The amount of carbon stored in the terrestrial biosphere waschanging for different reasons in the past. We here summarise what isknown about its changes during (1) seasonal variations, (2) theanthropogenic rise in CO2 during the last 250 years, (3) the lastglacial/interglacial transitions (20,000 to 10,000 years beforepresent), and (4) millennial-scale variability in the climate systemcaused by a reduction of the Atlantic thermohaline circulation duringthe last glacial period. For this aim we analyse mass balance equations,global budget considerations, and a dynamic global vegetation model.