Optimum human population size

Abstract

Based on prevailing biophysical and social conditions there is a solid scientific basis for determining the bounds of possibilities for the minimum and maximum viable population sizes for the earth. Above the minimum (perhaps 500 people) and within biophysical constraints determining the maximum depends upon social preferences. A possible criteria for this determination is that optimum population size 1) is not the same as the maximum population possible 2) should be small enough to guarantee the minimal physical ingredients of a decent life to everyone 3) should secure human rights from problems generated by too many people 4) should be large enough to sustain viable population in geographically dispersed parts of the world in order to preserve and foster cultural diversity 5) should be sufficiently large to provide a "critical mass" in each of a variety of densely population areas to stimulate creativity and 6) should be small enough to ensure the preservation of biodiversity. Using energy consumption to determine the upper bound of an optimum we find that current consumption is about 13 terawatts (TW) (70% in the rich countries). If the 1993 population of 5.5 billion stops at 14 billion in the next century and per capital energy use is 7.5 kilowatts (KWs) (two-thirds that in the US in the early 1990s) humanity would need 105 TW and would experience ecological collapse. The use of only 9 TW might be accomplished without environmental degradation. In a 6 TW world with all societies consuming 3 KW per capita the optimum population size would be 2 billion. 1.5 billion people using 4.5 TW of energy would carry an extra margin of safety. In light of the current population of 5.5 billion and growing the policy implications of this exercise are obvious.