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Abstract

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.