Sensitivity study of land biosphere CO(sub 2) exchange through an atmospheric tracer transport model using satellite-derived vegetation index data

摘要

We develop a simple, globally uniform model of CO(sub 2) 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 CO(sub 2) at various monitoring sites. CO(sub 2) 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 CO(sub 2) flux formulations together with some modifications of the modelled atmospheric transport. We find that while some formulations can be excluded, it cannot be decided whether or not to make CO(sub 2) uptake and release dependent on water stress. It appears that the seasonality of net CO(sub 2) fluxes in the tropics, which would be expected to be driven by water availability, is small and is therefore not visible in the seasonal cycle of atmospheric CO(sub 2). The latter is dominated largely by northern temperate and boreal vegetation, where seasonality is mostly temperature determined. We find some evidence that there is still considerable CO(sub 2) release from soils during northern-hemisphere winter. An exponential air temperature dependence of soil release with a Q(sub 10) 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 CO(sub 2) concentrations caused by changes in the wind data clearly outweigh those caused by year-to-year variability in the climate and vegetation index data. (orig.) (ERA citation 20:006534)