Garrett, Timothy J.

Grasselli, Matheus

Hanley, Brian P.

Keen, Steve

Lenton, Timothy M.

Rae, James W.B.

Proceedings of the National Academy of Sciences

English

St Andrews Research Repository

LETTEREstimates of economic and environmental damages from tippingpoints cannot be reconciled with the scientific literatureSteve Keena,1, Timothy M. Lentonb , Timothy J. Garrettc , James W. B. Raed , Brian P. Hanleye , and Matheus GrassellifTipping points reduce global consumption percapita by around . . . 1.4% upon 6 ◦C warming,based on a second-order polynomial fit of the data— Dietz et al. (1).As Nobel laureate Solow said to Congress when criticizingeconomic models for failing to anticipate the “Great Reces-sion,” “Every proposition has to pass a smell test: Does itreally make sense?” (2). The methods and conclusions inDietz et al. (1) do not make sense.Earth last experienced 6 ◦C warming in the Eocene epoch,≈40 million years ago (3). Asserting consumption would bejust 1.4% lower with all tipping points breached, i.e., criticalelements of the current climate destroyed—while also beingmuch larger than today—is inconceivable, and impossibleto reconcile with scientific literature (3–6).Dietz used the nonmarket damages function from theModel for Evaluating Regional and Global Effects (MERGE)of GHG reduction policies (7) as a “sensitivity check”(ref. 1, SI Appendix). MERGE does not include tipping points;how then can it provide a sensitivity check?Furthermore, MERGE assumed that damages werequadratic, for no better reason than “if damages changequadratically with temperature, the calibration requires onlya single point,” point (2.5 ◦C, 2%) (7). This makes no sense.The damages figure came from mid-1990s government en-vironmental expenditure, but the temperature figure wasentirely arbitrary because mid-1990s warming was 0.5 ◦C,not 2.5 ◦C.Dietz uncritically reproduced Manne’s assumptions: “Thecatastrophic warming temperature [of 17.68 ◦C] is derivedfrom the assumption that economic losses rise quadrati-cally, and are calibrated to a loss of 2% at 2.5 ◦C warming”(ref. 1, SI Appendix).Using temporally consistent mid-1990s warming of 0.5 ◦Cyields point (0.5 ◦C, 2%) forcing catastrophic damages at≈3.5 ◦C. Scientists anticipate severe cascading damages at3.5 ◦C (5, 6, 8, 9) but further emphasizes the inappropriate-ness of assuming quadratic damages.Fig. 1 compares two quadratics—through point (2.5 ◦C,2%) and point (0.5 ◦C, 2%)—and our suggested logistic.Dietz’s multiplicative quadratic-nonmarket-damages-modified utility function tautologically generates a fall inutility of 2% at 2.5 ◦C in high-income countries calibratedto Manne’s “speculative” coordinate, point (2.5 ◦C, 2%)(ref. 1, SI Appendix). However, a quadratic calibrated throughpoint (0.5 ◦C, 2%), and our suggested logistic, whichlogistic approximates Manne/Dietz’s quadratic to 0.6 ◦C,also yielding catastrophic damages at ≈3.5 ◦C, show thatDietz’s conclusions would have been entirely differentFig. 1. Manne and Dietz’s actual quadratic nonmarket damage function(0.0032 · ΔT2) versus a temporally consistent quadratic (0.08 · ΔT2), andcompared to a logistic alternative (100/[1 + e12/5 · (2 − ΔT)]). The point(0.5 ◦C, 2%) is the correct mid-1990’s warming when environmental expen-diture was 2%. Point (2.5 ◦C, 2%) is an arbitrary value without apparent basis.(ref. 1, SI Appendix).had they used more appropriate functions or points. Itmakes no sense to use a quadratic, whose third and higherderivatives are zero, to emulate tipping points, let alone onefitted to Manne’s arbitrary coordinates.Dietz et al. is also based on papers which are them-selves highly questionable. For example, Anthoff et al. (10)concludes that losing the Atlantic Meridional OverturningCirculation (AMOC) would actually increase global GDP. Thisdefies good sense and scientific research predicting a “catas-trophic” (6) decline in food production.We conclude that Dietz has done nothing to narrowthe “huge gulf between natural scientists’ understandingof climate tipping-points and economists’ representationsof climate catastrophes” (8). Future loss calculations byeconomists must be developed, not in isolation from cli-mate scientists, but in close collaboration with them.Author affiliations: aInstitute for Strategy, Resilience, and Security, University CollegeLondon, London WC1E 6BT, United Kingdom; bGlobal Systems Institute, University ofExeter, Exeter EX4 4QE, United Kingdom; cDepartment of Atmospheric Sciences, Universityof Utah, Salt Lake City, UT 84112; dSchool of Earth and Environmental Sciences, Universityof St. Andrews, St. Andrews KY16 9AJ, United Kingdom; eResearch, Butterfly Sciences,Davis, CA 95617; and fDepartment of Mathematics and Statistics, McMaster University,Hamilton, ON, L8S 4K1 CanadaAuthor contributions: S.K. designed research; S.K., T.M.L., T.J.G., J.W.B.R., B.P.H., and M.G.performed research; and S.K., T.M.L., T.J.G., J.W.B.R., B.P.H., and M.G. wrote the paper.The authors declare no competing interest.Copyright © 2022 the Author(s). Published by PNAS. This article is distributed underCreative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).1To whom correspondence may be addressed. Email: s.keen@isrs.org.uk.Published May 19, 2022.PNAS 2022 Vol. 119 No. 21 e2117308119 https://doi.org/10.1073/pnas.2117308119 1 of 2Downloaded from https://www.pnas.org by UNIVERSITY OF ST ANDREWS on April 4, 2024 from IP address 138.251.162.210.1. S. Dietz, J. Rising, T. Stoerk, G. Wagner, Economic impacts of tipping points in the climate system. Proc. Natl. Acad. Sci. U.S.A. 118, 10.1073/pnas.2103081118 (2021).2. R Solow, “Building a science of economics for the real world” (Hearing 111-106, House of Representatives, 2010).3. K. D. Burke et al., Pliocene and Eocene provide best analogs for near-future climates. Proc. Natl. Acad. Sci. U.S.A. 115, 13288–13293 (2018).4. J. W. Rae et al., Atmospheric CO2 over the past 66 million years from marine archives. Annu. Rev. Earth Planet. Sci. 49, 609–641 (2021).5. T. M. Lenton et al., Climate tipping points—Too risky to bet against. Nature 575, 592–595 (2019).6. OECD, Managing Climate Risks, Facing up to Losses and Damages (Organisation for Economic Co-operation and Development, Paris, 2021).7. A. Manne, R. Mendelsohn, R. Richels, Merge: A model for evaluating regional and global effects of GHG reduction policies. Energy Policy 23, 17–34 (1995).8. T. Lenton, J. C. Ciscar, Integrating tipping points into climate impact assessments. Clim. Change 117, 585–597 (2013).9. J. Hansen, M. Sato, P. Hearty, R. Ruedy, Ice melt, sea level rise and superstorms: Evidence from paleoclimate data, climate modeling, and modern observations that 2 ◦C global warming is highly dangerous.Atmos. Chem. Phys. Discuss. 15, 20059–20179 (2015).10. D. Anthoff, F. Estrada, R. S. J. Tol, Shutting down the thermohaline circulation. Am. Econ. Rev. 106, 602–606 (2016).2 of 2 https://doi.org/10.1073/pnas.2117308119 pnas.orgDownloaded from https://www.pnas.org by UNIVERSITY OF ST ANDREWS on April 4, 2024 from IP address 138.251.162.210.

Estimates of economic and environmental damages from tipping points cannot be reconciled with the scientific literature

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Estimates of economic and environmental damages from tipping points cannot be reconciled with the scientific literature

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