Do the math. Once we enter the post-fossil fuel era, the rule of thumb is that land-carrying capacity determines the maximum population size.

I don't have the US numbers, but for where I'm from, the Netherlands (Europe), it means back to the peak of the bio-based economy. So more or less back to the start of industrialisation.

For us that was around 1870, with 4-5 mln inhabitants. We are currently heading towards 20 mln. So the fascinating question is what will happen to the surplus of 15mln? (4/5) people? The nicest solution would be to aim for a reproduction rate of 1,6-1,7 and gradually phase them out over a period of 200 years. As the only other alternative, short of genocide, is to beg Russia to please let them in. (the fundamental reason Europeans may never alienate themselves from the realities of their continent or pay the highest price for arrogance and short-sightedness)

But, as historian Ian Morris points out, sufficient levels of economic growth are needed to prevent societal collapse during periods of turmoil and duress. The question is.......do we have enough economic growth to reach the 1/5 population value?