The European Union’s sweeping new plan to tackle climate change includes a proposal that if adopted could be the first of its kind: A carbon tariff on imports from countries that aren’t taking similarly aggressive steps to slash their own planet-warming greenhouse gas emissions. …In theory, a carbon border tax could help prevent that undercutting. If factories all over the world that wanted to sell steel, cement, aluminum or fertilizer to the E.U. had to pay a surcharge for the pollution they emit, they would have incentive to clean up their act too. Companies within Europe would have less incentive to shift operations overseas. And, if other countries adopted similar rules, that could put pressure on nations that are reluctant to curb their use of fossil fuels. …The E.U.’s proposal is an early test case… In the US, Democrats proposed their own version of a tax on imports…