The announcement in Washington on September 24 that President Xi Jinping is committing China to a national carbon trading system is the latest step in an important partnership between the two biggest carbon emitters in the world.

According to the International Energy Agency, China was responsible for 9.1 billion metric tons of carbon dioxide emissions in 2012; the United States emitted 5.2 billion metric tons. Together they produced more than a third of warming greenhouse gases.

The cap-and-trade system explains how China intends to meet the surprise commitment last year to “peak” emissions by 2030. After being piloted in seven provinces and cities since 2013, a nationwide system will go in effect in 2017, Jinping said.

China’s initial experiments with a carbon trading system have met with mixed success, says China Environment Forum Director Jennifer Turner. They “are truly laboratories for experimenting with different ways of doing cap and trade,” she told The Diplomat. “Not all are doing great, but the trial and error can help them speed through the learning curve.”

Beijing, Shanghai, and Guangdong have done better than other areas. Where pilot programs have run into issues, it’s because of a “lack of transparency on emissions and pricing,” Turner said. “To do a market system, markets like clear information,” she told the BBC.

But China has good reason to make the new initiative work. Air pollution is such a major public health problem that even the normally obdurate government is feeling pressure to make substantial changes. “The noose is tightening,” Turner said in The Diplomat. Coal-fired power plants are the largest offenders (China burns almost as much coal as the rest of the world combined) and a carbon cap will hit these plants the hardest, forcing air quality improvements and helping to blunt global climate change at the same time.

U.S.-China cooperation on climate change is one of the “bright lights” in an otherwise somewhat contentious relationship, Turner said, and the announcement now bolsters momentum towards a meaningful global climate agreement at the UN climate summit in Paris.