Impact of CO2 and climate on Last Glacial maximum vegetation - a factor separation

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 physio- logically available CO2. It is has been shown that the syn- ergy can be interpreted as a measure of the sensitivity of eco- physiological 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 vegeta- tion patterns. Raingreen shrubs benefit from the colder and drier climate. The pure ecophysiological effect of CO2 ap- pears 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 syn- ergy appears to be as strong as each of the pure contribu- tions locally, but weak on global average for most plant func- tional types. For tropical evergreen trees, however, the syn- ergy is strong on global average. It diminishes the difference between glacial and pre-industrial coverage of tropical ever- green trees, due to the pure climate effect and the pure eco- physiological CO2 effect, by approximately 50 per cent.