FIVE years ago next month, disagreement between America and China, the world’s biggest greenhouse-gas emitters, scuppered the UN’s Copenhagen climate-change conference. Yesterday Presidents Barack Obama and Xi Jinping announced a deal on emissions. That in itself is an achievement, but it may not be all it is cracked up to be since China has not conceded much, and Congress will do its best to prevent America from delivering what it has promised.

Because America is responsible for a far larger share of the greenhouse gases in the atmosphere than China, it was bound to accept sharper cuts. Even so, it seems to have made the bigger concessions. It has agreed to cut its emissions to 26-28% below its 2005 levels by 2025. Its emissions are already falling, and it might be able to meet its existing commitment to cut by 17% below 2005 levels by 2020, but this deal requires it to double the pace of cuts after 2020.

China has agreed that its emissions will peak in 2030, and that the percentage of non-fossil-fuels in its primary energy consumption will rise to 20% by 2030. Just getting a date out of the Chinese is an achievement, but American negotiators had been gunning for 2025, and the date the Chinese have agreed to may not be far off what would have happened anyway. In March this year He Jiankun of Tsinghua University reckoned that China’s carbon emissions would peak by “around 2030”, as GDP growth slows and urbanisation has mostly run its course.

The agreement gives both sides plenty of wiggle room, referring to the countries’ “best efforts” and intentions to reach their targets. And because it is not a treaty, it does not have to be ratified by Congress—which is just as well, since the new Republican-controlled Congress is determined to undermine the president’s efforts to cut America’s emissions.

Those efforts are in the hands of the Environmental Protection Agency (EPA), which is trying to regulate greenhouse gases as pollutants under the Clean Air Act. The Republican majority in the House of Representatives has already made it clear that it would like to wrest control of greenhouse gases from the EPA, and the new Republican-controlled Senate will now get in on the act.

Mitch McConnell, the Senate Republican leader, who hails from Kentucky, a coal-producing state, has complained in the past about “the president’s ideological war on coal” and has already attacked the deal. “This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs.” Senator Jim Inhofe, who is likely to head the Senate Environment and Public Works Committee, has called climate-change a hoax and compared the EPA to the Gestapo.

Whether the Republicans can do much is moot. They could presumably try to legislate to stop the EPA from controlling greenhouse gases, but the Senate Democrats would use the filibuster to prevent that from happening. Even if a bill got through the Senate the president would veto it. But newly powerful legislatures are full of energy and imagination when it comes to making life difficult for presidents.