The weak land carbon sink hypothesis

Background

This review examines the long-standing assumption that terrestrial ecosystems act as a strong global carbon sink. For decades, global carbon budgets have reported that land absorbs a large fraction of anthropogenic Carbon dioxide (CO₂) emissions, helping to offset fossil fuel outputs. These conclusions have primarily relied on atmospheric inversion models and ocean uptake estimates. However, recent discrepancies between atmospheric data, satellite biomass observations, and model simulations have raised questions about the true magnitude of the land sink. This study revisits global and hemispheric carbon fluxes from 2000 to 2019, arguing that current estimates may overstate land-based CO₂ absorption due to methodological biases and uncertainties in fossil fuel and ocean flux data.

Goals and Methods

This research aims to test the “weak land carbon sink hypothesis,” proposing that the land sink is substantially smaller than conventional estimates. The authors compared atmospheric inversion models, satellite-derived vegetation biomass time series (from Jet Propulsion Laboratory (JPL) and Chloris), and model outputs from CMIP6 (Coupled Model Intercomparison Project Phase 6) to estimate terrestrial carbon accumulation. They also adjusted the global carbon budget by modifying fossil fuel and ocean fluxes, reducing fossil fuel emissions by 6% and increasing ocean uptake by 8%, to assess consistency with observed Oxygen (O₂) and CO₂ trends. This synthesis integrates atmospheric, remote sensing, and modeling approaches to evaluate discrepancies and test alternative carbon budget closures.

Conclusions and Takeaways

This study concludes that the global land carbon sink is approximately 0.8 ± 0.7 Pg C yr⁻¹, which is about 46% weaker than the earlier reported values, challenging current carbon budget paradigms and underscoring the need for improved direct measurements of global ecosystem carbon stocks and fossil fuel emissions to refine climate projections. This “weak sink” aligns better with satellite biomass trends and global O₂ and CO₂ observations when combined with modest adjustments to fossil fuel and ocean fluxes. The authors highlight potential biases in emission inventories and the overestimation of vegetation and soil carbon accumulation in models, and propose refining Earth system models to better account for disturbance dynamics, soil carbon limitations, and decoupling between photosynthesis and long-term carbon storage.