INVESTIGATING PLAUSIBLE MECHANISMS TO ESCAPE A HARD SNOWBALL-EARTH

Among the problems raised by a globally ice-covered Earth, a major one is to establish the C02 threshold required for the deglaciation which is associated with the length of a snowball-earth. This problem has been addressed using Energy Balance Model (or EBM), which showed that for Neoproterozoic insolation (6% of the present day solar constant), a very high C02 concentration would be needed to trigger deglaciation. New results obtained with the Atmospheric General Circulation Model (AGCM) FOAM (for Fast Oceanic Atmospheric Model), demonstrate that even with a 0.2bar C02 atmosphere, the surface temperature remains far below 273°K at the equator, and therefore the model can not to simulate a deglaciation. The FOAM simulations argue for high C02 concentration to escape from a Snowball-Earth, this result has to be compared to other GCM experiments.

To achieve this goal, we compare the FOAM results with new simulations performed with LMDz (GCM of Laboratoire de Météorologie Dynamique) in a Hard Snowball Earth context.

We show that LMDz is much more sensitive to a C02 increase than FOAM. Many processus could explain this difference between both, but we attribute it to the convective scheme and the cloud parametrization. These results suggest that the C02 threshold is dependant on the parameterization, and could be lower than the one suggested by FOAM.