Since the atmosphere does not care about arbitrary political boundaries, per capita is in fact the correct way to measure when trying to decide whether a given country is contributing more or less than its fair share of the world's total for any kind of atmospheric pollution.

One simple way to see this is to consider what would happen if China were to split up into 100 separate smaller countries, each with the same per capita emissions as before.

Under a per country measurement, you get the absurd result that emissions have massively improved, even though the amount actually emitted into the atmosphere has not changed at all.