Abstract. The factor separation of Stein and Alpert (1993) is applied to simulations with the MPI Earth system model to determine the factors which cause the differences between vegetation patterns in glacial and pre-industrial climate. The factors firstly include differences in the climate, caused by a strong increase in ice masses and the radiative effect of lower greenhouse gas concentrations; secondly, differences in the ecophysiological effect of lower glacial atmospheric CO₂ concentrations; and thirdly, the synergy between the pure climate effect and the pure effect of changing physiologically available CO₂. It is has been shown that the synergy can be interpreted as a measure of the sensitivity of ecophysiological CO₂ effect to climate. The pure climate effect mainly leads to a contraction or a shift in vegetation patterns when comparing simulated glacial and pre-industrial vegetation patterns. Raingreen shrubs benefit from the colder and drier climate. The pure ecophysiological effect of CO₂ appears to be stronger than the pure climate effect for many plant functional types – in line with previous simulations. The pure ecophysiological effect of lower CO₂ mainly yields a reduction in fractional coverage, a thinning of vegetation and a strong reduction in net primary production. The synergy appears to be as strong as each of the pure contributions locally, but weak on global average for most plant functional types. For tropical evergreen trees, however, the synergy is strong on global average. It diminishes the difference between glacial and pre-industrial coverage of tropical evergreen trees, due to the pure climate effect and the pure ecophysiological CO₂ effect, by approximately 50 per cent.