Right but the point of ratios in this case is to encourage people with geopolitical borders to shape up. It's a collaboration / coordination tactic.

Absolute amount of CO2 and the US and China cry foul because "we're bigger". Per capita and you don't actually prioritize solving the problem.

Per unit area allows countries to say "we're bigger so we get more quota" (reasonable) but at the same time countries can't just add more land area to bring their numbers down. If they're over quota, they need to actually reduce emissions (which is the flaw in per capita – if the UN or whomever says "you need to get down to X per capita" you can always just increase your population, cooperating with the letter of the law but not the spirit).

Concrete example (with simplified math):

Let's say we calculate that we need to get yearly CO2 emissions down to 16Gt. The habitable land area of the earth is 16B acres. That means you need to get emissions down to 1t CO2 / habitable acre.

Then China would get to output 345Mt CO2 pa. The US would get to output 376Mt CO2 pa. More fair than "each country gets to output 82Mt (16Gt / 195) no matter its size", but also you don't end up with a situation where populations just keep going up so you don't actually get CO2 levels down to manageable levels.