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Catastrophic climate change, population ethics and intergenerational equity

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Abstract

Climate change threatens irreversible and dangerous impacts, possibly leading to extinction. The most relevant trade-off then may not be between present and future consumption but between present consumption and the mere existence of future generations. To investigate this trade-off, we build an integrated assessment model that explicitly accounts for the risk of extinction of future generations. Using the class of number-dampened utilitarian social welfare functions, we compare different climate policies that change the probability of catastrophic outcomes yielding an early extinction. We analyse the role of inequality aversion and population ethics. Low inequality aversion and a preference for large populations favour the most ambitious climate policy, although there are cases where the effect of inequality aversion on the preferred policy is reversed. This is due to the fact that a higher inequality aversion both decreases the welfare loss of reducing consumption of the current generation and also decreases the welfare gain of reducing the future risk of extinction.

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Notes

  1. The stochastic nature of catastrophes is also sometimes explicitly accounted for (Peck and Teisberg 1995; Lontzek et al. 2015; Lemoine and Traeger 2016).

  2. This modelling strategy follows a strand of literature (Cropper 1976; Clarke and Reed 1994; Gjerde et al. 1999; Tsur and Zemel 2009) that considers that abrupt climate change introduces an irreversible regime shift. Characteristic (ii) is not an accurate description of how human extinction might come about (it may take several years). But this simplification can be justified by saying that the extinction process is predetermined and rapidly yields a situation that is equivalent to extinction in terms of welfare (for instance because the extinction may involve a lot of suffering for many years where people live very bad lives).

  3. See for instance Jamieson (2007) or Shue (2014) for examples of such approaches to climate justice.

  4. The economic literature usually uses the term ‘generalised utilitarian’ rather than prioritarian as we do in the introduction. The formula here is prioritarian with respect to consumption; it may be viewed as utilitarian if function \(u(c)=\frac {c^{1-\eta }}{1-\eta }\) is the true utility function. There exists a limited literature in population ethics proposing alternatives to this generalised utilitarian formula, for instance equally distributed equivalent criteria (Fleurbaey and Zuber 2015), egalitarian criteria (Blackorby et al. 1996) or rank-dependent criteria (Asheim and Zuber 2014; 2016).

  5. See Ramsey (1928) and Stern (2007) for arguments on why we should treat generations in a symmetric way. Observe that one reason for discounting, namely the possibility that future generations may not exist, is not yet accounted for, given that the existence (or non-existence) of generations is for the moment known for sure. The next section will show that the extinction risk creates endogenous discounting.

  6. The notion of the neutral level of utility is important in the literature on population axiologies, as many paradoxes or principles (for instance ‘avoiding a repugnant conclusion’ or the ‘mere addition principle’), rely on the notion (see Arrhenius (2020)). The notion has been defended for instance by Holtug (2001). Still, there are discussions about the exact meaning of ‘a life is worth living’ and the level of \(\underline {c}\).

  7. Section S.A. in the Electronic Supplementary Material considers the possibility that policy choices affect population size from a theoretical point of view.

  8. See also Section S.A.2. of the Electronic Supplementary Material for a clarification of the discount rate and the underlying Ramsey formula in our framework.

  9. In this paper, we assume a decreasing Total Factor Productivity (TFP) growth due to the very long time horizon considered (of the order of 10,000 years). Indeed, assuming constant TFP growth would bring unrealistically high levels of consumption per capita at the time scales considered. This issue is usually not discussed in the literature due to the shorter time horizon used in models (typically a few hundred years).

  10. We do not include the 2 ∘C scenario in this section for the sake of clarity.

  11. The stream of consumption is lower over the whole period due to the fact that we do not account for climate damages. This is not the case if climate damages are accounted for, cf. Figure S.3 in the Electronic Supplementary Material

  12. See Section S.E.2. of the Electronic Supplementary Material for the comparison between the 3 ∘C and BAU scenarios, and between the 3 ∘C and 2 ∘C scenarios.

  13. See Section S.E.2 of the Electronic Supplementary Material for the comparison between the 3 ∘C and BAU scenarios, and between the 3 ∘C and 2 ∘C scenarios.

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Acknowledgements

We thank the reviewers and editors for their helpful comments. We thank Mark B. Budolfson, Francis Dennig, Maddalenna Ferranna, Noah Scovronick, Robert H. Socolow and audiences at workshops and seminars in Princeton and Paris for comments and advice.

Funding

This research has been supported by the Agence nationale de la recherche through the Fair-ClimPop project (ANR-16-CE03-0001-01) and Investissements d’Avenir program (ANR-10-LABX-93). Financial support from the Swedish foundation for humanities and social sciences (Anslag har erhallits från Stiftelsen Riksbankens Jubileumsfond) and from Princeton Universitys Program in Science, Technology and Environmental Policy (STEP) is also gratefully acknowledged.

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Authors and Affiliations

  1. CIRED (CNRS), 45 bis avenue de la Belle Gabrielle, 94736, Nogent-sur-Marne Cédex, France

    Aurélie Méjean

  2. CIRED (EHESS), Nogent-sur-Marne, France

    Antonin Pottier

  3. Paris School of Economics (CNRS), Paris, France

    Marc Fleurbaey & Stéphane Zuber

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  1. Aurélie Méjean
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A.M., A.P., M.F. and S.Z. jointly wrote the paper, designed the research and analysed the results. A.M. and A.P. conducted model simulations. A.M. generated figures and tables. A.P. developed the original model.

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Méjean, A., Pottier, A., Fleurbaey, M. et al. Catastrophic climate change, population ethics and intergenerational equity. Climatic Change 163, 873–890 (2020). https://doi.org/10.1007/s10584-020-02899-9

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