Extremeism is titillating, and sells books. But, if artfully wielded, moderation can be sexy as well. Joel Cohen, at the Laboratory of Populations of Rockefeller University, introduces his readers to the mischief of skepticism. How Many People Can the Earth Support? is as much about what demographers don't know as what they do. Armed with Cohen's Law of Prediction - that your confidence in a demographic projection should be in inverse proportion to the confidence with which it is made - you may read every population book after this one with narrowed eyes.
Cohen runs the gamut of the theoretical limits to human population, from less than a billion (Ehrlich, 1971) to a trillion (Marchetti, 1978, who suggested "floating cities on the sea"), entertaining even outlandish propositions. Then he patiently debunks each certainty, targeting the plucked-from-the-air numbers and unsubstantiated assumptions on which deceptively precise results depend: "It's so because I say it's so. There are no upper or lower limits (other than zero) on the estimates of the Earth's maximum population that can be achieved by this powerful technique."
A few certainties emerge. One is that population growth cannot continue indefinitely, whatever the ceiling. Another is that the quantity of our species is intimately related to the quality of the lives we choose to lead - whether we eat steak or pressed algae, whether we seek affluence or subsistence, whether we take long showers or sponge baths. Cohen boomerangs from dubious answers to salient questions: what distribution of wealth do we foresee, and what degree of global cooperation? How much wilderness will we preserve, and how many other species will keep us company?
Popular demography is often rabid, academic demography stupefyingly dry. Sharing both bookshelves, Cohen's level-headed work is distinguished by its clarity, eloquence, and (a relief) humour - in defiance of his own definition of a demographer as "somebody with a flair for numbers who doesn't have the personality to become an accountant". The prose is ranges from wry to lyrical. Spurning euphemistic jargon, the author refers refreshingly to developing nations as "poor countries". To illustrate the perils of our extinguishing thousands of species every year, he compares the Earth to a truck transporting human cargo along a treacherous mountain road. Some passengers are "throwing truck parts of unknown function out of the back of the truck so they will have a little more room to stretch out and relax . In one corner a few passengers are feverishly trying to decipher and understand the truck's instruction manual, even as others tear out odd pages to light their evening fires."
Among Cohen's rare certainties is the fact that from the momentum generated by age structure, if magically in 1990 our species had begun to reproduce at replacement rate worldwide, the resultant stable population would still be 8.4 billion. That's half again as many people as are alive today. Since the premise is absurd - current birthrates are much higher - the size of our population is likely soon to be larger. We probably have the fresh water to support about 10 billion - maybe a few billion more; maybe a few billion less.
Is that alarming? Cohen won't say so. Although this is one of those rare cross-over achievements - a contribution to its field, but punter-friendly - Cohen's refusal to answer his own question does become exasperating. Loath to commit the sin of certainty he derides, he will not be pinned down. But despite the coy demurral, this calm, droll volume is shocking in its effect. Cassandras can be dismissed as hysterics, Pollyannas as air-heads. Joel Cohen is neither, and for his very sensibleness the man is terrifying.