The variability in the carbon sinks as reconstructed for the last 1000 years

The atmospheric CO2 and delta(13)C records for the last millennium have bee n analyzed to reconstruct the evolution and the temporal variability in the terrestrial and oceanic carbon sinks and to identify natural variations in the marine carbon cycle. Reconstructed natural variations in sinks are usu ally less than +/- 0.2 Gt C yr(-1) on time scales of decades to centuries a nd thus one order of magnitude smaller than the sink fluxes driven by the a nthropogenic perturbation. The natural oceanic carbon cycle was generally c lose to steady state on a multi-decadal time scale. A large anomalous ocean ic carbon sink is found around 1940 that is attributed to a higher than usu al El Nino activity. Interannual variations in the oceanic sink as reconstr ucted for the 1980-1996 period are around +/- 1 Gt C yr(-1) and are signifi cantly correlated with the Southern Oscillation. The relatively low atmosph eric CO2 concentrations between 1600 and 1750 were caused by an additional terrestrial storage of 40 Gt C. The land biota acted as a carbon source bet ween 1750 and 1950 and as a sink afterwards. Terrestrial changes can be exp lained by land use emission up to 1920. Then, additional mechanisms such as CO2 fertilization are responsible for an estimated terrestrial sequestrati on of 100 Gt C between 1920 and 1996.