Final answer:
The equilibrium line in the northern hemisphere ice sheet model shifts north due to colder winters, increased snowfall, and higher albedo from snow cover reflecting sunlight, as influenced by Milankovich cycles. Lowering CO2 and changes in ocean circulation are also contributing factors.
Step-by-step explanation:
The condition that drives the equilibrium line north in the two-dimensional numerical model of northern hemisphere ice sheet behavior is primarily linked to climate variations driven by Milankovich cycles. These cycles involve changes in the Earth's orbit and axis tilt, leading to different climatic conditions over long time periods. As mentioned, resulting factors include colder winters, increased snowfall feeding the glaciers, and high albedo due to the snow cover reflecting sunlight, causing a decrease in warming after winter. This delicate balance can be upset by other factors such as a global cooling driven by lowering CO2 levels or changes in ocean circulation, contributing to the growth of ice sheets.
Furthermore, this understanding is supported by research, such as the findings from Cook, C., van de Flierdt, T., Williams, T. et al. in their study on the dynamic behavior of the East Antarctic ice sheet during Pliocene warmth. The model incorporates feedback mechanisms, where a radiative imbalance leads to surface temperature changes until the earth re-establishes a new equilibrium. Important thermal masses impact this, most notably the oceans and the ice sheets, the latter being crucial in this context.