Human population reduction is not a quick fix for environmental problems

Edited by Paul R. Ehrlich, Stanford University, Stanford, CA, and approved September 15, 2014 (received for review June 5, 2014)
October 27, 2014
111 (46) 16610-16615


The planet’s large, growing, and overconsuming human population, especially the increasing affluent component, is rapidly eroding many of the Earth’s natural ecosystems. However, 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-y 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 midcentury 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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Data Availability

Data deposition: Data available from the Aekos Data Portal, (


We thank B. Heard for databasing and C. Butler for helpful recommendations. C.J.A.B. and B.W.B. are both supported by Australian Research Council Future Fellowship Grants FT100100200 and FT110100306, respectively.

Supporting Information

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Go to Proceedings of the National Academy of Sciences
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Proceedings of the National Academy of Sciences
Vol. 111 | No. 46
November 18, 2014
PubMed: 25349398


Data Availability

Data deposition: Data available from the Aekos Data Portal, (

Submission history

Published online: October 27, 2014
Published in issue: November 18, 2014


  1. demography
  2. fertility
  3. catastrophe
  4. war
  5. mortality


We thank B. Heard for databasing and C. Butler for helpful recommendations. C.J.A.B. and B.W.B. are both supported by Australian Research Council Future Fellowship Grants FT100100200 and FT110100306, respectively.


This article is a PNAS Direct Submission.



Corey J. A. Bradshaw1 [email protected]
The Environment Institute and School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia
Barry W. Brook
The Environment Institute and School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia


To whom correspondence should be addressed. Email: [email protected].
Author contributions: C.J.A.B. and B.W.B. designed research; C.J.A.B. and B.W.B. performed research; B.W.B. contributed new reagents/analytic tools; C.J.A.B. analyzed data; and C.J.A.B. and B.W.B. wrote the paper.

Competing Interests

The authors declare no conflict of interest.

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