SEASONAL SNOWLINE INSTABILITY IN A CLIMATE MODEL WITH REALISTIC GEOGRAPHY - APPLICATION TO CARBONIFEROUS (APPROXIMATELY-300 MA) GLACIATION

Instabilities in the climate system are possible mechanisms for shifting the climate from one stable mode to another. One such instability involves the non-linearity associated with snow-albedo feedback. Previous studies have demonstrated the existence of a snowline instability in zero- and one-dimensional energy balance models (EBMs) as well as two-dimensional seasonal EBMs with idealized geography. In this paper we identify a snowline instability that has been found in a two-dimensional EBM utilizing a realistic geographic reconstruction of Gondwanaland for the Carboniferous (approximately 305 Ma), at the time of a major glacial period in Earth history. Several experiments were conducted, all with solar luminosity reduced 3% to reflect lower solar output. A baseline simulation for a "hot summer orbit" configuration yielded a South Polar temperature of 31-degrees-C. As the orbit was changed to a "cold summer orbit" and luminosity was gradually decreased, ice area linearly increased from 0.0 to 7.5 x 10(6) km2 (equivalent to about two-thirds the size of the present East Antarctic Ice Sheet). Additional small decreases in luminosity for the cold summer orbit configuration resulted in an abrupt increase in summer ice area to 26.0 x 10(6) km2. The transition occurs due to a perturbation of only 0.002 W/m2 - a perturbation much smaller than those due to Milankovitch variations or volcanic eruptions. The calculated area of expanded ice cover compares favorably with the distribution of glacial deposits in the Carboniferous. These results provide further support for the existence of the snowline instability mechanism and suggest a possible explanation for both initiation of glaciation and glacial oscillations during the Carboniferous.