The best estimate as to how much carbon the product released times the price of carbon of course. I'd guess the estimates would be on the high side to encourage domestic production.

> complacent countries where emission can be whatever you want them to be

and whom you gonna trust for measuring/estimate the carbon produced in the complacent country?

"yes sir we have all these kind of filters and capture schemes"

preemptive reply to "but we're going to check and fine them": Panama Papers happened and nothing was done about them.

You have domestic experts doing the estimation.

oh I bet the WTO would love that, unilateral levies from the country of destination based on capricious bureaucracy

People who know about such things generally think that the WTO would not be a problem. For example: https://www.climateadvisers.com/wp-content/uploads/2014/01/2...

You of course need to ensure that any export rebates and import taxes are fair.