In population dynamics (a term inexplicably missing from Cohen's extensive index) the traditional ecological notion is that of carrying capacity. Early on the author makes the point that humans are different from other animals, not because they're divine as previous centuries would have it, but simply because no other animal impacts the planet to the same extent. Simple example: irrigation. What other animal consumes as much water per head of population? The 21-page Introduction (Part 1) is full of such insights, anecdotes, and the semijoke at the end: 97.6% of all statistics are made up.
The 80-page Part 2, Chapters 2-6, looks to the past few thousand years. Hesitant population growth early on, plague, calories per hectare of old and new world crops, population growth by country in the last century or so. Chapter 5 is of particular interest as it treats four main models of population growth; the exponential or Malthusian curve with a constant CAGR or Compound Annual Growth Rate which blows up forever, the logistic or sigmoid curve which flattens out as the death rate approaches the birth rate due to limits on carrying capacity; the doomsday equation for which the CAGR is proportional to the population (and so becomes infinite in a finite time, leading to unspeakable but mercifully brief intimacies); and the sum-of-(two)-exponentials curve for which the CAGR increases with increasing population size as with the doomsday curve, but unlike the doomsday curve never becomes infinite. Chapter 6: Statistically, humans are immortal; evidence: most of them haven't died. (My rendering of that chapter.)
The 50-page Part 3 looks to the future. Shorter because mostly guesswork, various projections, etc.
The 200-page Part 4, "The Human Carrying Capacity of the Earth", is much longer because of course that's what the book claims to be about. Main equation of Chapter 9: "Methusaleh's Choice", namely that in a stationary population your life expectancy is the reciprocal of the birth rate. (So if you expect to live 50 years the birth rate must be 2% of the population, while if like Methusaleh you expect to live 900 years then it must be 0.11% of the population.)
Chapter 10: 8 estimates of the human carrying capacity. 1891: 6 billion. 1979: 2% more. In between, Weimar Republic scientists (Penke, pre-Hitler) estimated 8-9 billion max based on intuitive judgments about food supplies. The largest estimates were made in the 1960s by De Wit, highly conditional on various factors: a trillion people if only photosynthesis was the limit, but if people wanted a reasonable amount of room then a "mere" 80 billion, i.e. photosynthesis was not really the main constraint. But then Roger Revelle came up with a much lower limit predicated on there being far less arable land than allowed by De Wit.
In the 1970s it was calculated that Earth could only accommodate a billion Americans. (So with say 400 million Americans Earth could carry only 600 million other people living at the same standard. More than that and the inhabitants would not have sufficient caloric intake for prolonged heavy labor. This seemed the case for many of the people I saw myself in New Delhi some years ago who obviously had very little energy.)
In 1976 Roger Revelle estimated that, with careful management of all the available resources, 40 billion people could be fed with an investment of $700 billion dollars or $17 per person.
Chapter 11 takes a big step back to survey the last four centuries of estimates of human carrying capacity.
Chapter 12 considers carrying capacity in ecology, namely other species besides humans, and in applied ecology, where those species serve to meet human needs, e.g. fisheries. Here there are two important curves, the logistic curve from Chapter 5 describing a population (stock in fisheries terminology) that grows without human intervention up to its carrying capacity K, and the inverted U curve giving a fishery's revenue as a function of fishing effort: with no effort no revenue (the logistic curve prevails), with overfishing again no revenue (stock is driven to zero). Somewhere in between is the maximum sustainable (or steady) yield MSY which produces the greatest revenue for the fishery. Fisheries that act in concert can in principle achieve this, if not then the tragedy of the commons enters the picture.
There's a lot more in the book that would be fun to review here, such as defining carrying capacity, parallels with non-humans, technology, time horizon, etc. However it strikes me that with robots replacing people at the present rate a more interesting question might be, how many robots can the Earth support?
If, in the relatively near future, on a shorter time frame than typically considered in these human-carrying-capacity speculations, robots take over the "heavy labor" performed by humans, the caloric needs of humans could drop sharply, allowing for many more humans. The question would then turn from how much caloric energy humans need to how much robots need.
A human powered by 2000 (kilo)calories (= 8 megajoules) a day is operating at around the same average power as a 100 watt light bulb. Ten billion humans are therefore operating at one terawatt (a million million watts or 1,000,000,000,000 watts).
The grid on the other hand (and planes, cars, etc.) is operating at around 15 terawatts today. This is of course power that comes today largely from fossil fuels, but since those are not sustainable indefinitely then in the longer run from renewable sources like sun and wind.
So if a robot is equated to a hundred-watt lightbulb, even today our grid etc. could support 15 times as many robots as humans.
And given that Earth is absorbing around 120,000 terawatts of power from the Sun (after losing 50,000 to reflection), a hundred times more, even if only 1% of that power can be feasibly tapped into.
Robots are considerably less fussy than humans about where they get their energy. Vitamins, shvitamins.
Asimov notwithstanding, the matter of robots working alongside humans was not even a glint in Cohen's eye in his 1995 book, which predated not only Kurzweil's notion of the Singularity but the onset of the World Wide Web.
A lot can happen these days in a mere third of a century.
ISBN-13: 978-0393314953
ISBN-10: 0393314952
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