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.