It's almost as if you didn't read my post. We will be able to sustain our population pretty easily because we can adapt, farm further north, use more energy, etc. If anti-science people would stop blocking nuclear energy then we would have a clear path to low emissions energy for the entire planet.

If by 'sustainably' you mean some hand-wavy 'no impact to anything on the planet' then no, we cannot. We will certainly need to farm a lot and that takes land which will displace something.

Also, the ecosystem of the planet itself is not sustainable because of the competitive and evolutionary nature of life. Even without humans, animals and plants continually go extinct and have mass extinction events and yet life continues.