Gretchen C. Daily University of California (Berkeley) Anne H. Ehrlich and Paul R. Ehrlich Stanford University (July 1994)
At present, world energy use amounts to about 13 terawatts, about 70% of which is being used to support somewhat over a billion people in rich countries and 30% to support more than four billion people in developing countries.
This pattern is both ethically undesirable and biophysically unsustainable, because of the gross disparity between rich and poor societies, and because of the environmental damage that results.
The consumption of 13 TW of energy with current technologies is leading to the serious ecological impacts indicated above, all of which contribute to several forms of deleterious global change, including a continuous deterioration of ecosystems and the essential services they render to civilization.
An examination of probable future trends leads to dismal conclusions. The world population is projected to increase from 5.5 billion in 1993 to somewhere between 10 and 14 billion within the next century.
Suppose population growth halted at 14 billion and everyone were satisfied with a per-capita energy use of 7.5 kilowatts, the average in rich nations and about two thirds of that in the United States in the early 1990s.
A human enterprise that large would create a total impact of 105 TW, eight times that of today and a clear recipe for ecological collapse.
A scheme that might possibly avoid such a collapse was proposed by John Holdren of the Energy and Resources Group at the University of California, Berkeley.
The Holdren scenario postulates expansion of the human population to only 10 billion and a reduction of average per-capita energy use by people in industrialized nations from 7.5 to to 3 kilowatts, while increasing that of the developing nations from 1 to 3 kW.
The scenario would require, among other things, that citizens of the United States” cut their average use of energy from almost 12 kW to 3 kW.
That reduction could be achieved with energy efficient technologies now in hand and with an improvement in the standard of living.
While convergence on an average per-capita consumption of 3 kW of energy by 10 billion people would close the rich-poor gap, it would still result in a total energy consumption of 30 TW, more than twice that of today.
Whether the human enterprise can be sustained even temporarily on such a scale without devastating ecological consequences is unclear, as Holdren recognizes.
This will depend critically on the technologies involved in the future as reserves of fossil fuels, especially petroleum, are depleted.
Perhaps through funkier development and widespread application of more benign technologies (such as various forms of solar power and biomass-derived energy), environmental deterioration at the peak of human activities could be held to that of today.
Against that background, what might be said about the upper limits on an optimum population size, considering present attitudes and technologies?
In view of the environmental impacts of a civilization using 13 TW today, to say nothing of the threats to the future prospects of humanity, it is difficult to visualize a sustainable population that used more than 9 TW with present and foreseeable technologies.
One might postulate that, with careful choices of energy sources and technologies, 9 TW might be used without degrading environmental systems and dispersing nonrenewable resources any more rapidly than they could be repaired or substituted for.
Under similar assumptions, a 6 TW world would provide a 50% margin for error, something we deem essential considering the unexpected consequences that often attend even very benign-appearing technological developments (the invention and use of chlorofluorocarbons being the most instructive case to date).
A more conservative optimum would be based on a 4.5 TW world, giving a 100% margin for error.
Which upper limit one wished to choose would depend in pan on some sort of average social risk aversion combined with a scientific assessment of the soundness of the 9 TW maximum impact.
In the real world, the maximum sustainable population might well be determined in the course of reducing population size and overall impact—by discovering the scale of the human enterprise at which ecosystems and resources seemed to be holding their own.
For our thought experiment, let us consider a 6 TW world.
If we assume a convergence of all societies on 3 kW percapita consumption, that would imply an optimum population size of 2 billion people, roughly the number of human beings alive in 1930.
Such a number seems at first glance to be reasonable and well above the minimum number required to take advantage of both social and technical economies of scale.
In the first half of the twentieth century, there were many great cities and giant industrial operations.
A great diversity of cultures existed, and members of many of them were not in contact with industrializing cultures. Large bans of wilderness remained in many pans of the world.
A world with 1.5 billion people using 4.5 TW of energy seems equally plausible and would carry a larger margin of safety. This is about the same number of people as existed at the turn of the century.
To summarize this brief essay, determination of an “optimum” world population size involves social decisions about the life styles to be lived and the distribution of those life styles among individuals in the population.
To us it seems reasonable to assume that, until cultures and technology change radically, the optimum number of people to exist simultaneously in the vicinity of 1.5 to 2 billion people.
That number, if achieved reasonably soon, would also likely permit the maximum number of Homo sapiens to live a good life over the long run.
But suppose we have underestimated the optimum and it actually is 4 billion?
Since the present population is over 5.5 billion and growing rapidly, the policy implications of our conclusions are still clear. ( Link )