A reader writes:
The tariff measure is necessary eventually (at least if China continues to refuse to regulate its own emissions), or else any effort to curb carbon emissions will only cost the U.S. money without actually lowering global emissions. Businesses that cap and trade costs money will move to China, where they will pollute as much or more than they do here. Carbon emissions in China are the same as carbon emissions in the U.S. as far as global warming is concerned.
Ezra Klein also passes along a defense of the provision.