Thursday, October 23rd, Hans Renssen (Vrije Universiteit Amsterdam, The Netherlands) talks about the Holocene climate evolution as simulated by the LOVECLIM climate model.

ABSTRACT.
The ECBilt-CLIO-VECODE coupled atmosphere-ocean-vegetation model is used to study the impact of different forcings on the Holocene climate. We have performed several transient experiments covering the last 9,000 years, forced by annual changes in orbital parameters and atmospheric trace gas concentrations. We focus our analysis on three regions: the Arctic, Northern Africa and the South Polar region.
In the Arctic we find a long-term cooling trend in all seasons in response to the orbitally-forced decline in summer insolation. Over the oceans, the response is delayed by 2 months due to the large heat capacity, resulting in strongest Holocene cooling in autumn. We have also analysed the effect of the waning Laurentide Icesheet (LIS) on the early Holocene climate in the Arctic, by performing experiments including the effect of the freshwater forcing representing the background meltflux from the remnant LIS, and the effect of LIS surface albedo and topography. The LIS clearly causes a delay in the timing of the Holocene thermal maximum by about 2 to 3 thousand years in NE North America and NW Europe.
In Northern Africa we analysed the termination of the African Humid period at about 6 kyr BP. Between 9 and 7.5 kyr BP, we simulate a humid, grass-covered Sahara in agreement with proxy-based reconstructions. After 7.5 ka, the decrease in summer insolation produces a decline in the land-sea contrast, effectively reducing precipitation on the continent. This results in a breakdown of grass cover, and an increase of the surface albedo, which amplifies the drying of Northern Africa. However, in our model the termination of the African Humid Period between 7.5 and 5.5 kyr BP is an unstable phase in the NW Sahara, characterised by centennial fluctuations between grassland and desert.
In the Antarctic a long-term cooling trend during the Holocene is clearly apparent in proxy data. In the South Polar region, summer insolation is increasing over the course of the Holocene, so the relation between temperature and orbital forcing is less straight forward here than in the Arctic. Our results indeed show a long-term cooling trend in agreement with the proxy data. The model suggests that the Southern Ocean plays an important role as a heat storage in the early Holocene. Excess heat from the spring season is stored in the thermocline, which is released to the atmosphere in early winter. This effect is strongly amplified by sea ice.