BEIJING — A groundbreaking agreement struck by the United States and China is putting the world’s two worst polluters on a faster track to curbing the heat-trapping gases blamed for global warming. With the clock ticking on a worldwide climate treaty, the two countries are seeking to put their troubled history as environmental adversaries behind them in hopes that other nations will be spurred to take equally aggressive action.

The U.S., a chief proponent of the prospective treaty, is setting an ambitious new goal to stop pumping as much carbon dioxide into the air. China, whose appetite for cheap energy has grown along with its burgeoning economy, agreed for the first time to a self-imposed deadline of 2030 for when its emissions will top out.

Yet it wasn’t clear how either the U.S. or China would meet their goals, nor whether China’s growing emissions until 2030 would negate any reductions in the U.S.

Still, the dual announcements from President Barack Obama and Chinese President Xi Jinping, unveiled today in Beijing, came as a shock to environmentalists who had pined for such action but suspected China’s reluctance and Obama’s weakened political standing might interfere. In Washington, Republicans were equally taken aback, accusing Obama of dumping an unrealistic obligation on the next president.

In fact, the deal had been hashed out behind the scenes for months. U.S. officials said U.S. Secretary of State John Kerry floated the idea during a visit to China in February, and Obama followed up by writing Xi in the spring to suggest that the world’s two largest economies join forces.

Obama pressed the issue again during a meeting with China’s vice premier on the sidelines of a U.N. climate summit in September, and the two countries finally sealed the deal late Tuesday — just in time to announce it in grand fashion at the Great Hall of the People as Obama’s trip to China was coming to an end.

“This is a major milestone in the U.S.-China relationship,” Obama said, with Xi at his side. “It shows what’s possible when we work together on an urgent global challenge.”

Under the agreement, Obama set a goal to cut U.S. emissions between 26 and 28 percent by 2025, compared with 2005 levels. Officials have said the U.S. is already on track to meet Obama’s earlier goal to lower emissions 17 percent by 2020, and that the revised goal meant the U.S. would be cutting pollution roughly twice as fast during a five-year period starting in 2020.

China set a target for its emissions to peak by about 2030 — earlier if possible — with the idea being that its emissions would then start falling.