Helpfully, the World Trade Organisation (WTO) has made it clear that members can use border tax adjustments to deal with other jurisdictions that lack carbon pricing. For instance, steelmakers that are subject to a domestic carbon tax or cap-and-trade scheme could have their profitability protected from steelmakers in unregulated jurisdictions, through the use of an import tax.
The standard WTO position on environmental rules is that they are fine if applied equally to both domestic and international firms. For instance, you can require that both domestic fishers and those trying to sell imported fish use nets that are designed not to catch sea turtles. What you cannot do is impose the restriction on foreign firms in other WTO countries, but not impose it on domestic firms. Of course, as with all international legal issues, the practicalities of implementation and enforcement are complex.
More discussion of the statement is on the Free Exchange blog.