PNAS: Human population reduction is not a quick fix

Human population reduction is not a quick fix for environmental problems

by Corey J. A. Bradshaw and Barry W. Brook

Proceedings of the National Academy of Sciences (PNAS). Edited by Paul R. Ehrlich, Stanford University, and approved September 15, 2014

The planet’s large, growing, and over-consuming human population, especially the increasing affluent component, is rapidly eroding many of the Earth’s natural ecosystems.

Society’s only real policy lever to reduce the human population humanely is to encourage lower per capita fertility.

How long might fertility reduction take to make a meaningful impact?

We examined various scenarios for global human population change to the year 2100 by adjusting fertility and mortality rates (both chronic and short-term interventions) to determine the plausible range of outcomes.

Even one-child policies imposed worldwide and catastrophic mortality events would still likely result in 5–10 billion people by 2100.

Because of this demographic momentum, there are no easy ways to change the broad trends of human population size this century.


The inexorable demographic momentum of the global human population is rapidly eroding Earth’s life-support system. There are consequently more frequent calls to address environmental problems by advocating further reductions in human fertility. To examine how quickly this could lead to a smaller human population, we used scenario-based matrix modeling to project the global population to the year 2100. Assuming a continuation of current trends in mortality reduction, even a rapid transition to a worldwide one-child policy leads to a population similar to today’s by 2100.

Even a catastrophic mass mortality event of 2 billion deaths over a hypothetical 5-year window in the mid-21st century would still yield around 8.5 billion people by 2100.

In the absence of catastrophe or large fertility reductions (to fewer than two children per female worldwide), the greatest threats to ecosystems—as measured by regional projections within the 35 global Biodiversity Hotspots—indicate that Africa and South Asia will experience the greatest human pressures on future ecosystems.

Humanity’s large demographic momentum means that there are no easy policy levers to change the size of the human population substantially over coming decades, short of extreme and rapid reductions in female fertility; it will take centuries, and the long-term target remains unclear. However, some reduction could be achieved by mid-century and lead to hundreds of millions fewer people to feed. More immediate results for sustainability would emerge from policies and technologies that reverse rising consumption of natural resources.

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One Response to PNAS: Human population reduction is not a quick fix

  1. Darryl says:

    I think it was Gary Peters who said that if we don’t control our numbers, nature will. And we’re not going to control them.

    Thank-you for maintaining this excellent blogsite.