It was some point in May last year, shortly after ISIS surged into the city of Ramadi, and I was working on a story about Iraq’s fast-disappearing Mesopotamian Marshes. Keen to fact-check a few statistics with the Ministry of Water Resources and to hear the government line on the wetlands’ struggles, I dialed its Baghdad offices. After being passed from official to official like a hot potato, a young employee, Hussein, finally gave it to me straight. “No, no, we don’t have this sort of information,” he said, clearly impatient to get off the phone. “There are much more important things in Iraq right now.”

In December 2015, Beijing issued its first-ever “red alert” for smog, its highest air pollution warning, which closed schools and restricted the number of cars on the road. Less than two weeks later, it issued its second.

The recent news that China has been underreporting its already globe-leading coal consumption by nearly 20 percent for the last decade underscores the scale of its air pollution crisis.

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.