Sensitivity study of land biosphere CO2 exchange through an atmospheric tracer transport model using satellite-derived vegetation index data

We develop a simple, globally uniform model of CO2 exchange between the atmosphere and the terrestrial biosphere by coupling the model with a three- dimensional atmospheric tracer transport model using observed winds, and checking results against observed concentrations of C02 at various monitoring sites. C02 fluxes are derived from observed greenness using satellite-derived Global Vegetation Index data, combined with observations of temperature, radiation. and precipitation. We explore a range of C02 flux formulations together with some modifications of the modelled atmospheric transport. It appears that the seasonality of net C02 fluxes in the tropics, which would be expected to be driven by water availability, is remarkably small, because C02 uptake and release are reduced simultaneously during the dry season. Consequently, tropical vegetation contributes only very little to the seaonal cycle of atmospheric C102, which is dominated by northern temperate and boreal vegetation, where seasonality is mostly temperature determined. We find some evidence that there is still considerable C02 release from soils during the northern—hemisphere winter. An exponential air temperature dependence of soil release with a Q10 of 1.5 is found to be most appropriate, with no cutoff at low freezing temperatures. This result is independent of the year from which observed winds were taken. This is remarkable insofar as year-to-year changes in modelled CO2 concentrations caused by changes in the wind data clearly outweigh those caused by year-to—year variability in the climate and vegetation index data