[tim333]

That sounds sensible but the devil's in the details. How do you figure how much of X% is environmental? Do you insist the Chinese have similar pay and conditions for workers? How do you check where the rare earths you are buying in say Singapore actually came from?

[yoz-y]

In an ideal world some entity could force country A to include externalities such as environment in the price of the goods.

[lsd5you]

Which is why precautionary and moderate tariffs are suitable, with some discretion for cases where things are especially egregious and/or where the economics are different (low margin vs luxury goods need different levels of tariff).

Probably ideally tariff levels should not directly made by the executive, but rather some kind of review system - (although quangos are another problem).