CHANGES IN THE SEASONAL AMPLITUDE OF ATMOSPHERIC CARBON DIOXIDE CONCENTRATION: CAUSES AND FUTURE PROJECTIONS
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The atmospheric carbon dioxide (CO2) observations reveal a seasonal cycle that is dominated by the growth and decay of land vegetation. Ground-based and aircraft-based observations indicate that the amplitude of this seasonal cycle has increased over the past five decades, suggesting enhanced biosphere activity. Previous studies have tried to explain the amplitude increase with stimulated vegetation growth by higher concentrations of CO2 and warming, but the understanding of all the important mechanisms and their relative contribution is still lacking. This work comprises of three individual studies that contribute to better understanding of the CO2 amplitude increase over time and space. With improved crop simulation scheme in a terrestrial carbon model, a new mechanism—the intensive farming practices of the agricultural Green Revolution—is presented as a driver of changes in the seasonal features of the global carbon cycle. Results are further compared with eight other models’ simulations and a number of observation-based datasets on the seasonal characteristics of simulated carbon flux, and on the relative contribution of rising CO2, climate and land use/cover change. In addition, future projections on the amplitude change of CO2 seasonal cycle are examined using simulations from 10 Coupled Model Intercomparison Project Phase 5 (CMIP5) earth system models. Results from this work demonstrate that human land-management activities are powerful enough to modify the basic seasonal characteristics of the biosphere, as reflected by atmospheric CO2. Models attribute 83±56%, −3±74% and 20±30% of global carbon flux amplitude increase to the CO2, climate and land use/cover factors, respectively. Additionally, the models’ underlying mechanisms for the simulated carbon flux amplitude increase in different regions are substantially different. Strong productivity increase under higher CO2 concentration is also seen in the CMIP5 models, leading to 62±19% global mean CO2 amplitude increase in 2081-2090 compared to 1961-1970. Both groups of models suggest that models simulating larger amplitude increase tend to show a larger gain in land carbon sink (with a cross-model R2 of ~0.5 in both cases). Overall, this work presents significant insights in the change of CO2 amplitude and model representation of global carbon cycle.