Climate forcings and the initiation of low-latitude ice sheets during the Neoproterozoic Varanger glacial interval

The GISS GCM was used to determine if a diverse set of climate forcings, al one or in combination, could have initiated the low-latitude ice sheets of the Varanger (similar to 600 Ma) glacial interval. The simulations use a re alistic reconstruction of the paleocontinental distribution and test the fo llowing forcings, alone and in combination: 6% solar luminosity decrease, f our atmospheric CO2 scenarios (1260, 315, 140, and 40 ppm), a 50% increase and a 50% decrease in ocean heat transports, and a change in obliquity to 6 0 degrees. None of the forcings, individually, produced year-round snow acc umulation on low-latitude continents, although the solar insolation decreas e and 40 ppm CO2 scenarios allowed snow and ice to accumulate at high and m iddle latitudes. Combining forcings further cools the climate: when solar l uminosity, CO2, and ocean heat transports were all decreased, annual mean f reezing and snow accumulation extended across tropical continents. No simul ation would have initiated low-latitude glaciation without contemporaneous glaciation at higher latitudes, a finding that matches the distribution of glacial deposits but which argues against high obliquity as a cause of the Varanger ice age. Lour-level clouds increased in most scenarios, as did sur face albedo, while atmospheric water vapor amounts declined; all are positi ve feedbacks that drive temperatures lower. In the most severe scenario, gl obal snow and ice cover increased to 68%, compared to 12% under modern cond itions, and water vapor dropped by 90%. These results do not necessarily pr eclude a "snowball" Earth climate scenario for the Varanger glacial interva l. However, either more severe forcings existed or radical changes occurred in the ocean/atmosphere system which are unaccounted for by the GCM. Also, as sea ice extent increased in these experiments, snow accumulation began to decline, because of an increasingly dry atmosphere. Under snowball Earth conditions, glaciation would be impossible, since the hydrological cycle w ould all but cease if the atmosphere's primary moisture source were cut off .