Factors in Long-Term Evolution of Ice Sheets
			q The long-term evolution of ice sheets reflects the interaction of two factors: changes in summer insolation that drive shorter-term changes in ice sheet mass balance and a much more gradual global cooling represented by a slowly changing glaciation threshold. q Ice sheets accumulate when summer insolation falls below a critical glaciation threshold and melt when it rises above it.
		Due to CO2 changes, for example.