Satellite-Constrained Reanalysis Reveals CO₂ Versus Climate Process Compensation Across the Global Land Carbon Sink

Abstract

Terrestrial ecosystems annually absorb ~30% of anthropogenic C emissions. The degrees to which contemporary CO₂ and climate trends drive this absorption are uncertain, as are the governing mechanisms. To reduce uncertainty, we use Bayesian model-data integration (CARbon DAta MOdel fraMework) to retrieve a terrestrial biosphere reanalysis where Earth Observations optimally inform mechanistic model processes: observations include satellite- and inventory-based constraints on distributions and change in terrestrial C (including live biomass, dead organic C, and land-atmosphere CO₂ exchanges) and underlying mechanisms (including photosynthesis, deforestation, water storage anomalies, and fire). We find that the impact of 2001–2021's atmospheric CO₂ increase on terrestrial C (+39.4 PgC) opposes and far outweighs the impact of climate trends over this period (10.5 PgC). Globally, C gains are mostly attributable to live biomass growth (+31.2 PgC), while CO₂-induced dead organic C gains (+7.8 PgC) are compensated by climate-induced losses (8.8 PgC). The distribution of compensating dead C changes induces an aggregate shift in dead C from high- and mid-latitudes (3.5 PgC) to tropical ecosystems (+2.6 PgC). We additionally find global residence time reductions attributable to CO₂ (2.6%) and climate (1.3%) reflected across latitudes, irrespective of reservoir C changes. In aggregate, these changes reveal an acceleration and redistribution of terrestrial C stores in response to CO₂ and climate trends, which together reflect a gradual but fundamental reorganization of the terrestrial C cycle. Tracking this reorganization—through robust and continual diagnosis of ecosystem function—is essential for accurately resolving the compensating dynamics governing the strength and resilience of the terrestrial C sink.

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