Contribution of climate variability, land-use and Southern Ocean dynamics to changes in atmospheric CO2 concentration over the past two millennia

By performing an ensemble of sensitivity experiments with the LOVECLIM model, we confirm the earlier results showing that temperature changes had a large influence on the land carbon cycle over the preindustrial Common Era. 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 12^th and the 17^th 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. Those wind changes also have a strong impact on the difference in ¹⁴C between the northern and southern hemisphere, presenting strong similarities with observed changes. 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 17^th 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 11^th and 16^th centuries.