.2022 Aug 23;119(34):e2108146119.

doi: 10.1073/pnas.2108146119. Epub 2022 Aug 1.

Climate Endgame: Exploring catastrophic climate change scenarios

Luke Kemp^{1 2}, Chi Xu³, Joanna Depledge⁴, Kristie L Ebi⁵, Goodwin Gibbins⁶, Timothy A Kohler^{7 8 9}, Johan Rockström¹⁰, Marten Scheffer¹¹, Hans Joachim Schellnhuber^{10 12}, Will Steffen¹³, Timothy M Lenton¹⁴

Affiliations

¹ Centre for the Study of Existential Risk, University of Cambridge, Cambridge CB2 1SB, United Kingdom.
² Centre for the Study of Existential Risk, and Darwin College, University of Cambridge, Cambridge CB2 1SB, United Kingdom.
³ School of Life Sciences, Nanjing University, Nanjing 210023, China.
⁴ Cambridge Centre for Environment, Energy and Natural Resource Governance, University of Cambridge, Cambridge CB2 3QZ, United Kingdom.
⁵ Center for Health and the Global Environment, University of Washington, Seattle, WA 98195.
⁶ Future of Humanity Institute, University of Oxford, Oxford OX2 0DJ, United Kingdom.
⁷ Department of Anthropology, Washington State University, Pullman, WA 99164-4910.
⁸ Santa Fe Institute, Santa Fe, NM 87501.
⁹ Cluster of Excellence ROOTS - Social, Environmental, and Cultural Connectivity in Past Societies, Christian-Albrechts-Universität, Kiel, 24118 Germany.
¹⁰ Potsdam Institute for Climate Impact Research, 14473 Potsdam, Germany.
¹¹ Department of Environmental Sciences, University of Wageningen, 6708PB Wageningen, The Netherlands.
¹² Earth System Science Department, Tsinghua University, Beijing 100190, China.
¹³ Fenner School of Environment and Society, The Australian National University, Canberra, ACT 2601, Australia.
¹⁴ Global Systems Institute, University of Exeter, Exeter EX4 4QE, United Kingdom.

PMID:35914185
PMCID: PMC9407216
DOI: 10.1073/pnas.2108146119

Climate Endgame: Exploring catastrophic climate change scenarios

Luke Kemp et al. Proc Natl Acad Sci U S A.2022.

.2022 Aug 23;119(34):e2108146119.

doi: 10.1073/pnas.2108146119. Epub 2022 Aug 1.

Authors

Affiliations

¹ Centre for the Study of Existential Risk, University of Cambridge, Cambridge CB2 1SB, United Kingdom.
² Centre for the Study of Existential Risk, and Darwin College, University of Cambridge, Cambridge CB2 1SB, United Kingdom.
³ School of Life Sciences, Nanjing University, Nanjing 210023, China.
⁴ Cambridge Centre for Environment, Energy and Natural Resource Governance, University of Cambridge, Cambridge CB2 3QZ, United Kingdom.
⁵ Center for Health and the Global Environment, University of Washington, Seattle, WA 98195.
⁶ Future of Humanity Institute, University of Oxford, Oxford OX2 0DJ, United Kingdom.
⁷ Department of Anthropology, Washington State University, Pullman, WA 99164-4910.
⁸ Santa Fe Institute, Santa Fe, NM 87501.
⁹ Cluster of Excellence ROOTS - Social, Environmental, and Cultural Connectivity in Past Societies, Christian-Albrechts-Universität, Kiel, 24118 Germany.
¹⁰ Potsdam Institute for Climate Impact Research, 14473 Potsdam, Germany.
¹¹ Department of Environmental Sciences, University of Wageningen, 6708PB Wageningen, The Netherlands.
¹² Earth System Science Department, Tsinghua University, Beijing 100190, China.
¹³ Fenner School of Environment and Society, The Australian National University, Canberra, ACT 2601, Australia.
¹⁴ Global Systems Institute, University of Exeter, Exeter EX4 4QE, United Kingdom.

PMID:35914185
PMCID: PMC9407216
DOI: 10.1073/pnas.2108146119

Abstract

Prudent risk management requires consideration of bad-to-worst-case scenarios. Yet, for climate change, such potential futures are poorly understood. Could anthropogenic climate change result in worldwide societal collapse or even eventual human extinction? At present, this is a dangerously underexplored topic. Yet there are ample reasons to suspect that climate change could result in a global catastrophe. Analyzing the mechanisms for these extreme consequences could help galvanize action, improve resilience, and inform policy, including emergency responses. We outline current knowledge about the likelihood of extreme climate change, discuss why understanding bad-to-worst cases is vital, articulate reasons for concern about catastrophic outcomes, define key terms, and put forward a research agenda. The proposed agenda covers four main questions: 1) What is the potential for climate change to drive mass extinction events? 2) What are the mechanisms that could result in human mass mortality and morbidity? 3) What are human societies' vulnerabilities to climate-triggered risk cascades, such as from conflict, political instability, and systemic financial risk? 4) How can these multiple strands of evidence-together with other global dangers-be usefully synthesized into an "integrated catastrophe assessment"? It is time for the scientific community to grapple with the challenge of better understanding catastrophic climate change.

Keywords: Anthropocene; Earth system trajectories; catastrophic climate change; climate change; tipping elements.

PubMed Disclaimer

Conflict of interest statement

The authors declare no competing interest.

Figures

**Fig. 1.**
Overlap between future population distribution and extreme heat. CMIP6 model data [from nine GCM models available from the WorldClim database (45)] were used to calculate MAT under SSP3-7.0 during around 2070 (2060–2080) alongside Shared SSP3 demographic projections to ∼2070 (46). The shaded areas depict regions where MAT exceeds 29 °C, while the colored topography details the spread of population density.

**Fig. 2.**
Fragile heat: the overlap between state fragility, extreme heat, and nuclear and biological catastrophic hazards. GCM model data [from the WorldClim database (45)] was used to calculate mean annual warming rates under SSP3-7.0 and SSP5-8.5. This results in a temperature rise of 2.8 °C in ∼2070 (48) for SSP3-7.0, and 3.2 °C for SSP5-8.5. The shaded areas depict regions where MAT exceeds 29 °C. These projections are overlapped with the 2021 Fragile State Index (FSI) (49). This is a necessarily rough proxy because FSI only estimates current fragility levels. While such measurements of fragility and stability are contested and have limitations, the FSI provides one of the more robust indices. This Figure also identifies the capitals of states with nuclear weapons, and the location of maximum containment Biosafety Level 4 (BS4) laboratories which handle the most dangerous pathogens in the world. These are provided as one rough proxy for nuclear and biological catastrophc hazards.

**Fig. 3.**
Cascading global climate failure. This is a causal loop diagram, in which a complete line represents a positive polarity (e.g., amplifying feedback; not necessarily positive in a normative sense) and a dotted line denotes a negative polarity (meaning a dampening feedback). SeeSI Appendix for further information.

See this image and copyright information in PMC

Comment in

Reply to Burgess et al: Catastrophic climate risks are neglected, plausible, and safe to study.
Kemp L, Xu C, Depledge J, Ebi KL, Gibbins G, Kohler TA, Rockström J, Scheffer M, Schellnhuber HJ, Steffen W, Lenton TM.Kemp L, et al.Proc Natl Acad Sci U S A. 2022 Oct 18;119(42):e2214884119. doi: 10.1073/pnas.2214884119. Epub 2022 Oct 10.Proc Natl Acad Sci U S A. 2022.PMID:36215481Free PMC article.No abstract available.
Catastrophic climate risks should be neither understated nor overstated.
Burgess MG, Pielke R Jr, Ritchie J.Burgess MG, et al.Proc Natl Acad Sci U S A. 2022 Oct 18;119(42):e2214347119. doi: 10.1073/pnas.2214347119. Epub 2022 Oct 10.Proc Natl Acad Sci U S A. 2022.PMID:36215483Free PMC article.No abstract available.
Reply to Kelman: The foundations for studying catastrophic climate risks.
Kemp L, Xu C, Depledge J, Ebi KL, Gibbins G, Kohler TA, Rockström J, Scheffer M, Schellnhuber HJ, Steffen W, Lenton TM.Kemp L, et al.Proc Natl Acad Sci U S A. 2022 Oct 18;119(42):e2214794119. doi: 10.1073/pnas.2214794119. Epub 2022 Oct 10.Proc Natl Acad Sci U S A. 2022.PMID:36215499Free PMC article.No abstract available.
From Climate Endgame to Climate Long Game.
Bhowmik A, McCaffrey MS, Rooney Varga J.Bhowmik A, et al.Proc Natl Acad Sci U S A. 2022 Nov 8;119(45):e2214975119. doi: 10.1073/pnas.2214975119. Epub 2022 Nov 2.Proc Natl Acad Sci U S A. 2022.PMID:36322727Free PMC article.No abstract available.
Reply to Bhowmik et al.: Democratic climate action and studying extreme climate risks are not in tension.
Kemp L, Xu C, Depledge J, Ebi KL, Gibbins G, Kohler TA, Rockström J, Scheffer M, Schellnhuber HJ, Steffen W, Lenton TM.Kemp L, et al.Proc Natl Acad Sci U S A. 2022 Nov 8;119(45):e2216034119. doi: 10.1073/pnas.2216034119. Epub 2022 Nov 2.Proc Natl Acad Sci U S A. 2022.PMID:36322745Free PMC article.No abstract available.
Designing extreme climate change scenarios for anticipatory governance.
Ruhl JB, Craig RK.Ruhl JB, et al.Proc Natl Acad Sci U S A. 2022 Dec 6;119(49):e2216155119. doi: 10.1073/pnas.2216155119. Epub 2022 Nov 29.Proc Natl Acad Sci U S A. 2022.PMID:36445959Free PMC article.No abstract available.

References

1. Intergovernmental Panel on Climate Change, Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Field C. B., et al., Eds. (Cambridge University Press, 2014).
1. Jehn F. U., Schneider M., Wang J. R., Kemp L., Breuer L., Betting on the best case: Higher end warming is underrepresented in research. Environ. Res. Lett. 16, 084036 (2021).
1. Simpson N. P., et al. , A framework for complex climate change risk assessment. One Earth 4, 489–501 (2021).
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1. Phillips C. A., et al. , Compound climate risks in the COVID-19 pandemic. Nat. Clim. Chang. 10, 586–588 (2020).

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Climate Endgame: Exploring catastrophic climate change scenarios

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Climate Endgame: Exploring catastrophic climate change scenarios

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Conflict of interest statement

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