The way I see it, the major barrier to countries implementing carbon taxes is the fear their economic competitors won’t do the same, therefore hindering their economic growth needlessly. A valid concern.
Why don’t some nations build an ‘opt in’ style Free Trade Agreement that allows any country to join as long as they prove they have implemented and enforced a carbon tax. Those countries then have high financial incentives to only trade within the ‘carbon tax block’ and any country outside is at a serious trade disadvantage.
I’ve (quickly) looked and have not found anything like this proposed (which is frankly crazy).
Would you support your country jumping into this FTA?
What are the unforeseen downsides or objections to a plan like this?
It might be a big tripping hazard to go full "free trade agreement" just to get a carbon tax. The better approach is probably going to be some sort of mutual taxation/tariff/duty pledge. Something where all the countries that opt in would levy a duty of some sort on all goods that involve carbon emissions in their lifecycle outside the transportation of said goods (this is a trade agreement after all), and waive that duty on all member nations' exports.
When people hear "free trade" they think of a system that waives all import duties, which may or may not be what is desired here. I can think of some bad actors passing a "carbon tax" just to get all the other duties on their goods dropped.
The alternative of course would be an actual free trade agreement but with a lot more qualifications than just "carbon tax." Like union support, a living minimum wage, free education through age 18 (for example), environmental protections, reasonable intellectual property protections, no wars of aggression, etc etc., PLUS a carbon tax.
Honestly, this is exactly what I was thinking when I formulated this question. While I agree with your comprehensive list, we may not have time for that. Even a 10 or 20 year deal of a "carbon tax free trade agreement" may be all we need to course correct. If it is effective (at curbing carbon emission and as a political tool) a new FTA with the qualifications you listed could be crafted. The more qualifications, the slower nations would be to adapt/enroll and I'd be wary of adding too many if the goal is fast action now.
Bad actors' intent matters little, as long as their actions align with world goals.