Climate-driven reduction in biomass production of the Eurasian steppe coincides with nomadic migration during the first millennium CE

Although it is generally accepted today that climate and other environmental factors can affect human societies at different spatiotemporal scales, direct linkages are difficult to determine, and correlation should not be confused with causation. Here, we use a tree-ring width network of multi-millennial chronologies from inner Eurasia to reconstruct annual changes in Net Primary Productivity (NPP) back to 200 BCE. Our findings reveal that episodes of diminished NPP around the 70s–100s CE, 360s–380s CE, and 470s–560s CE likely contributed to the westward and southward migration of nomadic people from their homelands in northwestern China and Mongolia. Although prolonged multi-decadal periods of climate-induced low NPP served as tipping points for agricultural and pastoral subsistence systems, the inherent mobility of nomadic communities not only enabled them to adapt to adverse environmental conditions but also facilitated a widespread dispersal of ethnic groups.尽管如今学界普遍认为，气候与其他环境因子可在不同时空尺度上对人类社会产生影响，但二者间的直接关联仍难以确定，且需注意切勿将相关性与因果性混为一谈。本研究利用欧亚内陆区域的千年树轮宽度年表网络，重建了公元前200年以来的净初级生产力（Net Primary Productivity, NPP）年际变化。研究结果显示，公元70-100年代、360-380年代以及470-560年代的净初级生产力下降事件，可能推动了中国西北与蒙古地区游牧族群从原居地向西、向南迁徙。尽管持续数十年的气候驱动型低净初级生产力时期，成为农业与游牧生计系统的关键临界点，但游牧社群与生俱来的流动性不仅使其能够适应不利的环境条件，还推动了族群的广泛扩散。