CO2 levels required for deglaciation of a "Near-Snowball" Earth

Geologic evidence suggests that in the Late Neoproterozoic (similar to 600 Ma) almost all land masses were glaciated, with sea-level glaciation existi ng even at the equator. A recent modeling study has shown that it is possib le to simulate an ice-covered Earth glaciation with a coupled climate/ice-s heet model. However, separate general circulation model experiments suggest that a second solution may exist with a substantial area of ice free ocean in the tropics. Although 0.1 to 0.3 of an atmosphere of CO2 (similar to 30 0 to 1000 X) is required for deglaciation of a "Snowball Earth," the "exit" CO, levels for an open water solution could be significantly less. In this paper we utilize a coupled climate/ice sheet model to demonstrate four poi nts: (1) the open water solution can be simulated in the coupled model if t he sea ice parameter is adjusted slightly; (2) a major reduction in ice vol ume from the open water/equatorial ice solution occurs at a CO, level of ab out 4X present values about two orders of magnitude less than required for exit from the "hard" snowball initial state; (3) additional CO2 increases a re required to get fuller meltback of the ice; and (4) the open water solut ion exhibits hysteresis properties, such that climates with the same level of CO2 may evolve into either the snowball, open water, or a warmer world s olution, with the trajectory depending on initial conditions. These results set useful targets for geochemical calculations of CO2 changes associated with the open-water solution.