Spatiotemporal Characteristics of the 4.2 ka Climate Event Across Eurasia and Its Implications for Early Societal Trajectories

Reconstructing the spatiotemporal features of past climate variability and assessing its influence on societal change are essential for understanding long-term human–environment co-evolution and for informing contemporary climate adaptation. The “4.2 ka event” (around 4.2 ka BP) has long been regarded as a major hydroclimatic anomaly marking the onset of the Late Holocene and has frequently been invoked to explain major societal disruptions across multiple regions. However, expanding and increasingly detailed proxy records have challenged both the presumed global uniformity of this event and the magnitude of its societal impacts.

To address these debates, this study conducts a global transient simulation for 4.5–3.5 ka BP using the Community Earth System Model (CESM) to obtain continuous climate fields across Eurasia and to evaluate whether the 4.2 ka anomaly represents coherent regional change or spatially heterogeneous variability. In parallel, we compile archaeological cultural sequences and key regional syntheses across Eurasia, and delineate seven sub-regions based on subsistence strategies and environmental settings. By comparing the spatiotemporal pattern of climate anomalies with trajectories of social change within each sub-region, we examine plausible pathways through which climate perturbations may have shaped early societal dynamics.

Our results indicate that the 4.2 ka signal is widespread across Eurasia but is far from uniform: anomaly intensity, persistence, and hydroclimatic expression exhibit pronounced spatial heterogeneity. Such heterogeneity implies region-specific societal consequences, ranging from amplified stress and risk accumulation in some socio-ecological settings to the reorganization of resources and interregional connectivity in others. These differential impacts may have contributed to divergent developmental pathways among early societies, including those in early China, the Harappan world, Mesopotamia, the Mediterranean, and the Eurasian steppe. Overall, our findings underscore the need to move beyond deterministic “collapse vs. flourishing” narratives and toward process-based, regionally explicit mechanisms linking climate variability and social change.