In response to:

Will Slower Population Growth Stop Global Warming? from the December 20, 2007 issue

To the Editors:

Bill McKibben’s reply to Allen Schill [Letters, December 20, 2007] is not inaccurate, but it fails to get to the essence of the issue. The projected 2.5 billion further increase in the human population will almost certainly have a much greater environmental impact than the last 2.5 billion added since 1975. Our species has already plucked the low-hanging resource fruit and converted the richest lands to human uses. To support the newcomers, metals will have to be won from ever-poorer ores, while oil, natural gas, and water will need to be obtained from ever-deeper wells and transported further. So-called “marginal” lands, often the last strongholds of the biodiversity on which we all depend for essential ecosystem services, increasingly will be converted into yet more crops to feed people, livestock, or (as biofuels) SUVs. These changes, plus the alterations that will be needed to cope with fossil fuel problems and new geographic patterns of drought and precipitation, will require accelerating energy use with its attendant destructive consequences for the global environment in general and climate stability in particular.

Climate change is a major threat, even if it may not be the greatest environmental problem. Land-use change, toxification of the planet, increased probability of vast epidemics, or conflicts over scarce resources, involving, possibly, use of nuclear weapons—all population-related—may prove more menacing. To ameliorate any of these threats there are no panaceas; a portfolio approach is required. And any truly effective portfolio must contain measures to slow and eventually reverse human population growth. McKibben is certainly correct that curbing overall consumption is critical. The world’s poorest need more, yet the world’s most affluent should use considerably less. But consumption too has a tight population connection, as McKibben himself is certainly aware. No matter how you slice it, we’re living beyond Earth’s long-term ability to support even the present population. It is not enough to break the momentum of population increase, we’ve got to move more rapidly toward population reduction.

Paul R. Ehrlich

Bing Professor of Population Studies President, Center for Conservation Biology

Stanford University

Anne H. Ehrlich

Associate Director/Policy Coordinator, Center for Conservation Biology

Stanford University

Stanford, California

Bill McKibben replies:

Many thanks to the Ehrlichs, not only for their useful letter but for their long work on this question.

The point I was trying to make in response to Allen Schill is that the connection between population growth and fossil fuel use is actually quite weak—that is, heavy population growth is expected to occur in the areas where fossil fuel use is extremely low and likely to remain so. Thus, in the fight against climate change, which was the question he asked about, consumption is the first imperative. This does not change the fact that a world that strains to supply six billion with everything from water to food to school desks and hospital beds will have a harder time with nine billion.

This Issue

February 14, 2008