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Impact of population growth and population ethics on climate change mitigation policy

  1. Fabian Wagnera,i,j
  1. aWoodrow Wilson School, Princeton University, Princeton, NJ 08544;
  2. bDepartment of Philosophy, University of Vermont, Burlington, VT 05405;
  3. cYale–NUS College, Singapore 138527;
  4. dCenter for Human Values, Princeton University, Princeton, NJ 08544;
  5. eInternational Research Institute for Climate and Society, Columbia University, Palisades, NY 10964;
  6. fDepartment of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544;
  7. gDepartment of Economics, University of Texas at Austin, Austin, TX 78712;
  8. hEconomics and Planning Unit, Indian Statistical Institute, Delhi, India, 110016;
  9. iAndlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544;
  10. jInternational Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria A-2361
  1. Edited by William C. Clark, Harvard University, Cambridge, MA, and approved September 26, 2017 (received for review November 4, 2016)

Significance

We investigate how future population growth is relevant to climate change policy. The answer depends importantly on ethical questions about whether our ultimate goal should be to increase the number of people who are happy or rather to increase the average level of people’s happiness. We calculate the best (optimal) emissions reduction pathway given each of these two different goals that society might have and calculate how much cheaper it would be to avoid dangerous interference with the climate given a smaller rather than a larger population. We also show that whether it is ultimately better to have a smaller population in response to climate change depends on which of these two goals society chooses.

Abstract

Future population growth is uncertain and matters for climate policy: higher growth entails more emissions and means more people will be vulnerable to climate-related impacts. We show that how future population is valued importantly determines mitigation decisions. Using the Dynamic Integrated Climate-Economy model, we explore two approaches to valuing population: a discounted version of total utilitarianism (TU), which considers total wellbeing and is standard in social cost of carbon dioxide (SCC) models, and of average utilitarianism (AU), which ignores population size and sums only each time period’s discounted average wellbeing. Under both approaches, as population increases the SCC increases, but optimal peak temperature decreases. The effect is larger under TU, because it responds to the fact that a larger population means climate change hurts more people: for example, in 2025, assuming the United Nations (UN)-high rather than UN-low population scenario entails an increase in the SCC of 85% under TU vs. 5% under AU. The difference in the SCC between the two population scenarios under TU is comparable to commonly debated decisions regarding time discounting. Additionally, we estimate the avoided mitigation costs implied by plausible reductions in population growth, finding that large near-term savings ($billions annually) occur under TU; savings under AU emerge in the more distant future. These savings are larger than spending shortfalls for human development policies that may lower fertility. Finally, we show that whether lowering population growth entails overall improvements in wellbeing—rather than merely cost savings—again depends on the ethical approach to valuing population.

Footnotes

  • ?1N.S., M.B.B., and D.S. contributed equally to this work.

  • ?2To whom correspondence should be addressed. Email: Noah.Scovronick{at}princeton.edu.
  • Author contributions: N.S., M.B.B., and D.S. designed the research; M.B.B. led the computer modeling; N.S., M.B.B., F.D., M.F., D.S., and F.W. performed research; N.S., M.B.B., F.D., M.F., A.S., R.H.S., D.S., and F.W. analyzed data; and N.S., M.B.B., and D.S. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • See Commentary on page 12103.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1618308114/-/DCSupplemental.

Online Impact

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