Krugman suggests Carbon Tariffs on China

if and when wealthy countries take serious action to limit greenhouse gas emissions, they’re very likely to start imposing “carbon tariffs” on goods imported from countries that aren’t taking similar action. Such tariffs should be legal under existing trade rules — the World Trade Organization would probably declare that carbon limits are effectively a tax on consumers, which can be levied on imports as well as domestic production. Furthermore, trade rules give special consideration to environmental protection. So China would find itself with strong incentives to start limiting emissions.

I really like the idea. It make the argument about China not taking greenhouse gasses seriously mostly irrelevant. They need to trade with the industrialized west and those tariffs would be a huge incentive to sign on to a climate agreement.