20% by 2070-2090, on the other hand, is a lost cause. That would put atmospheric at 580 PPM by 2080. That's >4C of warming, if we don't hit any positive feedback loops.
I'm not particularly concerned about plastics. They can be synthesized from biomass.
US coal use is down to where it was around 1930. Petroleum is around 1960's usage levels. https://www.eia.gov/todayinenergy/detail.php?id=26912
Natrual gas has spiked, but it's realitvly speaking less of a global warming threat due to C(x) H(x) turning into H20 (x) + C02 (x) which produces less CO2 per heating vs just C into CO2 from coal.
So, while there are a huge range of predictions. What happens in the next 20 years is more important than what's going on 40 - 60 years from now.
Note it does break down into more CO2 than the wieght of Methane because Oxygen molecules are heavy.
The real risk is if warming the arctic starts to release a lot of stored methane, which would be very bad.
The way I see it, the limit is at 1 billion people if we want to preserve current situation, and even less if we want to let the nature regenerate.