I'm not introducing any goal posts. These are things assumable with common sense. "If a metric becomes a goal, it ceases to be a metric" applies in such case. For progress, the only thing that matters is the total amount going down---neither per km area nor per capita have any value in measuring meaningful progress. But they could provide a good snapshot of present impact of each country.

Per capita is a better snapshot because it measures impact of a citizen in the country. Per landmass isn't great because it ignores countries with outsized impacts.