The US Supreme Court on 9 February blocked President Barack Obama's plan to regulate carbon dioxide emissions from power plants, pending the outcome of a legal challenge by more than two dozen states.

The emissions rules, known as the Clean Power Plan, are one of Obama's primary tools in the battle against climate change. Finalized in August 2015 by the US Environmental Protection Agency (EPA), the regulations would require states and electric utilities to reduce emissions by lowering demand, deploying low-carbon energy sources or increasing the efficiency of existing fossil-fuel power plants. By 2030, the regulations would reduce CO2 emissions from the power sector by an estimated 32% compared with 2005 levels. 

The Supreme Court overruled a 21 January decision by a federal appeals court that would have allowed the EPA to implement the regulations pending legal review. If the appeals court ultimately sides with the Obama administration and the states appeal against the verdict, the regulations would remain on hold pending a Supreme Court hearing. In that case, the high court could issue a ruling as early as 2017.

“Make no mistake — this is a great victory for West Virginia,” the state's attorney general, Patrick Morrisey, said in a prepared statement. West Virginia is one of the states that has sued to overturn the EPA regulations.

Under the rules, each state must craft its own plan to reduce emissions beginning in 2022. Before the Supreme Court ruling, States had until 6 September 2016 to submit those plans to the EPA, although they could also apply for a two-year extension. The Supreme Court’s decision puts that process on hold, although Obama administration officials say that the EPA will continue working with states to prepare for the eventual implementation of the regulations.

Jeffrey Holmstead, a lawyer at the firm Bracewell in Washington DC, who worked at the EPA under President George W. Bush, says that this is the first time that the Supreme Court has stepped in to block a major environmental regulation before a lower court ruling. This suggests that the Supreme Court is sympathetic to arguments that the EPA overstepped its authority in crafting power-plant regulations, he says.

“I think the likelihood that [the EPA rules] will ever go into effect is pretty low,” Holmstead says. “There needs to be some sort of congressional action to decide how the United States is going to deal with climate change.”

White House officials are disappointed with the Supreme Court’s decision, but say they are confident that the EPA regulations will ultimately be upheld by the court. Officials also say that the ruling will not affect the country's ability to meet the commitment to reduce greenhouse-gas emissions it made as part of the Paris climate agreement.

“This rule is on sound legal footing,” a senior administration official said during a background briefing late on 9 February. “We remain completely confident in our ability to meet those commitments, and we will continue making steps to make sure that the United States is doing its part.”