Developed countries would need to add import emissions taxes for countries that aren't part of the global emissions trading system. It wouldn't be perfect, but

Developed countries would need to add import emissions taxes for countries that aren't part of the global emissions trading system. It wouldn't be perfect, but the total amount taxed should roughly correspond to their estimated emissions. We should do the same with international shipping emissions. I'm guessing this would require a change at the WTO level however, making it relatively unlikely.