Impact of drought stress and other factors on seasonal land biosphere CO₂ exchange studied through an atmospheric tracer transport model

Abstract

We develop a simple, generic global model of carbon cycling by terrestrial vegetation driven by climate data, observed greenness from global vegetation index data, and a drought-stress indicator calculated with a one-layer bucket model. Modelled CO₂ fluxes are then fed into a three-dimensional atmospheric tracer transport model, so that results can be checked against observed concentrations of CO₂ at various monitoring sites. By exploring a wide range of model formulations, we find an optimal fit of the model to atmospheric CO₂ data. To check these results, we use a second model with a coupled photosynthesis-hydrology scheme, using the Penman-Monteith equation to calculate evapotranspiration. This model simulates feedbacks between drought-stress and photosynthesis through the soil-water balance. We show that CO₂ measurements at tropical and southern-hemisphere stations can be used to constrain the seasonal atmosphere-biosphere carbon exchange in the wet-dry tropics. In both models, this seasonality is strongly suppressed, more strongly in fact than predicted by some complex terrestrial-vegetation models. We also find some evidence of a considerable CO₂ 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. The results of this study should indicate in how far measurements of atmospheric CO₂ concentration can provide a constraint on global models of terrestrial-vegetation activity.