Strength and limits of transient mid to late Holocene simulations with dynamical vegetation

Abstract. We discuss here the first 6000 years long Holocene simulations with fully interactive vegetation and carbon cycle with the IPSL Earth system model. It reproduces the long term trends in tree line in northern hemisphere and the southward shift of Afro-Asian monsoon precipitation in the tropics in response to orbital forcing. The simulation is discussed at the light of a set of mid Holocene and pre industrial simulations performed to set up the model version and to initialize the dynamical vegetation. These sensitivity experiments remind us that model quality or realism is not only a function of model parameterizations and tuning, but also of experimental set up. They also question the possibility for bi-stable vegetation states under modern conditions. Despite these limitations the results show different timing of vegetation changes through space and time, mainly due to the pace of the insolation forcing and to internal variability. Forest in Eurasia exhibits changes in forest composition with time as well as large centennial variability. The rapid increase of atmospheric CO 2 in the last centuries of the simulation contributes to enhance tree growth and counteracts the long term trends induced by Holocene insolation in the northern hemisphere. A complete evaluation of the results would require being able to properly account for systematic model biases and, more important, a careful choice of the reference period depending on the scientific questions.