Description: A significant influence of changes in the westerly winds over the Southern Ocean was proposed as a mechanism to explain a large portion of the glacial atmospheric pCO2 drawdown (Toggweiler et al., 2006). However, additional modelling studies with Earth System Models of Intermediate Complexity do not confirm the size and sometimes even the sign of the impact of southern hemispheric winds on the glacial pCO2 as suggested by Toggweiler (Men- viel et al., 2008; Tschumi et al., 2008, d’Orgeville et al., 2010). We here add to this discussion and explore the potential contribution of changes in the latitudinal position of the winds on Southern Ocean physics and the carbon cycle by using a state-of-the-art ocean general circulation model (MITgcm) in a spatial resolution increasing in the Southern Ocean (2◦ longitude; northern hemisphere: 2◦ latitude; southern hemisphere: 2◦cos(α)). We discuss how the change in carbon cycling is related to the upwelling strength and pattern in the Southern Ocean and how they depend on the changing wind fields and/or the sea ice coverage. While the previous studies explored the impact of the westlies starting from present day or pre-industrial back- ground conditions, we here perform simulations from LGM background climate. Ocean surface conditions are for reasons of consistency taken from output of the COSMOS Earth System model for a pre-industrial control and two LGM runs (Zhang et al., in preparation). Additionally, a northwards shift (by 10◦) of the westerly wind belt as proposed by Toggweiler is investigated.

Description: We investigate the potential of a specific geoengineering technique: the carbon sequestration by artificially en- hanced silicate weathering via the dissolution of olivine. This approach would not only operate against rising temperatures but would also oppose ocean acidification. If details of the marine chemistry are taken into consid- eration, a new mass ratio of CO2 sequestration per olivine dissolution of about 1 is achieved, 20% smaller than previously assumed. We calculate that this approach has the potential to sequestrate up to 1 Pg of C per year di- rectly, if olivine is distributed as fine powder over land areas of the humid tropics, but this rate is limited by the saturation concentration of silicic acid. These upper limit sequestration rates come at the environmental cost of pH values in the rivers rising to 8.2 in examples for the rivers Amazon and Congo (Köhler et al., 2010). The secondary effects of the input of silicic acid connected with this approach leads in an ecosystem model (Re- COM2.0 in MITgcm) to species shifts aways from the calcifying species towards diatoms, thus altering the biolog- ical carbon pumps. Open ocean dissolution of olivine would sequestrate about 1 Pg CO2 per Pg olivine from which about 8% are caused by changes in the biological pumps (increase export of organic matter, decreased export of CaCO3). The chemical impact of open ocean dissolution of olivine (the increased alkalinity input) is therefore less efficient than dissolution on land, but leads due to different chemical impacts to a higher surface ocean pH enhancement to counteract ocean acidification. We finally investigate open ocean dissolution rates of up to 10 Pg olivine per year corresponding to geoengineering rates which might be of interest in the light of expected future emission (e.g. A2 scenario with emissions rising to 30 PgC/yr in 2100 AD). Those rates would still sequestrate only less than 20% of the emission until 2100, but would require that the nowadays available shipping capacity of tankers and bulk carriers is entirely used for olivine dissolution ten times a year.