Background: The amount of carbon dioxide in the atmosphere steadily increases as a
consequence of anthropogenic emissions but with large interannual variability caused by the
terrestrial biosphere. These variations in the CO2 growth rate are caused by large-scale climate
anomalies but the relative contributions of vegetation growth and soil decomposition is uncertain.
We use a biogeochemical model of the terrestrial biosphere to differentiate the effects of
temperature and precipitation on net primary production (NPP) and heterotrophic respiration
(Rh) during the two largest anomalies in atmospheric CO2 increase during the last 25 years. One
of these, the smallest atmospheric year-to-year increase (largest land carbon uptake) in that period,
was caused by global cooling in 1992/93 after the Pinatubo volcanic eruption. The other, the largest
atmospheric increase on record (largest land carbon release), was caused by the strong El Niño
event of 1997/98.
Results: We find that the LPJ model correctly simulates the magnitude of terrestrial modulation
of atmospheric carbon anomalies for these two extreme disturbances. The response of soil
respiration to changes in temperature and precipitation explains most of the modelled anomalous
CO2 flux.
Conclusion: Observed and modelled NEE anomalies are in good agreement, therefore we suggest
that the temporal variability of heterotrophic respiration produced by our model is reasonably
realistic. We therefore conclude that during the last 25 years the two largest disturbances of the
global carbon cycle were strongly controlled by soil processes rather then the response of
vegetation to these large-scale climatic events