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

Abstract

Terrestrial ecosystems annually absorb $\sim 30$% of anthropogenic C emissions. The degrees to which contemporary ${\text{CO}}_2$ 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 ${\text{CO}}_2$ exchanges) and underlying mechanisms (including photosynthesis, deforestation, water storage anomalies, and fire). We find that the impact of 2001–2021's atmospheric ${\text{CO}}_2$ 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 ${\text{CO}}_2$-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 ${\text{CO}}_2$ ($-$2.6%) and climate ($-$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 ${\text{CO}}_2$ 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.