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8.2: When climate physicists discovered the ice-albedo instability through energy-balance calculations in the 1960’s, they assumed that the ice-covered planet would be stable in perpetuity on account of its high albedo. Kirschvink (1992), building on concepts developed by the planetary atmosphere chemist James C.G. Walker and associates in the 1980’s, proposed that a “snowball earth” (his name) was self-reversing because of silicate-weathering feedback in the tectonically-driven carbon cycle.

8.3: Modeled increase in Solar luminosity over geologic time (Endahl & Sofia, 1981) and the compensatory decrease in atmospheric CO₂ partial pressure (assuming negligible methane) required if surface temperatures were quasi-stable over billions of years as is observed geologically.

8.4:The geochemical carbon cycle showing major sources and sinks of carbon dioxide to the ocean-atmosphere system. Silicate weathering feedback adjusts atmospheric CO₂ level to balance CO₂ sources and sinks. Plate tectonics recycles sedimentary carbon (carbonate and organic matter) as volcanic and metamorphic CO₂.

8.5: Volcanic eruption on Iceland during winter.

8.6: Ice-line latitude on an all-ocean planet as a function of the Solar flux (the observed interannual variability of which is <0.1%) and the equivalent CO₂ radiative forcing (the “greenhouse” effect), according to simple energy-balance calculations of the Budyko-Sellers type. If radiative forcing declined, ice lines follow the solid green line until they reach ~30Å latitude, after which ice-albedo feedback is self-sustaining and ice lines move rapidly (<103 years) to the equator (snowball earth). After millions of years, dependent on the magnitude of the CO₂ hysteresis loop, normal volcanic outgassing combined with reduced silicate weathering causes CO₂ to reach the critical level for deglaciation. Meltdown occurs rapidly (<104, driven by reverse ice-albedo and other feedbacks, resulting in an ice-free state with greatly elevated CO₂, which takes 10⁵ to 10⁷ years to draw down through silicate weathering of the glaciated landscape.

8.7: The geochemical carbon cycle on a snowball Earth. Volcanic and metamorphic CO₂ sources continue unaffected, but removal of CO₂ from the atmosphere is limited by the absence of rainfall. Silicate weathering is reduced by ice cover and cold ground temperatures.

8.8: Tectonically-driven carbon cycle on a snowball earth, where volcanism continues to emit CO₂ into the atmosphere and ocean, but CO₂ consumption is limited by the absence of rainfall and by cold-base ice.

8.9: Hypothetical depiction of the Sturtian snowball earth scenario in terms of global mean surface temperature (Pierrehumbert, 2002) and ice cover (pale blue) on a 750-Ma paleogeography (Powell et al., 2001). Note abrupt onset and termination of glaciation at low-latitudes, and the hot aftermath due to CO₂. Temperature rise during snowball interval is non-linear due to non-linear CO₂ radiative forcing.