Note that China has no ability to impact global CO2 emissions either.

Let’s split China population in k Denmark-sized groups, plus one smaller-than-Denmark reminder.

None of the k groups has any ability to impact global CO2 emissions (same as Denmark).

We can reasonably assume that a smaller group has even less ability to impact global CO2 emissions than a bigger group. Hence the smaller-than-Denmark reminder has no ability to impact global CO2 emissions either.

Thus China is made of groups that have no ability to impact global CO2 emissions either. And therefore China as a whole has no ability to impact global CO2 emissions. (Otherwise at least one group within China would have to impact global emissions and we just saw that it isn’t possible).

This is known as the CO2 impossibility theorem, loosely based on Arrow’s concept of “(in)decisive” set.