power Congress gave it to respond to "the most pressing environmental challenge of our time".
Kagan said the stakes in the case are high. She said: "The Court appoints itself - instead of Congress or the expert agency - the decisionmaker on climate policy. I cannot think of many things more frightening."
Biden, in a statement, called the ruling "another devastating decision that aims to take our country backwards. While this decision risks damaging our nation's ability to keep our air clean and combat climate change, I will not relent in using my lawful authorities to protect public health and tackle the climate crisis."
West Virginia Attorney General Patrick Morrisey, who led the legal challenge to EPA authority, said the "EPA can no longer sidestep Congress to exercise broad regulatory power that would radically transform the nation's energy grid and force states to fundamentally shift their energy portfolios away from coal-fired generation".
But University of Georgia meteorology professor Marshall Shepherd, a past president of the American Meteorological Society, said of the decision: "It feels like a gut punch to critical efforts to combat the climate crisis which has the potential to place lives at risk for decades to come."
Richard Revesz, an environmental expert at the New York University School of Law, called the decision "a significant setback for environmental protection and public health safeguards".
But he said in a statement that EPA still has authority to address greenhouse gas emissions from the power sector.
EPA spokesman Tim Carroll said the agency is reviewing the decision. "EPA is committed to using the full scope of its existing authorities to protect public health and significantly reduce environmental pollution, which is in alignment with the growing clean energy economy."
The court held that Congress must speak with specificity when it wants to give an agency authority to regulate on an issue of major national significance.
Senate Democratic leader Chuck Schumer of New York said the decision would have a wide effect. "The consequences of this decision will ripple across the entire federal government, from the regulation of food and drugs to our nation's health care system, all of which will put American lives at risk," Schumer said.