10%, chances are very good that you’re it.
Not that it matters much, postfossil indefinitely sustainable planetary population is a lot less than one billion, perhaps as low as 100 million. It would be just 500000 if we were a just another species our size.



Yes, we haven’t been longterm sustainable even since preindustrial times. Worse, the carrying capacity after all is sung and done will be a lot lower than the preindustrial one.