Imbalance Trajectories of GPP-TER Coupling Under Global Warming

The stability of the terrestrial carbon sink depends on the dynamic coupling between gross primary production (GPP) and total ecosystem respiration (TER). Droughts and heatwaves can disrupt this coordination and trigger short-lived sink-to-source shifts. Here, we propose a three-dimensional diagnostic framework based on coupling mean, coupling slope, and coupling variability, combined with optimal-lag detection, an Otsu-based composite threshold, and an incremental metric to identify regions with weakening GPP-TER coordination. Using gridded datasets for 1980-2017, we find a latitude-banded pattern of strong mean coupling, low variability, and widespread weakening; tropical regions show looser coupling with near-synchronous responses, whereas subtropical and mid- to high-latitude regions commonly exhibit a 1-3 month lag of TER behind GPP. The three-dimensional space separates ecosystems into eight state classes and enables attribution of state differences to environmental drivers. When extended to CESM2-WACCM projections under SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 for 2025-2100, the framework indicates contraction of robust classes and expansion of vulnerable and degraded classes, especially under high emissions. These patterns point to increasing destabilization of GPP-TER coupling under warming and drying and provide a practical basis for hotspot identification and model evaluation.