Conference Agenda

Ongoing Earth observation missions bring unprecedented detail and comprehensiveness for understanding and quantifying the role of terrestrial ecosystem on the global carbon cycle. Yet, limited for longer term analysis given their contemporaneous shorter period in orbit. Legacy missions and datasets are essential to study dynamics and processes at longer time scales. Here, analysing global long-term datasets on vegetation aboveground biomass dynamics, spanning from 1992 to 2019, and atmospheric CO2 measurements, we study (1) the contribution of biomass dynamics to the atmospheric CO2 growth rate and (2) the CO2 fertilization effect on plant biomass.

Adopting a fully data-driven three-box model that simulates carbon dynamics within live vegetation, woody debris and soil organic carbon pools, and considers wildfires and spatio-temporal changes in primary productivity, we are able to explain over 60% of the observed variability in atmospheric CO2 growth rate over the period of 1997-2019 (R = 0.78, p-value < 0.05), with a low RMSE of 1.0 PgC yr-1. Our results show that, globally, lagged effects from heterotrophic pools account for 50% of the variability in atmospheric CO2 growth rate, exceeding three times the direct contribution of transient effects from the live biomass pool. These findings highlight the importance of quantifying tree mortality and cascading carbon release from litter and soils in shaping the terrestrial carbon balance.

We further leverage these Earth observations to isolate the specific contribution of elevated CO2 concentration to the biomass dynamics using both local multiple regression and residual methods. The approach is evaluated across an ensemble of dynamic global vegetation model simulations, showing low errors (RMSE: 0.04 and 0.02) and high correlation (R2: 0.79 and 0.88; p-value < 0.005). Globally, satellite-derived estimates indicate a global increase in AGB of 16.9% [13.9–18.8%] per 100 ppm rise in CO2 concentration. These observation-based estimates are close to those estimated by current land surface models (16.3 ± 5.0 %) but exceed estimates from global Earth system models (12.7 ± 6.5% for CMIP5, 13.2 ± 4.6% for CMIP6), suggesting an underestimation of Earth system models on the contribution of the land ecosystems in dampening anthropogenic CO2 emissions.

Overall, vegetation-atmosphere interactions from annual to decadal time scales show both the strong role of carbon loss processes and legacy dynamics, alongside a modest though larger CO2 fertilization effect on biomass when compared to global models. Ultimately, we contextualize these results on the possible future benefits from integrating BIOMASS, NISAR and the GEDI missions to better quantify and understand processes controlling growth, disturbance and recovery processes in terrestrial ecosystems.