We aren't talking about a tariff on renewable energy production though are we. A Tariff on goods produced by companies/factories that are powered by coal or natural gas due for export to the rest of the world. Prove you use renewable energy and you dont pay the tariff, this would motivate companies who export to adopt greener energy. It would also make trade freer for companies who do this and more difficult for those who do not, therefore there is incentive to innovate to use renewable energy.