Transient Climate Simulations With a Complex 3D-Earthsystem Model for the Eemian and Holocene

The model consists of an atmospheric and oceanic general circulation model, a dynamical terrestrial vegetation model, and a marine carbon cycle model. The model has a prognostic calculation of atmospheric CO2. The model was forced with time-varying insolation from 129 to 113 kyBP, Eemian warm period) and from 10 to 0 kyBP. The main results are: 1) A strong biophysical climate feedback from terrestrial biosphere: Strong increases in monsoonal precipitation and temperature in boreal regions are found during the Eemian (126 kyBP). These features are amplified by more than a factor of 2 compared to simulations where the land surface conditions were kept fixed according to present day conditions. This amplification refers to strong albedo feedbacks due to increases in vegetation coverage in the African - Asian monsoon regions and due to a northward expansion of boreal forests. 2) The simulated storage of carbon in the terrestrial biosphere is increased by more than 10% during the Eemian compared to the following cold period. The effect on the atmospheric pCO2 is, however, rather low (~ 20 ppm) because most of the carbon released to the atmosphere is taken up by the ocean. In the cause of the Eemian almost the entire ocean acts as carbon sink for the atmosphere. This effect is strongest in the eastern tropical Pacific. In this area characterized by upwelling the export production increases as a result of increasing nutrient supply to the surface. For the Holocene climate response and the changes in the carbon cycle are similar to those found in the Eemian but are generally weaker by about 30 %.