If 90% of the plastic comes from 10 rivers... wouldn't it make the most sense to build giant filters at the estuary of each river, and stop the plastic from reaching the Ocean with 10 giant recycling plants?
The top 10 rivers contribute 90% to RIVER-DRIVEN plastic input, not TOTAL input. It is still a large problem, but a vast amount of ocean trash is dumped by coastal cities in Asia.
[1] https://www.ufz.de/export/data/2/149396_EGU_2017_Schmidt.pdf
Rivers pass through multiple countries. In my country, which is fairly narrow, many of the rivers that end up on our shores don't begin inside our territory, and we've had conflicts with our neighbors over the levels of pollution of the water entering the country.
I'm baffled why you think these sorts of accommodations are not what would routinely be done if such a tariff scheme were implemented.
The whole idea of tariffs is you impose them on other countries without their assent.
Or maybe I’m wrong about a majority floating.