America is not exactly the most reliable negotiating partner on these issues, with climate denialists controlling Congress and the only prospect of action in the near future, and maybe for many years, coming from executive orders. (Not to mention the possibility that the next president could well be an anti-environmentalist who could reverse anything President Obama does.) Meanwhile, China’s leadership has to deal with its own nationalists, who hate any suggestion that the newly risen superpower might be letting the West dictate its policies. So what we’re getting here is more a statement of principle than the shape of policy to come.

Until now, those of us who argued that China could be induced to join an international climate agreement were speculating. Now we have the Chinese saying that they are, indeed, willing to deal — and the opponents of action have to claim that they don’t mean what they say.

Needless to say, I don’t expect the usual suspects to concede that a major part of the anti-environmentalist argument has just collapsed. But it has. This was a good week for the planet.