The solution there is to create tariffs based on emissions so that the costs of the emissions get accounted when people import goods.

I don't understand the word "tarries", somehow even after looking it up :-). (Not my first language, sorry)

I meant that in practice it's very difficult to track, because it involves a lot of actors in a lot of countries.

This is irrelevant for looking at the effectiveness of climate policy. Restrictions on emissions in the US and EU are not going to reduce the emissions from factories in China even if the resulting products are sold in the US/EU.

How do you mean that? China would not produce iPads for the rest of the world if the rest of the world was not buying iPads, right?

The graal of climate policy would actually be to be able to know precisely the impact of every single action we do. So that we could optimize properly. But because we can't do that, we have to rely on proxies, and that makes it error prone and practically very challenging.