The state of the terrestrial biosphere during the Holocene and the Last Glacial Maximum (LGM) was estimated from data bases and steady state simulations in former studies. Here, we use these previous estimates and run a simple globally averaged box model of the terrestrial carbon stocks driven by various paleorecords (temperature, pCO₂, sea level) from the LGM across termination I to the Holocene to determine which forcing might be appropriate to explain observed changes in the biosphere. Former forcing strength of this type of model on recent climate changes is not transferable to our problem of glacial/interglacial variations. The modelled terrestrial carbon stock at the LGM is about 1600 PgC, 600 PgC less than in preindustrial times. The oceanic release of carbon during the last 20 ky seems to be in phase with the atmospheric pCO₂ record, but four times larger than the pCO₂ increase due to the build-up of the terrestrial stocks. Calculated changes in oceanic δ¹³C correspond well with data and suggest not only a significant role of the biosphere on atmospheric δ¹³C during stable climate conditions such as the LGM or the Holocene, but also during the transition. A final identification of the relative importance of either climate change or CO₂ fertilization for fixation of carbon in the terrestrial biosphere is not yet possible.