General Discussion
In reply to the discussion: Why Is Population Growth Seldom Discussed? [View all]GliderGuider
(21,088 posts)As I said above, living (reproductive) populations tend to follow a logistic curve.
The logistic curve starts off with a long flat "toe", that mirrors the slow growth of human population until the 1600s.
Then there is an increasing quasi-exponential section in which the population rises very swiftly, like we saw from about 1700 to 1980;
That is followed by a quasi-linear portion that resembles the world's population growth since 1980;
Finally there is the shoulder of the curve, in which population growth declines to zero, asymptotic to the 'carrying capacity' of the planet; we haven't got there yet.
The definition of a human carrying capacity is related to the definition of the term "sustainable". Both are exceedingly imprecise and slippery terms.
At the moment I prefer this definition of the word sustainable:
A sustainable system is an open system whose characteristics can be preserved ("sustained" for a given period, assuming no change in the system's environment.
This definition requires us to specify both a bounded time period and the environment the system is operating in. In other words, it requires us expand the system boundaries to include its environment, and to to evaluate that larger system as a dynamic process operating over a bounded time frame.
Using an approach like that it's possible to understand how we could arrive at estimates for a sustainable human population (aka "planetary carrying capacity" that range all the way from our current 7.2 billion (or even more) all the way down to less than 10 million people living a hunter-gatherer lifestyle.
The closest I've come to estimating a carrying capacity for the world biosphere is "200 MT of terrestrial animal biomass". That carrying capacity is redistributed between species (including humans) over time as conditions change. I arrived at this conclusion through analyzing Vaclav Smil's estimates of wild animal, domesticated animal and human biomass over time. My estimate of 200 MT is based on the assumed wild animal biomass in 10,000 BCE.
I agree with evolutionary biologist Ernst Mayr who said that intelligence is a lethal mutation. In other words, human beings are an inherently unsustainable species, and there is no "sustainable carrying capacity" per se for homo sapiens.