Changes in atmospheric CO2 concentration over the past two millennia: contribution of climate variability, land-use and Southern Ocean dynamics

The fluctuations of atmospheric CO2 concentrations over the preindustrial Common Era are generally attributed to changes in land carbon storage, caused primarily by changes in surface air temperature but also by changes in land use. This dominant influence of the land carbon cycle is consistent with the negative correlation between atmospheric CO2 concentrations and δ13CO2 variations recorded in ice cores. By performing an ensemble of sensitivity experiments with the LOVECLIM model, we confirm the potentially large role that temperature changes have on the land carbon cycle. However, this process alone cannot explain the magnitude of the reconstructed atmospheric CO2 and δ13CO2 variations. In particular, even when the model is constrained to follow reconstructed temperature changes by data assimilation, and when applying relatively large values of the climate-carbon feedback parameter, it can only explain about 50% of the atmospheric CO2 decrease between the 12th and the seventeenth century. We find that land use changes are likely responsible for most of the observed long term atmospheric CO2 trend over the first millennium of the Common Era, and for up to 30% of the decrease observed after 1600 CE. In addition, in our experiments, changes in southern hemisphere westerly winds induce slightly smaller changes in atmospheric CO2 concentrations than those associated with land use change, and variations in δ13CO2 of the same order of magnitude as the observed ones. Combining the effects of changes in temperature, land use and winds over the Southern Ocean provides a reasonable agreement with reconstructions for atmospheric CO2 concentrations and δ13CO2, especially for the low CO2 values observed during the seventeenth century. This underlines the important contribution of both land and ocean carbon processes. Nevertheless, some uncertainties remain on the origin of the relatively high CO2 concentrations reconstructed during the eleventh and sixteenth centuries