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Proc Natl Acad Sci U S A. 2017 Nov 14;114(46):12338-12343. doi: 10.1073/pnas.1618308114. Epub 2017 Oct 30.

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

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Woodrow Wilson School, Princeton University, Princeton, NJ 08544;
Department of Philosophy, University of Vermont, Burlington, VT 05405.
Yale-NUS College, Singapore 138527.
Woodrow Wilson School, Princeton University, Princeton, NJ 08544.
Center for Human Values, Princeton University, Princeton, NJ 08544.
International Research Institute for Climate and Society, Columbia University, Palisades, NY 10964.
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544.
Department of Economics, University of Texas at Austin, Austin, TX 78712.
Economics and Planning Unit, Indian Statistical Institute, Delhi, India, 110016.
Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544.
International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria A-2361.


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.


climate change; emissions; population; social cost of carbon; social welfare

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