Sorry I missed this earlier. I personally find 1 to 2B optimistic. I wrote in The Biochar Solution in 2009 that a population that size likely existed in the Western Hemisphere before the Columbian Encounter and that would have been entirely without benefit of fossil fuel energy slaves (albeit they were fond of the human variety). Alas, that was back in the Holocene when climate favored warm blooded mammals such as ourselves and the proteins required to sustain a growing population. The era is now past. Post-Holocene I would imagine a global population of a few hundred million in the next century and after that NTHE.