[InclinedPlane]

Humans are technological. We do not have an ecological niche as other animals do, we make our own. On the one hand we use agriculture and industry to make our own food and homes. On the other hand we choose our own behaviors based on our culture, economy, etc. That makes the idea of a simple carrying capacity meaningless. We can change our diet and the effective carrying capacity will change. Or we can change our technology, our land use, etc, etc, etc.

As far as the absolute limits, it's unlikely we're anywhere near them. If humans switched to becoming significantly more vegetarian in their diet we could support quite a lot more people with current levels of food production. Yet food production is not even remotely running up against fundamental limits at current levels.