Climate change, population pressure, and wars in European history

Recent studies show that wars were more prevalent during colder periods in human history. Nevertheless, the temporal consistency of the climate-war correlation in Europe over extended period has rarely been examined systematically. In this study, we extended the European violent conflict record in the Conflict Catalog [Brecke 1999. "Violent conflicts 1400 A.D. to the present in different regions of the world." Paper presented at the 1999 Meeting of the Peace Science Society (International), Ann Arbor, MI, 8–10 October 1999] back to the year AD900, and examined quantitatively the climate-war consistency in Europe in AD900–1999. The period covers the Medieval Warm Period, Little Ice Age, and twentieth-century warming. Grounded on a total number of 2309 recorded violent conflicts in Europe over the last 1100 years, our statistical results were: (1) the negative temperature-war correlation was significant in terms of multi-decadal cycles; (2) in the second half of the period (AD1450–1999): the climate-war relationship was more apparent during longer cycles; a large spatial extent of slight cooling was more pertinent than a small spatial extent of severe cooling in affecting social stability in Europe; and the overall temperature-war correlation was stronger; and (3) the climate-war association was temporarily distorted when population pressure was drastically reduced. The association became significant again once the population system pushed against its Malthusian constraints. In sum, the climate-war association in Europe was statistically significant at the multi-decadal timescale. Yet, its strength varied across different periods and was contingent upon population pressure during the time. The findings in this study may provide some hints in assessing the effectiveness of human adaptations to climate change in the long-term. Copyright © 2018 Hong Kong Geographical Association.