In very rough terms, the human population of planet Earth reached one billion around the middle of the nineteenth century. It hit two billion around the middle of the twentieth century. By the end of that century, it was around six billion. In other words, population doubled in a century, then tripled in half that time.
From observation, the planet has degraded substantially since the middle of the twentieth century. It looks like we passed sustainable limits at some time in the first half of that century. That would put the sustainable human population somewhere below two billion.
We’ve been running an unsustainable population for the better part of a century. As any livestock farmer knows, you can run an overstock for a while, but it degrades the pasture. The longer the overstock persists, the greater the degradation. Normally, we’d move the stock and allow the pasture to recover. Sadly, we don’t have anywhere to go. Earth is it.
In nature, populations die down to the level that the environment can sustain. That will probably happen to us. With the damage that we’ve done, the carrying capacity of Earth is probably far less than the two billion that it might have sustained in its prime.
My guess is that the human population will crash below one billion. Civilisation is not likely to recover.
From a reply to a question on Quora.
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