Comment by fastball

Yes, that. But also that these different externalities will have different responses to reduction at scale, which also impacts how effective action on a more "personal" level really is.

For example the OP was talking about plastic. A 2% reduction in plastic waste has a clear benefit, because any amount of plastic reduction is a bonus. However it is not clear that a 5% reduction in CO2 emissions due to Americans driving their cars less will have any meaningful difference when it comes to climate change.