Global climate change, war, and population decline in recent human history

Abstract

Although scientists have warned of possible social perils resulting from climate change, the impacts of long-term climate change on social unrest and population collapse have not been quantitatively investigated. In this study, high-resolution paleo-climatic data have been used to explore at a macroscale the effects of climate change on the outbreak of war and population decline in the preindustrial era. We show that long-term fluctuations of war frequency and population changes followed the cycles of temperature change. Further analyses show that cooling impeded agricultural production, which brought about a series of serious social problems, including price inflation, then successively war outbreak, famine, and population decline successively. The findings suggest that worldwide and synchronistic war-peace, population, and price cycles in recent centuries have been driven mainly by long-term climate change. The findings also imply that social mechanisms that might mitigate the impact of climate change were not significantly effective during the study period. Climate change may thus have played a more important role and imposed a wider ranging effect on human civilization than has so far been suggested. Findings of this research may lend an additional dimension to the classic concepts of Malthusianism and Darwinism.

Fig. 1.

Paleo-temperature variation, war frequency, and population growth rate, A.D. 1400–1900. (A) Temperature anomaly (°C) in the NH that is smoothed by 40-year Butterworth low pass filter (17). (B) Number of wars in the NH (bright green), Asia (pink), Europe (turquoise), and the arid areas in the NH (orange). (C) Number of wars worldwide as recorded by Wright (39) (turquoise), Luard (38) (orange), and Brecke (bright green). (D) Twenty-year population growth rate in Europe (turquoise), Asia (pink), and the NH (blue) and the NH 50-year fatality index (bright green). Cold phases are shaded as gray stripes. All war time series are in 10-year units, and the data are listed in SI Table 1. The bright green curves correspond to the right y axis.

Fig. 2.

Paleo-temperature variation and its impact on Europe and China, A.D. 1500–1800. (A) Temperature anomaly (°C) in the NH (17). (B–E) European data are in green and Chinese data are in red. The data are in normalized units that indicate the relative amplitude of change only. (B) Detrended estimated values of agricultural production (solid lines) and detrended grain yield ratio in Europe (green-dotted curve) (18). The estimated values of agricultural production are calculated by dividing population size by food price. (C) Detrended population size (solid lines) and population growth rate (dotted curves) (22, 24). (D) Detrended wheat price index in Europe and detrended rice price in China (41, 42). (E) Detrended total war frequency ( and ref. 40). All data are smoothed by 40-year Butterworth low pass filter. All of the dotted curves correspond to the right y axis.