Satellite constraints reveal CO2 fertilization outweighed climate change impacts on the 2001–2021 land carbon sink (Manuscript, supplement, and accompanying dataset to be submitted to Global Change Biology)

The rate at which terrestrial ecosystems absorb atmospheric CO2 depends on complex and spatially variable responses to changes in climate and rising atmospheric CO2. Understanding and disentangling these responses is essential for climate projections and planning nature-based climate solutions, yet their recent impacts on terrestrial carbon (C) uptake remains uncertain. To address this uncertainty, we apply a Bayesian model data integration framework (CARDAMOM), using contemporary (2001–2021) global observations to optimize each pixel’s mechanistic parameters within a terrestrial ecosystem model (DALECCWE). The observations include satellite- and inventory-based constraints on distributions and change in terrestrial C (e.g., live biomass, soil organic C states, and net biosphere exchange of CO2) and mechanisms of that change (e.g., photosynthesis, deforestation, terrestrial water storage anomalies, or fire). We find that the impact of 2001–2021’s atmospheric CO2 increase on terrestrial C (+38 PgC) opposes and far outweighs the impact of climate trends over the same period (−8.2 PgC). Terrestrial C growth due to CO2 increase is primarily in live biomass (+29 PgC) rather than dead organic C (+9.4 PgC); by contrast, terrestrial C decline due to climate trends is mainly in dead organic C (−7.5 PgC), with regionally divergent impacts to live biomass that are minimal in the global mean (−0.7 PgC). In the last twenty years, the global response to combined climate and CO2 trends has led to increased total terrestrial C (+30 PgC), with regional responses largely positive despite scattered regions of net decreases in C storage (∼9% of global land area). Notably, live and dead organic C states exhibit spatially distinct and often compensating responses, indicating that accurately resolving ecosystem function and mechanisms underlying the terrestrial C cycle’s emergent behavior is crucial for estimating the strength and resilience of the land C sink over the coming decades.