Present‐day and ice‐covered equilibrium states in a comprehensive climate model

We show that in a comprehensive climate model both the current climate and a completely ice‐covered Earth are stable states under today's total solar irradiance (TSI) and CO 2 level. We employ the Max Planck Institute for Meteorology coupled atmosphere‐ocean general circulation model ECHAM5/MPI‐OM, at relatively high resolution (horizontally T63 in the atmosphere and 1.5 degrees in the ocean). Setting TSI to near‐zero causes a transition from realistic present‐day climate to a completely ice‐covered state within 15 years; this state persists even when TSI re‐assumes today's value. A break‐up of the complete ice cover occurs with today's TSI and 100 times – but not with 10 times – today's atmospheric CO 2 level. While TSI is near‐zero, extremely strong meridional overturning ensues in both the Atlantic and the Pacific Oceans. Our results imply that a snowball Earth is possible, in principle, with inception possibly triggered by a brief dark spell.