Cenozoic Antarctic glaciation: An integrated atmosphere - Ocean - Ice Sheet Model Approach
Final Report Abstract
The performed modelling studies performed within this project are helpful to understand the sensitivity of the Antarctic continent to the major changes, which occurred during the Eocene-Oligocene (E-O) transition. The causal mechanism for this fundamental shift is controversial, the most popular explanations being that glaciation was caused either (1) by cooling of Antarctica due to plate tectonic reorganisation and related changes in ocean circulation; or (2) by a response to declining atmospheric pCO2 favoured by the Earth’s orbital configuration. Models and proxy records are being interpreted very differently to bring support to both theories. In this study no transient model run from a greenhouse to an icehouse climate has been considered, but rather several sets of sensitivity experiments have investigated the response of the Antarctic continent to changes in the global climate conditions. This project provides results from experiments with a new climate-ice sheet model simulating present and past AIS as well as the global climate. The results support the idea that the establishment of the ACC and low atmospheric pCO2 levels could have comparable significance in creating the conditions for a wide continental glaciation. On the other hand, orbital forcings, although they certainly support the initial AIS formation, do not seem to have a major impact. Within this project, a complete representation of the Antarctic climate system, including the ocean, the atmosphere and the Antarctic cryosphere, has been obtained. This has been possible by a new modelling approach, involving four numerical models in a coupled modelling system. The iterative coupling procedure applied in this study accounts for the different time scales at which the climate subsystems (ocean, atmosphere and cryosphere) evolve. Main outcomes can be summarized as following: The opening of the Drake Passage is followed by a change in the global ocean circulation and in the establishment of the Antarctic Circumpolar Current. The reorganization of the oceanic flow results in a decrease of the water and air temperatures in the Southern Ocean region and in an increase of the atmospheric vortex around the continent. These changes cooperate in decreasing the Antarctic surface atmospheric temperature (SAT) by approximately 0.4 K. Subsequent to this is the development of an AIS which has a volume of 1.56x10^6 km3 larger than the one obtained with closed Drake Passage. The development of the AIS under Late Eocene climate boundary conditions has been found sensitive to the concentration of atmospheric carbon dioxide. The results show that reducing the atmospheric pCO2 level by 50% would decrease the Antarctic surface atmospheric temperatures by 0.45 K. This leads to an increase of the AIS volume of about 1.45x10^6 km3 after 10 ky. Under the same Late Eocene and Late Oligocene climate boundary conditions the development of an early AIS is supported by a favourable orbital configuration. An astronomical set-up yielding the lowest austral summer insolation decreases the Antarctic SAT by 0.17 K for the Late Eocene continental configuration and by 0.09 K for the Late Oligocene with respect to a modern orbit. The smaller decrease in temperature suggests that the orbital forcing does not seem to be critical for the early development of the AIS. The modelling results achieved with the sensitivity experiments support the idea that the two main processes (i.e. the opening of the Drake Passage and the decrease in atmospheric pCO2 favoured by the Earth’s orbital configuration) could have had comparable importance in creating the conditions for an Antarctic glaciation. Since the planned collaboration with University of Hamburg couldn’t be established during the first year of the project, the workplan had to be rearranged. Instead the University of Bremen (MARUM) has joined the project, which guaranteed for the input of ocean simulations as initialization and forcing of the coupled model. The only difference to the original plan is that transient simulations of the Eocene-Oligocene transition have to be skipped in favour of detailed time slice experiments. The developed coupling scheme for an integrated Earth System Model including atmosphere, ocean, and ice is now base for further studies at the Alfred-Wegener-Institute and can be seen as fundamental approach for future climate studies on long time scales.
Publications
- (2007). The evolution of the Antarctic Ice Sheet under different climate boundary conditions. European Geosciences Union, General Assembly, 15-20 April, 2007, Vienna (Austria)
Cristini, L., Grosfeld, K., Lohmann, G., Huybrechts, P.
- (2008). Simulating the Antarctic Ice Sheet with a Cenozoic Antarctic Glaciation coupled Atmosphere - Ice Sheet model approach. 23. Internationale Polartagung der Deutschen Gesellschaft für Polarforschung, 10-14 March, 2008, Münster (Germany)
Cristini, L., Huybrechts, P., Grosfeld, K., Lohmann, G.
- (2009). Driving the Cenozoic Antarctic Glaciation: role of atmospheric CO2 and uninterrupted circumpolar current. First Antarctic Climate Evolution Symposium, 7-11 September 2009, Granada (Spain)
Cristini, L., Butzin, M., Grosfeld, K., Lohmann, G., Huybrechts, P.
