To actually answer both of your rhetorical questions.
“Why per capita?” - many people have an innate desire for ‘fairness’ and assigning some degree of accountability (or expectation of impact of mitigating changes) based 1-1 per person, appeals to that. It also bypasses thinking about the larger overall problem, while giving a degree of apparent control to them as individuals. And let’s them blame other individual for the problem if it doesn’t get fixed.
“Why per country?” - because anyone trying to actually accomplish anything realizes that issues of this scale are not really resolvable through individuals making decisions against their incentives, but at best national policy making - but really international treaties to change incentives. The atmosphere is the largest ‘commons’ we have, except perhaps the oceans.
Perhaps not even then. Since every country has an incentive to avoid actual accountability for their own actions, while pawning off the consequences onto someone else, and managing that type of situation typically requires an overarching authority. And no sovereign (for good reasons!) wants to bend the knee to a foreign power. And any international authority is necessarily going to be foreign to everyone to some extent.
So attempting to assign accountability at the coarsest level of sovereign control on that front feels most actionable.
Really I expect things will get much worse until individual countries start doing emergency geo-engineering to attempt to stop widespread famine, out of control flooding, or lethal heat waves, making it worse for some, better for others but rather randomly.
I’m honestly surprised India hasn’t already started. I’m guessing within the next decade or two.
This will lead to other countries starting wars with them to attempt to control such behavior, until some sort of international accords can be worked out - where the rich countries will prosper, and the poor countries will be fucked. As usual.
But people call me an Optimist. You don’t want to hear what I say when I’m being cynical.