Skip to main content

Main menu

  • Home
  • Articles
    • Current
    • Special Feature Articles - Most Recent
    • Special Features
    • Colloquia
    • Collected Articles
    • PNAS Classics
    • List of Issues
  • Front Matter
    • Front Matter Portal
    • Journal Club
  • News
    • For the Press
    • This Week In PNAS
    • PNAS in the News
  • Podcasts
  • Authors
    • Information for Authors
    • Editorial and Journal Policies
    • Submission Procedures
    • Fees and Licenses
  • Submit
  • Submit
  • About
    • Editorial Board
    • PNAS Staff
    • FAQ
    • Accessibility Statement
    • Rights and Permissions
    • Site Map
  • Contact
  • Journal Club
  • Subscribe
    • Subscription Rates
    • Subscriptions FAQ
    • Open Access
    • Recommend PNAS to Your Librarian

User menu

  • Log in
  • My Cart

Search

  • Advanced search
Home
Home
  • Log in
  • My Cart

Advanced Search

  • Home
  • Articles
    • Current
    • Special Feature Articles - Most Recent
    • Special Features
    • Colloquia
    • Collected Articles
    • PNAS Classics
    • List of Issues
  • Front Matter
    • Front Matter Portal
    • Journal Club
  • News
    • For the Press
    • This Week In PNAS
    • PNAS in the News
  • Podcasts
  • Authors
    • Information for Authors
    • Editorial and Journal Policies
    • Submission Procedures
    • Fees and Licenses
  • Submit
Letter

Plausible reductions in future population growth and implications for the environment

Brian C. O’Neill, Leiwen Jiang, and Patrick Gerland
  1. aClimate & Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80305; and
  2. bUnited Nations Population Division, Department of Economic and Social Affairs, United Nations, New York, NY 10017

See allHide authors and affiliations

PNAS February 10, 2015 112 (6) E506; first published January 23, 2015; https://doi.org/10.1073/pnas.1421989112
Brian C. O’Neill
aClimate & Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80305; and
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
  • For correspondence: boneill@ucar.edu
Leiwen Jiang
aClimate & Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80305; and
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Patrick Gerland
bUnited Nations Population Division, Department of Economic and Social Affairs, United Nations, New York, NY 10017
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
  • Article
  • Info & Metrics
  • PDF
Loading

Bradshaw and Brook (1) conclude that a substantial slowing of global population growth is infeasible over this century short of drastic reductions in fertility, and therefore could not contribute much to lessening pressure on the environment. However, their methodology is too crude to produce credible results.

Bradshaw and Brook’s (1) principal results are produced with a population model that treats all people in the world as identical. Rather than treating Nigeria, which has a total fertility rate of 6.0 births per woman, differently from Italy, with a fertility rate of 1.4, Bradshaw and Brook use a single global region, essentially treating everyone in the world as if they were from Mexico (which is close to the global average). It is not possible to specify meaningful scenarios about future paths of fertility or mortality in a highly diverse world in such a model. For example, Bradshaw and Brook’s “Scenario 1” assumes constant global fertility and mortality rates and projects a population in 2100 of 10 billion. However, a scenario that truly held rates constant in countries around the world would actually produce 20 billion people by the end of the century (2).

Furthermore, in their “realistic” Scenario 2a, Bradshaw and Brook (1) make the unrealistic assumption that mortality rates for every age group decline by half by the end of century. It is well known that as countries transition from lower to higher life expectancy, mortality rates decline first in very young age groups and only later at older ages, a dynamic with substantial effects on population growth. Similarly, Bradshaw and Brook’s (1) simulation of increasing age at childbirth by allocating half of the fertility of women aged 15–24 evenly to those aged 25–49 is arbitrary and not based on evidence.

Bradshaw and Brook (1) also make selective use of scenarios. Their low-fertility scenarios include either the assumption that global fertility declines only to two births per woman, similar to central or best-guess scenarios from demographic institutions (2, 3), or that it declines all of the way to one birth per woman, simulating the imposition of a global one-child policy. It is no surprise then that the authors conclude that there are only two possibilities: little change in the expected global population growth path or draconian limits on child-bearing. Bradshaw and Brook (1) ignore the possibility of plausible low-fertility outcomes in between these scenarios (2, 3), which are consistent with experience in many countries that have already achieved low fertility and show that there is scope for slowing future population growth modestly by midcentury and substantially toward the end of the century. Although such scenarios are not considered the most-likely outcomes (3, 4), they would be facilitated by accelerated progress in female education, delayed first unions and first births, increased time interval between births, and expanded access to family-planning options.

We have used such scenarios to conclude that although slowing population growth would not be the largest nor necessarily most important factor in reducing greenhouse gas emissions, it could make a significant contribution, particularly in the long run (5). We stand by that conclusion.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: boneill{at}ucar.edu.
  • Author contributions: B.C.O., L.J., and P.G. wrote the paper.

  • The authors declare no conflict of interest.

  • Views expressed in this letter are those of the authors and do not necessarily reflect those of NCAR or the UN.

References

  1. ↵
    1. Bradshaw CJA,
    2. Brook BW
    (2014) Human population reduction is not a quick fix for environmental problems. Proc Natl Acad Sci USA 111(46):16610–16615
    .
    OpenUrlAbstract/FREE Full Text
  2. ↵
    1. United Nations, Department of Economic and Social Affairs, Population Division
    (2013) World Population Prospects: The 2012 Revision (United Nations, New York) . Available at esa.un.org/unpd/wpp/index.htm. Accessed November 11, 2014
    .
  3. ↵
    1. Lutz W,
    2. Butz WP,
    3. Samir KC
    , eds (2014) World Population and Human Capital in the Twenty-First Century (Oxford Univ Press, Oxford)
    .
  4. ↵
    1. Gerland P, et al.
    (2014) World population stabilization unlikely this century. Science 346(6206):234–237
    .
    OpenUrlAbstract/FREE Full Text
  5. ↵
    1. O’Neill BC, et al.
    (2010) Global demographic trends and future carbon emissions. Proc Natl Acad Sci USA 107(41):17521–17526
    .
    OpenUrlAbstract/FREE Full Text
PreviousNext
Back to top
Article Alerts
Email Article

Thank you for your interest in spreading the word on PNAS.

NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address.

Enter multiple addresses on separate lines or separate them with commas.
Plausible reductions in future population growth and implications for the environment
(Your Name) has sent you a message from PNAS
(Your Name) thought you would like to see the PNAS web site.
CAPTCHA
This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.
Citation Tools
Plausible reductions in future population growth
Brian C. O’Neill, Leiwen Jiang, Patrick Gerland
Proceedings of the National Academy of Sciences Feb 2015, 112 (6) E506; DOI: 10.1073/pnas.1421989112

Citation Manager Formats

  • BibTeX
  • Bookends
  • EasyBib
  • EndNote (tagged)
  • EndNote 8 (xml)
  • Medlars
  • Mendeley
  • Papers
  • RefWorks Tagged
  • Ref Manager
  • RIS
  • Zotero
Request Permissions
Share
Plausible reductions in future population growth
Brian C. O’Neill, Leiwen Jiang, Patrick Gerland
Proceedings of the National Academy of Sciences Feb 2015, 112 (6) E506; DOI: 10.1073/pnas.1421989112
del.icio.us logo Digg logo Reddit logo Twitter logo CiteULike logo Facebook logo Google logo Mendeley logo
  • Tweet Widget
  • Facebook Like
  • Mendeley logo Mendeley

Article Classifications

  • Biological Sciences
  • Population Biology

This Letter has a Reply and related content. Please see:

  • Relationship between Letter and Reply - January 23, 2015
  • Human population reduction is not a quick fix for environmental problems - October 27, 2014

See related content:

  • Population stabilization potential underestimated
    - Jan 23, 2015
Proceedings of the National Academy of Sciences: 112 (6)
Table of Contents

Submit

Sign up for Article Alerts

Jump to section

  • Article
    • Footnotes
    • References
  • Info & Metrics
  • PDF

You May Also be Interested in

Setting sun over a sun-baked dirt landscape
Core Concept: Popular integrated assessment climate policy models have key caveats
Better explicating the strengths and shortcomings of these models will help refine projections and improve transparency in the years ahead.
Image credit: Witsawat.S.
Model of the Amazon forest
News Feature: A sea in the Amazon
Did the Caribbean sweep into the western Amazon millions of years ago, shaping the region’s rich biodiversity?
Image credit: Tacio Cordeiro Bicudo (University of São Paulo, São Paulo, Brazil), Victor Sacek (University of São Paulo, São Paulo, Brazil), and Lucy Reading-Ikkanda (artist).
Syrian archaeological site
Journal Club: In Mesopotamia, early cities may have faltered before climate-driven collapse
Settlements 4,200 years ago may have suffered from overpopulation before drought and lower temperatures ultimately made them unsustainable.
Image credit: Andrea Ricci.
Steamboat Geyser eruption.
Eruption of Steamboat Geyser
Mara Reed and Michael Manga explore why Yellowstone's Steamboat Geyser resumed erupting in 2018.
Listen
Past PodcastsSubscribe
Birds nestling on tree branches
Parent–offspring conflict in songbird fledging
Some songbird parents might improve their own fitness by manipulating their offspring into leaving the nest early, at the cost of fledgling survival, a study finds.
Image credit: Gil Eckrich (photographer).

Similar Articles

Site Logo
Powered by HighWire
  • Submit Manuscript
  • Twitter
  • Facebook
  • RSS Feeds
  • Email Alerts

Articles

  • Current Issue
  • Special Feature Articles – Most Recent
  • List of Issues

PNAS Portals

  • Anthropology
  • Chemistry
  • Classics
  • Front Matter
  • Physics
  • Sustainability Science
  • Teaching Resources

Information

  • Authors
  • Editorial Board
  • Reviewers
  • Subscribers
  • Librarians
  • Press
  • Site Map
  • PNAS Updates
  • FAQs
  • Accessibility Statement
  • Rights & Permissions
  • About
  • Contact

Feedback    Privacy/Legal

Copyright © 2021 National Academy of Sciences. Online ISSN 1091-6490