large-crowdPopulation growth, now at roughly 78 million extra people per year, is the don't-go-there zone of modern environmentalism and political discourse.

But let's go there for the moment: The biodiversity crisis. The water crisis. The climate crisis. Lurking behind all these crises is at least one shared factor: human population. Species extinction? Think land clearing for agriculture to feed a growing population of 6.8 billion people. Water? The majority of water goes directly to growing that same food supply. And giving a helping hand to all these other crises as a result of all the fossil fuel burning needed to power our lives and lift billions out of poverty: anthropogenic climate change.

So is birth control policy and access the answer to the environmental challenges of our time? So argues an editorial in The Lancet, as well as recent research from the London School of Economics, and statisticians at Oregon State University, just to name a few recent examples.

This suggestion is rarely received warmly. The Sierra Club has been troubled in recent years by U.S. nativists opposed to population growth, particularly via immigration. "The Population Bomb," a controversial book by environmental scientist Paul Ehrlich predicting widespread starvation as a result of population growth, turned off a generation of thinkers, in part by being wrong (at least in the short term) and in part by seeming anti-human, continuing a tradition that stretches back to the "Dismal Theorem" of Thomas Malthus. And, of course, the specter of population control looms like an Orwellian nightmare, ceding control over what we think of as one of the most basic freedoms a human being has.

Philosophical Transactions of the Royal Society B—a journal from the Royal Society whose motto is "Take nobody's word for it" —wades into these treacherous waters this week with a special issue, "The Impact of Population Growth on Tomorrow's World." As Roger Short of the University of Melbourne writes in the introduction, "The inexorable increase in human numbers is exhausting conventional energy supplies, accelerating environmental pollution and global warming, and providing an increasing number of failed states where civil unrest prevails," among other faults. And he goes so far as to call for a halt to future population growth.

Then again, ask other contributors to the special issue, is population growth even a problem? After all, as various nations have developed, birth rates have fallen—in some cases so much so that populations are shrinking—thanks, in large part, to empowering women to control their reproduction. Or so argue public health scientists Martha Campbell and Kathleen Beford of the University of California, Berkeley in the special issue.

Yet, this demographic transition does not hold everywhere. And, as political scientist Bradley Thayer of Baylor University argues in the same issue, national population bombs trigger war, especially of the internecine civil variety, as well as terrorism as "youth bulges" in Middle Eastern countries leave large masses of young men without economic prospects. In fact, notes Steven Sinding of the Gutmacher Institute in Manchester, Vt., controlling population growth can actually help individuals and families escape poverty. Witness the exceptional economic rise of China in recent decades, in part helped along by the controversial One Child Policy instituted by Mao Zedong.

And family planning has proven effective in the past, from Thailand to Iran, yet funding for such programs has dwindled in recent years. Partially as a result, developing countries in eastern Africa—Kenya, Tanzania, Uganda and Zimbabwe—have seen their populations begin to increase rapidly again in recent years.

Lurking behind all this is a potential crisis in the very resource that has enabled this unprecedented expansion of human numbers: fossil fuels. Thanks to growing population and dwindling supplies, fossil fuel production per capita may peak by mid-century—ending the two centuries of unlimited growth in energy production that is at the root of modern civilization, consultant Richard Nehring writes in the journal.

Yet the issue leaves unexplored vexing questions such as: Who gets to set the limiting number for population growth? Who are the targets of restrained fertility and is this just? And, ultimately, is there an ideal number for human population on this planet?

While an increase in population from 6.8 billion today to closer to 10 billion by mid-century will make sustainable living on the planet a challenge, especially since the bulk of that growth will be among those living in poverty who have a moral claim to economic development, the real problem may not be human numbers so much as human behavior. After all, we have doubled the population in the past 50 years while the world economy measured as global gross domestic product has increased seven-fold and resource use has increased nearly four-fold.

The real problem may be consumption, which, to date, begets ever more consumption.

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