Have you ever heard of the words 'per capita'? China is only the biggest carbon producer because it has four times as many people as the US, not to mention being still in a much less advanced stage of the industrial revolution, though moving forward fast. And as for crippling one's economy - I don't know about you, but personally I'm involved with Chinese manufacturers on a daily basis in my job. It may have taken a long time for them to move, but today the Chinese government sure as hell has given a dramatically higher priority to the environment and CO2 emissions, and isn't too bothered if it makes their manufacturers bleed. I suppose if you're a dictatorship you can do that - I don't think the way they're cracking down would fly in the US, nor in Europe.

Of course they have more concrete and more urgent reasons for doing so than global warming alone, their people are literally suffocating, but the point remains that you'd have to be wilfully blind to pretend it's in any way fair to impose the same targets in absolute emission numbers on China as on the US.

They signed an agreement with the United States of America, and expect the United States of America to live up to what it committed to, just like any other signatory (to the extent that there was any real commitment from anyone to begin with, as everyone's targets are voluntary and non-binding anyway). Or not, as the case may be, but then don't bother with feeble excuses about internal politics. If you wouldn't accept that as an excuse for other countries reneging on their commitments, why would other countries accept it as an excuse for the USA?