Aff causes protectionism, escalating tariffs, and a trade war. It also turns the case. Trade and international cooperation are key to decarbonization measures.

But other countries whose exports get hit with the border taxes might not see it that way. China, for example, the U.S.’s number one trading partner and the world’s largest emitter, could bring complaints to the World Trade Organization, which prohibits protectionist tariffs and could authorize China to retaliate. (After Trump instituted heavy tariffs on foreign steel, China and the European Union launched WTO challenges and retaliatory taxes against many U.S. imports, including soybeans and bourbon.)

That’s the problem with a potential carbon border tax: At best, it could help protect critical industries during a difficult switch to green energy; at worst, it could spark a trade war that would slow efforts to cut emissions.

“A trade war with China is not good for decarbonization,” Allan said. “They’ve done really excellent work in driving down the cost of wind and solar. To turn around and try to exclude them from the global economy at this point … it could be bad for decarbonization, because we’d have to try to recreate those cost reductions.”