Abstract
Time series of the atmospheric CO 2 concentration 1957-1980 show a distinct oscillatory
behaviour, related primarily to the seasonal cycle of the land vegetation on the northern
hemisphere, which is superimposed on an almost exponential increase, originating in the
release of CO 2 from industrial activities. Subtraction of these regular features reveals
anomalies of the atmospheric CO 2 concentration of considerable magnitude, reflecting
transient imbalances of the natural carbon exchange fluxes between atmosphere, terrestrial
biosphere and ocean which are induced by climate fluctuations such as the El Nifio-Southern
Oscillation (ENSO) phenomenon. Reconstruction of these concentration anomalies may
provide a means of validating climate-driven carbon cycle models. We employed the
Osnabriick Biosphere Model (OBM) to explore this approach. We ran the model from a
preindustrial steady state to 1980 prescribing emissions of CO 2 from fossil fuel burning with
observed climate data. The simulated anomalous CO 2 fluxes 1958-1980 correlate with
observational estimates. However, the simulated amplitude is about a factor 3 too small,
indicating a 1oo small sensitivity of the model to the climate perturbations. The spatial pattern of the anomalous net biospheric flux reflects those regions where anomalies are anticorrelated to ENSO indices.
