Aquifers are drying up, topsoil thinning, rainforests disappearing, forest and savannah desertifying, oceans are polluted, dying and rising; rivers and lakes are polluted; petroleum reserves are dwindling and remaining sources expensive and hazardous to access. All this is due to us.
How is any of this consistent with a sustainable population?
We are using resources faster than they can be replenished. How can that continue? You can argue that technology will come up with miraculous fixes, but the technology depends on those resources too and, with half the world's population in poverty, doesn't seem to be keeping up.
Our population is increasing as the carrying capacity of the planet is decreasing. Sustainable -- I don't think so.
The biosphere is a dynamic, interconnected system of myriad life forms and chemical reactions. It's like a huge machine or single massive organism. I don't think most people appreciate the intricacy of its various systems, if they're aware of them at all. I don't think most people have any inkling of how quickly and seriously these systems are being degraded.
A sustainable population, you'd think, would require a stable support system. Are overpopulation deniers arguing that we have a stable biosphere, that we're not impacting the biosphere, or that the biosphere's unnecessary?