Now I characterize my lowball estimates as one end of a probability distribution curve. Because if this I'd say that we are in overshoot by anywhere between 60% (according to the Global Footprint Network estimate, which I think id extremely optimistic) and 1,000,000% (one of my lowest estimates: quite pessimistic). In order to put numbers on both cases I use the I=P(AT) formula, where AT is the average human activity/impact level, measured through the proxy of exosomatic energy consumption in watts per person.
Arithmetically speaking:
Correcting a 60% overshoot would require a world population reduction to 4.5 billion people at today's world average energy consumption levels, or 1 billion average Americans.
Correcting a 1,000,000% overshoot would require a reduction to 15 million foragers, 1 million people with today's world average energy consumption levels, or 225,000 average Americans.
The "real" number, if there is or even could be one, would likely fall somewhere between those two limits.
The reason I gravitate towards the lowest number is that 15 million was about the population of the world in about 5,000 BCE, not long after the beginnings of agriculture. I could perhaps be talked into raising that lower bound to 50 million foragers - the estimated population in 1,000 BCE. However I'm reluctant to do so, because anthropogenic damage like the desertification of the Fertile Crescent was already in progress by then, so that situation doesn't meet a strict definition of "sustainable". In addition, supporting 15 million foragers would certainly require hunting for meat, and most of the big wild animals that provided meat in the past are now gone. Animal husbandry is at least as ecologically damaging as crop agriculture, so I don't see it as having a widespread role in such a future.
This assessment ignores the long-term effects of climate change and pollution from our extraction, manufacturing and energy production industries (abandoned mines, leaking hydrocarbon wells, industrial waste, coal slag, nuclear reactor waste etc.) which will certainly lower the planet's carrying capacity by some significant amount. IMO when those effects are factored in, there is no longer a lower bound.
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