The global carbon cycle plays a significant role in glacial/interglacial transitions. On one hand because carbon reservoirs and exchange rates are subject to external climateconditions, on the other hand because changes in carbon dioxide concentrations lead to amplification and mediation of regional climate variations. Time slice experimentswere so far unable to unambiguously explain the driving forces of the glacial/interglacial change in atmospheric CO2 of about 100 ppmv. Additional information can begained from the temporal evolution of the carbon cycle using transient model runs. Here we used a coupled atmosphere/biosphere/ocean box model of the global carboncycle to quantify changes in CO2 and d13C observed in Antarctic ice core records. To this end the model is transiently driven by various proxy records overthe last 25,000 years. First results show that the estimated increase in the terrestrial biosphere together with the measured atmospheric pCO2 are difficult to reconcilewith current theories of oceanic changes in the carbon cycle. In addition a significant role of the biosphere on changes in the isotopic composition of atmospheric CO2is supported.