Spatiotemporal shifts of population and war under climate change in imperial China

Studies on the spatiotemporal relationship between historical climate change and the patterns of population and war are rare. In this research, statistical methods (such as correlation test and Granger causality analysis) and visualization technique are applied to demonstrate how temperature, in terms of long-term trend and cyclic mode, fundamentally affects the temporal-spatial variations of population center and war center during imperial China (5–1911 CE). Results show that (1) the consistent southward migration of population center and war center overall accords with the macro-trend of temperature cooling over the last two millennia. (2) The extent of the outward expansion of the Chinese Empire is measured by the population center–war center distance that lengthens during warm periods but shortens in cold phases, which correspond to the north/west/northwestward advancement and south/east/eastward retreatment of war center, respectively, while population center moves within a small range. (3) The shift of population latitude precedes that of war latitude, indicating the change from ecological-demographic to social-political sphere in space. We suggest that similar to population center, the temperature-influenced ancient Hu Line, which symbolizes the disparity of population density in different regions of China, may shift by several hundred kilometers; latitudinal rather than longitudinal variations of population center and war center are more robust in history. We also find that precipitation controls war center and population center on the multicentennial scale, but not the scale focused upon in this study. These findings provide new insights and theoretical implications into the in-depth understanding of the nature–human nexus.