How Many People Can the Earth Support?by Joel E. Cohen
532 pages; (1995) W.W. Norton & Company, New York; ISBN: 0393314952
Reviewed by John P. Ebersole, Biology Dept, UMass Boston
The famous tag line from Walt Kelly's "Pogo" comic strip, "We have met the enemy and he is us," comes to my mind at least six times a week as I see people edging out more and more of the natural world. Then I think of Paul Ehrlich's warning that "whatever your cause, it's a lost cause without population control." Nevertheless, modern conservation biologists and conservation organizations rarely confront the issue of human population growth, maybe because the issue is too sensitive, maybe because it's too large and intimidating, maybe because it's too depressing. In How Many People Can the Earth Support, Joel Cohen addresses this issue with terrific intelligence, humanity, and humor -- and finally gives us some cause for optimism.
The mathematical and human dimensions of the overpopulation problem are reviewed in the early chapters of the book. Wrenching scenes from India, Africa, and New York precede a careful and cautious mathematical treatment that includes some great surprises. Contrary to what many of us have learned (and maybe taught) human population growth is NOT a simple exponential -- it is worse than that. On the other hand, it isn't so bad as depicted in the 1960 "Doomsday" formula that prompted Pogo's mutilation of Caesar. In his discussion of the population 'evolutions' that have produced this explosive growth, Cohen presents us with a glimmer of hope: the fourth evolution -- still in progress -- is a worldwide decline in fertility that began in the eighteenth century.
In all his illuminating and penetrating analysis, Cohen emphasizes how difficult it is to project into the ecological future, especially a future where human behavior and perceived living requirements affect growth rates and limits. His discussion of different concepts of carrying capacity used by theoretical and applied ecologists makes it easy to understand how estimates of human carrying capacity and projections of population size can vary so widely. However, that there must be a carrying capacity, and that this limit poses a terrible dilemma, is kept at the forefront of all Cohen's considerations. While stressing that current growth rates are so much greater than ever before that they defy historical comparison and so stretch human comprehension, Cohen's elegant and accessible math clearly expresses our most basic choice: reduce birth rate or reduce life span.
In later chapters that consider solutions to the population dilemma, Cohen makes it clear that the trade-offs and choices to be made will determine the kind of world our children and grandchildren will live in. Another surprise for superficial students of human population growth is that some past evidence, and much recent evidence, indicates that voluntary reduction in fertility can be achieved before economic development has occurred.
How Many People Can the Earth Support is studded with gems of scholarship and intellect. Who could not delight to discover that according to one Greek epic, Zeus engineered the birth of Helen and provoked the Trojan War so that "the load of death might empty the world," relieving "the all-nurturing earth of men [who] oppressed the surface of the deep-bosomed earth?" Teachers dealing with student anxiety and distrust of math will like Cohen's straightforward demonstration of the value of taking logs -- to linearize curves so they may be more easily analyzed and compared. Finally, in a beautiful appendix that has also appeared in the journal, Science, Cohen shows how the claim of Bush I that "every human being represents hands to work, and not just another mouth to feed" may be included in population growth models to improve the fit to existing data -- but without solving the basic population dilemma.
Cohen's book on human population is unique in for its careful, elegant approach to the math of population growth -- and for its compilation and analysis of social research and historical information on voluntary birth control. A general reader will probably find that some of the math is demanding, but compensation for rising to this challenge is more than adequate. Seven years old now, this work is a classic.