WASHINGTON • In a blow to the fight against climate change, the Supreme Court on Thursday limited how the nation’s main anti-air pollution law can be used to reduce carbon dioxide emissions from power plants, and in an immigration ruling, the Biden administration can scrap a Trump-era immigration policy that was at the center of effors to deter asylum-seekers, forcing some to wait in Mexico.

By a 6-3 vote, 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.

The court’s ruling could complicate the administration's plans to combat climate change. Its detailed proposal to regulate power plant emissions is expected by the end of the year. Though the decision was specific ific to the EPA, it was in line with the conservative majority’s skepticism of the

power of regulatory agencies and it sent a message on possible future effects 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% 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.”

The justices’ immigration decision came in a case involving former President Trump’s “Remain in Mexico” policy, formally know as Migrant Protection Protocols, which enrolled about 70,000 people after it was launched in 2019.

President Joe Biden suspended the program on his first day in office in January 2021. But lower courts ordered it reinstated in response to a lawsuit from Republican-led Texas and Missouri. The current administration has sent far fewer people back to Mexico than did the Trump administration.

The heart of the legal fight in the immigration case was about whether U.S. immigration authorities, with far less detention capacity than needed, had to send people to Mexico or whether those authorities had the discretion under federal law to release asylum-seekers into the United States while they awaited their hearings.

After Biden’s suspension of the program, Homeland Security Secretary Alejandro Mayorkas ended it in June 2021.