A reassessment of the impact of temperature change on European conflict during the second millennium CE using a bespoke Bayesian time-series model


Recently, there has been a lot of discussion about the impact of climate change on human conflict. Here, we report a study in which we revisited the findings of a paper that has been cited many times in the discussion. The paper in question focused on the association between temperature and conflict in Europe between 1000 and 1980 CE and suggested that colder temperatures led to more conflict. However, there are reasons to be skeptical of this finding. Most importantly, the analytical technique used by the paper’s authors was not suitable for the conflict dataset because the dataset is count-based and contains autocorrelation. With this in mind, we developed a Bayesian time-series model that is capable of dealing with these features, and then we reanalysed the dataset in conjunction with several temperature reconstructions. The results we obtained were unambiguous. None of the models that included temperature as a covariate outperformed a null hypothesis in which conflict levels at any given time were determined only by previous levels. Thus, we found no evidence that colder temperatures led to more conflict in Europe during the second millennium CE. When this finding is placed alongside the results of other studies that have examined temperature and conflict over the long term, it is clear that the impact of temperature on conflict is context dependent. Identifying the factor(s) that mediate the relationship between temperature and conflict should now be a priority.

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  1. Briffa KR, Osborn TJ, Schweingruber FH (2004) Large-scale temperature inferences from tree rings: A review. volume 40, pp 11–26, Elsevier B.V.

  2. Buntgen U, Tegel W, Nicolussi K, McCormick M, Frank D, Trouet V, Kaplan JO, Herzig F, Heussner K-U, Wanner H, Luterbacher J, Esper J (2011) 2500 Years of European climate variability and human susceptibility. Science 331(6017):578–582

    Article  Google Scholar 

  3. Burke MB, Miguel E, Satyanath S, Dykema JA, Lobell DB (2009) Warming increases the risk of civil war in Africa. Proc Natl Acad Sci USA 106(49):20670–20674

    Article  Google Scholar 

  4. Burke, Marshall, Solomon MH, Miguel E (2015) Climate and Conflict. Annual Review of Economics 7(1):577–617. https://doi.org/10.1146/annurev-economics-080614-115430.

    Article  Google Scholar 

  5. Carleton WC, Campbell D, Collard M (2017) Increasing temperature exacerbated Classic Maya conflict over the long term. Quat Sci Rev 163:209–218

    Article  Google Scholar 

  6. Crowley TJ, Lowery TS (2000) How warm was the medieval warm period?. AMBIO: A Journal of the Human Environment 29(1):51–54

    Article  Google Scholar 

  7. D’Arrigo R, Wilson R, Jacoby G (2006) On the long-term context for late twentieth century warming. J Geophys Res 111(D3):D03103

    Google Scholar 

  8. Edwards TW, Hammarlund D, Newton BW, Sjolte J, Linderson H, Sturm C, Amour NAS, Bailey JN, Nilsson AL (2017) Seasonal variability in northern hemisphere atmospheric circulation during the medieval climate anomaly and the little ice age. Quat Sci Rev 165:102–110

    Article  Google Scholar 

  9. Gabry J, Simpson D, Vehtari A, Betancourt M, Gelman A (2019) Visualization in Bayesian workflow. Journal of the Royal Statistical Society, Series A: Statistics in Society 182(2):389–402

    Article  Google Scholar 

  10. Gelman A, Carlin JB, Stern HS, Dunson DB, Vehtari A, Rubin DB (2014) Bayesian data analysis, 3rd edn. Boca Raton, CRC Press

    Google Scholar 

  11. Geweke J (1992) Evaluating the accuracy of sampling-based approaches to the calculation of posterior moments. In: Bernardo JM, Berger JO, Dawid AP, Smith AFM (eds) Bayesian stastistics. Clarendon Press, Oxford, pp 169–193

  12. Glaser R, Riemann D (2009) A thousand-year record of temperature variations for Germany and central Europe based on documentary data. J Quat Sci 24(5):437–449

    Article  Google Scholar 

  13. High Representative of the European Union for Foreign Affairs and Security Policy (2013) Joint communication to the European Parliament and the Council: The EU’s Comprehensive Approach to External Conflict and Crises

  14. Hsiang SM, Burke M, Miguel E (2013) Quantifying the influence of climate on human conflict. Science 341(6151):1235367

    Article  Google Scholar 

  15. IPCC (2014) climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC

  16. Keele L, Kelly NJ (2006) Dynamic models for dynamic theories: the ins and outs of lagged dependent variables. Polit Anal 14(2):186–205

    Article  Google Scholar 

  17. Koubi V (2019) Climate change and conflict. Annu Rev Polit Sci 22(1):343–360

    Article  Google Scholar 

  18. Lobell DB, Bänziger M, Magorokosho C, Vivek B (2011) Nonlinear heat effects on African maize as evidenced by historical yield trials. Nat Clim Chang 1(1):42–45

    Article  Google Scholar 

  19. Lockwood M, Owens M, Hawkins E, Jones GS, Usoskin I (2017) Frost fairs, sunspots and the Little Ice Age. Astron Geophys 58(2):2.17–2.23

    Article  Google Scholar 

  20. Luterbacher J, Werner JP, Smerdon JE, Fernández-Donado L, González-Rouco FJ, Barriopedro D, Ljungqvist FC, Büntgen U, Zorita E, Wagner S, Esper J, McCarroll D, Toreti A, Frank D, Jungclaus JH, Barriendos M, Bertolin C, Bothe O, Brázdil R, Camuffo D, Dobrovolný P, Gagen M, García-Bustamante E, Ge Q, Gómez-Navarro JJ, Guiot J, Hao Z, Hegerl GC, Holmgren K, Klimenko VV, Martín-Chivelet J, Pfister C, Roberts N, Schindler A, Schurer A, Solomina O, von Gunten L, Wahl E, Wanner H, Wetter O, Xoplaki E, Yuan N, Zanchettin D, Zhang H, Zerefos C (2016) European summer temperatures since Roman times. Environ Res Lett 11(2):024001

    Article  Google Scholar 

  21. Mach KJ, Kraan CM, Adger WN, Buhaug H, Burke M, Fearon JD, Field CB, Hendrix CS, Maystadt JF, O’Loughlin J, Roessler P, Scheffran J, Schultz KA, von Uexkull N (2019) Climate as a risk factor for armed conflict. Nature 571(7764):193–197

    Article  Google Scholar 

  22. Mann ME, Jones DP (2003) Global surface temperatures over the past two millennia. Geophys Res Lett 30(15):15–18

    Article  Google Scholar 

  23. Mann ME, Zhang Z, Rutherford S, Bradley RS, Hughes MK, Shindell D, Ammann C, Faluvegi G, Ni F (2009) Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326 (5957):1256–1260

    Article  Google Scholar 

  24. Martín-Chivelet J, Muñoz-García MB, Edwards RL, Turrero MJ, Ortega AI (2011) Land surface temperature changes in northern Iberia since 4000yrbp, based on 13c of speleothems. Glob Planet Chang 77:1–12

    Article  Google Scholar 

  25. Maystadt J-F, Ecker O, Mabiso A (2012) Extreme Weather and Civil War in Somalia Does Drought Fuel Conflict through Livestock Price Shocks? Technical report, Center for Institutions and Economic Performance, KU Leuven

  26. NIMBLE Development Team (2018) Nimble user manual. R package manual version 0.6-12

  27. NOAA (2020) Global Climate Report: State of the Climate

  28. Ogilvie AE, Barlow LK, Jennings AE (2000) North Atlantic climate c. AD 1000: Millennial reflections on the Viking discoveries of Iceland, Greenland and North America. Weather 55(2):34–45

    Article  Google Scholar 

  29. O’Loughlin J, Witmer FD, Linke AM, Laing A, Gettelman A, Dudhia J (2012) Climate variability and conflict risk in East Africa, 1990-2009. Proc Natl Acad Sci USA 109(45):18344–18349

    Article  Google Scholar 

  30. Porter JR, Gawith M (1999) Temperatures and the growth and development of wheat: a review. Eur J Agron 10(1):23–36

    Article  Google Scholar 

  31. R Core Team (2019) R: A Language and Environment for Statistical Computing

  32. Theisen OM, Holtermann H, Buhaug H (2012) Climate wars? assessing the claim that drought breeds conflict. Int Secur 36:79–106

    Article  Google Scholar 

  33. Tol RSJ, Wagner S (2010) Climate change and violent conflict in Europe over the last millennium. Clim Chang 99(1):65–79

    Article  Google Scholar 

  34. US Department of Defense (2010) Quadrennial Defense Review Report 2010

  35. Von Uexkull N, Croicu M, Fjelde H, Buhaug H (2016) Civil conflict sensitivity to growing-season drought. Proc Natl Acad Sci USA 113:12391–12396

    Article  Google Scholar 

  36. Wagenmakers EJ, Lodewyckx T, Kuriyal H, Grasman R (2010) Bayesian hypothesis testing for psychologists: A tutorial on the Savage-Dickey method. Cogn Psychol 60(3):158–189

    Article  Google Scholar 

  37. Wand M (2015) KernSmooth: Functions for Kernel Smoothing Supporting Wand & Jones (1995). R package version 2.23-15

  38. Wang J, Vanga SK, Saxena R, Orsat V, Raghavan V (2018) Effect of climate change on the yield of cereal crops: a review. Climate 6(2):41

    Article  Google Scholar 

  39. Wilkins AS (2018) To lag or not to lag?: Re-evaluating the use of lagged dependent variables in regression analysis

  40. Zhang DD, Brecke P, Lee HF, He Y-QQ, Zhang J, Jim CY, Lin GCSS, He Y-QQ, Wang JJ, Lee HF (2006) Climatic change, wars and dynastic cycles in China over the last millennium. Climatic Change 76(3-4):459–477

    Article  Google Scholar 

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We thank the deputy editor and the anonymous reviewers for their advice about how to improve our paper. Our work was supported by the Social Sciences and Humanities Research Council of Canada (895-2011-1009), the Canada Research Chairs Program (228117 and 231256), the Canada Foundation for Innovation (203808 and 36801), the British Columbia Knowledge Development Fund (862-804231 and 962-805808), and Simon Fraser University (14518).

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Correspondence to Mark Collard.

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Carleton, W.C., Campbell, D. & Collard, M. A reassessment of the impact of temperature change on European conflict during the second millennium CE using a bespoke Bayesian time-series model. Climatic Change 165, 4 (2021). https://doi.org/10.1007/s10584-021-03022-2

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  • Climate change
  • Conflict
  • Temperature
  • European history
  • Bayesian time-series analysis