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STANFORD -- Until cultures change radically, the optimum number of people to exist on the planet at any one time lies in the vicinity of 1.5 billion to 2 billion people, about a third of the present number, three California ecologists estimated in an article published in the journal Population and Environment.
Paul and Anne Ehrlich of the Center for Conservation Biology at Stanford University and Gretchen Daily of the Energy and Resources Group of the University of California-Berkeley said that figure, "if achieved reasonably soon, would also likely permit the maximum number of Homo sapiens to live a good life over the long run."
"Determination of an 'optimum' world population size involves social decisions about the lifestyles to be lived and the distribution of those lifestyles among individuals in the population," the scientists wrote.
Between a minimum viable population size (one just large enough to ensure against extinction) and the maximum number that can be supported by Earth's life-support systems ("housed and nurtured by methods analogous to those used to raise battery chickens"), determining an optimum becomes a problem of choosing what lifestyles are to be led. Community-level, national and international discussions of lifestyle preferences will be required before population size targets can be established.
The team predicated their estimate on a desire to preserve the great diversity of human cultures and also to secure basic human well- being for all the world's people, including future generations.
"In general, we would choose a population size that maximizes very broad environmental and social options for individuals," they wrote. "For example, the population of the United States should be small enough to permit the availability of large tracts of wilderness for hikers and hermits, yet large enough to create vibrant cities that can support complex artistic, educational and other cultural endeavors that lift the human spirit."
Daily and the Ehrlichs said that, since the present population has already exceeded 5.5 billion, even if the optimum were actually 4 billion, "the policy implications of our conclusions are still clear." In other words, any reasonable optimum already has been passed; thus, not only a halt to growth, but subsequent shrinkage, is required.
It will be decades before growth can be halted and shrinkage begins - time to reach a consensus on what is the optimum size where the shrinkage should be stopped.
The Daily-Ehrlich team arrived at their estimate of the optimum by "using humanity's energy consumption as a rough indirect measure of the total impact of civilization on Earth's life-support systems."
The use of energy, "especially that provided by fossil fuel and biomass combustion," directly causes or underpins activities that cause many global environmental problems, they wrote: air and water pollution, acid precipitation, land degradation, emissions of carbon dioxide and other greenhouse gases, and production of toxic and hazardous materials and wastes.
Today's technologies and total energy consumption of 13 trillion watts (13 terawatts) already cause serious environmental deterioration and are clearly not sustainable in the long run. Each American, on average, contributes almost 12,000 watts (12 kilowatts) to that total, more than 10 times that of the average citizen of a poor nation, they wrote.
"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 terawatts, eight times that of today and a clear recipe for ecological collapse," the research team wrote.
The article describes energy expert John Holdren's optimistic scenario in which population growth stops at 10 billion and both rich and poor nations converge at a level of energy use less than one-third of the current American level. That level could be reached with technologies now in hand and with an increase in the quality of life for Americans.
Since the current 13-terawatt world is clearly not sustainable, "one might postulate that, with careful choices of energy sources and technologies, 9 terawatts might be used without degrading environmental systems and dispersing non-renewable resources any more rapidly than they could be repaired or substituted for. Under similar assumptions, a 6-terawatt world would provide a 50 percent margin for error," Daily and her colleagues wrote.
They considered a generous margin of error essential, in view of the history of unforeseen environmental threats such as the destruction to the ozone layer. At 3 kilowatts per person, a 6-terawatt world implies 2 billion people, about the number alive in 1930. That was a sufficient number of people to allow for "many great cities, giant industries, and thriving arts and letters. A great diversity of cultures existed, and members of many of them were not in contact with industrialized cultures. Large tracts of wilderness remained in many parts of the world.
"A world with 1.5 billion people using 4.5 terawatts 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."
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