Synchronous coupling dominates human–nature interactions in China: Evidence from nine-quadrant trajectory analysis

Prevailing coupled human-natural systems theory assumes ecological responses to anthropogenic disturbances lag by years to decades due to ecosystem inertia—a foundational assumption underlying Environmental Kuznets Curve models. However, this framework inadequately captures dynamics in rapidly developing economies with abrupt policy transitions and satellite-based monitoring. We tested whether synchronous coupling—concurrent human-ecological changes—dominates such contexts by constructing 1-km resolution indices (Anthropogenic Activity Magnitude Index, AAMI; Ecological Environment Condition Index, EECI) for China (2000−2020) and applying a nine-quadrant trajectory framework with provincial quantile thresholds. Three mechanisms explain synchronous coupling: direct biophysical coupling (land-use events simultaneously modifying human and ecological indicators), policy-driven transitions (top-down regulations generating systemic adjustments within 1–2 years), and climate-mediated forcing (hydrometeorological extremes constraining both human activities and ecosystems). Results reveal 69.94% of China exhibited synchronous AAMI-EECI changes, fundamentally challenging lag assumptions. Spatially, the Hu Line functions as a governance capacity threshold: eastern regions achieved coordination despite higher pressure through effective institutions, while conflict zones expanded 24.3% in central-western cities where development outpaced governance. Temporal analysis shows ecological turning points varied systematically (2010–2011 eastward vs. 2013–2015 westward), reflecting governance differentials rather than uniform trajectories. These findings establish that in policy-driven economies, governance regime type—not only development stage—restructures coupling timescales, necessitating spatially differentiated sustainability frameworks and trajectory-based analytical approaches for satellite-observable human-nature systems. • Developed novel indices to quantify human activity and ecological conditions at 1-km resolution. • Introduced an eight-quadrant method to analyze human-ecological dynamics from 2000 to 2020. • Identified 70% of China experiencing concurrent changes in human and ecological systems. • Revealed the Hu Line as a key boundary between coordination- and conflict-dominated zones. • Provided a scalable framework for assessing sustainability in large coupled human-natural systems.