Snowball Earth
Kirschvink responded to the positive Elatina fold test with a theory he first aired in 1989 (Maugh, 1989). It eventually appeared as an article, seven paragraphs long, embedded in a 1348-page book (Kirschvink, 1992). It postulated that conditions amenable to global glaciation were set up by an unusual preponderance of land masses within middle to low latitudes, a situation that has not been encountered at any subsequent time in Earth history. One effect of such a paleogeography would be a substantially higher albedo in the sub-tropics, where clouds are least important (Kirschvink, 1992). Any resultant glaciation would further increase the Earth's albedo by lowering sea level, exposing continental shelves and inland seas (Kirschvink, 1992). Placing more continents in the tropics would also increase the silicate weathering rate (provided tectonic uplift was not mysteriously absent there), leading to a colder planet with lower atmospheric pCO2 (Marshall et al., 1988; Worsley and Kidder, 1991). In addition, the meridional heat transport by the Hadley cells would be weakened because tropical air masses would be drier on average with greater continentality. Oceanic heat transport might also be lessened, but this is less certain. These combined effects might lead to the growth of large ice caps, nucleated on islands or continents bordering the polar seas.

Ice caps create a positive feedback on climate change through ice-albedo feedback (Croll, 1867; Budyko, 1966; Manabe and Broccoli, 1985). Early energy-balance climate models revealed a fundamental instability in the climate system caused by this positive feedback (Eriksson, 1968; Budyko, 1969; Sellers, 1969; North et al., 1981), an effect reproduced in a number of GCMs (Wetherald and Manabe, 1975; Jenkins and Smith, 1999; Hyde et al., 2000; Pollard and Kasting, 2001). If more than about half the Earth's surface area were to become ice covered, the albedo feedback would be unstoppable (Fig. 6). Surface temperatures would plummet (Fig. 7), and pack ice would quickly envelope the tropical oceans (Hyde et al., 2000; Baum and Crowley, 2001; Warren et al., 2002). With reduced Neoproterozoic solar forcing, the ice-albedo instability occurs in the GENESIS v2 GCM between 1.0 and 2.5 times modern pCO2 with different paleogeographies (Baum and Crowley, 2001; Pollard and Kasting, 2001). The time scale of the sea-ice advance is uncertain—the ocean's thermal inertia resists the ice advance initially (Poulsen et al., 2001), but the ocean is a finite heat reservoir and cooling over thousands of years would leave it powerless to resist the ice encroachment. It is also uncertain if the tropical ocean would ever become entirely ice covered (Hyde et al., 2000; Baum and Crowley, 2001)—Kirschvink (1992) speculated that areas of open water (polynyas) would remain, tracking the zone of highest solar incidence back and forth across the equator and imparting a strongly seasonal climate even at low latitude, consistent with geological observations (Williams and Tonkin, 1985). This is distinct from the tropical "loophole" model (Hyde et al., 2000; Baum and Crowley, 2001; Crowley et al., 2001), in which the ice fronts miraculously approach but never cross the ice-albedo instability threshold [but the continents are glaciated because they are mostly placed in middle and high latitudes, contrary to paleomagnetic evidence (Evans, 2000)].

Assuming an albedo runaway did occur, the climate would be dominated by the dry atmosphere and the low heat capacity of the solid surface (Walker, 2001). It would be more like Mars (Leovy, 2001) than Earth as we know it, except that the greater atmospheric pressure would allow surface meltwater to exist. Diurnal and seasonal temperature oscillations would be strongly amplified at all latitudes because of weak lateral heat transfer and extreme "continentality" (Walker, 2001). Despite mean annual temperatures well below freezing everywhere, afternoon temperatures in the summer hemisphere would reach the melting point (Walker, 2001). Evaporation of transient melt water would contribute, along with sublimation, to maintain low levels of atmospheric water vapor, and glaciers would feed on daily updrafts of this moisture (Walker, 2001). The global mean thickness of sea ice depends strongly on sea-ice albedo (~1.4 km for albedo 0.6) and meridional variability is a complex function of solar incidence, greenhouse forcing (see below), zonal albedo, ablation or precipitation, and equatorward flowage of warm basal ice (Warren et al., 2002).

Climate physicists originally assumed that no ice-albedo catastrophe ever actually occurred because it would be permanent: the high planetary albedo would be irreversible. A saviour exists, however, and Kirschvink (1992) identified it as the buildup of an intense atmospheric CO2 greenhouse through the action of plate tectonics in driving the long-term carbon cycle (Walker et al., 1981; Kirschvink, 1992; Caldeira and Kasting, 1992). On a snowball Earth, volcanoes would continue to pump CO2 into the atmosphere (and ocean), but the sinks for CO2—silicate weathering and photosynthesis—would be largely eliminated (Kirschvink, 1992). Even if dry ice condensed at the poles in winter, it would likely sublimate away again in summer (Walker, 2001). CO2 levels would inexorably rise and surface temperatures would follow, most rapidly at first and more slowly later on (Fig. 7) due to the non-linear relation between CO2 concentration and the resultant greenhouse forcing (Caldeira and Kasting, 1992). With rising surface temperatures, sea ice thins but ground ice sheets expand in some areas due to a stronger hydrologic cycle. If CO2 outgassing rates were broadly similar to today (a reasonable assumption for 600-700 Ma), then the time needed to build up the estimated 0.12 bar CO2 required to begin permanent melting at the equator, assuming a planetary albedo of 0.6, would be a few million years (Caldeira and Kasting, 1992; Crowley et al., 2001). This estimate (Fig. 7), while subject to large uncertainties, is of the same order as the estimated time-scale of LNGD from paleomagnetic (Sohl et al., 1999) and stratigraphic (Hoffman et al., 1998a) evidence. Once the tropical ocean begins to open up perennially, deglaciation proceeds rapidly due to reverse ice-albedo feedback (Caldeira and Kasting, 1992; Crowley et al., 2001). The fall in planetary albedo occurs far faster than the excess atmospheric CO2 can be consumed by silicate weathering, with the result that a transient heat wave must follow the ice retreat (Kirschvink, 1992; Caldeira and Kasting, 1992). Kirschvink (1992) concluded that a cold planet with many tropical continents and large polar sea-ice caps "would be a rather unstable situation, with the potential for fluctuating rapidly between the 'ice house' and 'greenhouse' states".