Climate evolution during the Holocene: a study with an Earth system model of intermediate complexity

An Earth system model of intermediate complexity, MoBidiC, has been used to simulate the transient variations in continental temperature, sea-surface temperature (SST), thermohaline circulation (THC) and sea-ice cover over the last 9000 years (9 kyr). Experiments were designed to determine (a) the deviation of the climatic system with respect to equilibrium over the last 9 kyr, (b) the individual contributions of oceans and vegetation to climatic changes, as well as the potential synergies between these components, and (c) the relative importance of precession, obliquity and CO2 concentration changes during this period. Results show a monotonous cooling trend in the northern high latitudes between 9 kyr BP and the present day, both over the oceans and the continents. North of 60°N, this cooling is noticed throughout the year, but the largest variations appear in spring and summer (up to 6 °C over continents). Along with this cooling, the model exhibits a southward shift of the northern treeline by about 600 km. Most of this shift takes place between 4 and 1 kyr BP. During this period, reorganisations of the boreal forest introduce a lag of about 200 years in the system with respect to a state in equilibrium with the external forcing. Sensitivity experiments illustrate the strong impact of this vegetation shift both on the oceans and the continents, especially in spring and early summer. However, the model exhibits a weak synergy between vegetation and ocean throughout the Holocene. Finally, a sensitivity study to the forcing components shows the dominant role of the astronomical forcing with respect to CO2, as well as the non-linear behaviour of climate in response to obliquity and precession.