Of course they're estimates, you can't exactly plug a meter on every exhaust pipe in the country and take a reading every couple of days. And of course the methodology varies from one study to another. But modern estimates do account for things like types of power plants, typical personal (and public, and business/industrial) transportation usages etc.. It's not like the whole scientific world waited for us to discuss this on HN before figuring out that maybe 10 people who go to work by bike isn't quite the same as 10 people who drive a jeep to work.
There is less data available for China and therefore the estimates are likely wrong by a higher margin than the estimates for the US. But the figures do very much fit the demographics, economy and geography of China so I doubt that the margin is that high.
There is less data available for China and therefore the estimates are likely wrong by a higher margin than the estimates for the US. But the figures do very much fit the demographics, economy and geography of China so I doubt that the margin is that high.