The Milankovitch theory provides an explanation for the growth of the Northern Hemisphere ice sheet. It proposes that ice growth in occurs during times when summer insolation is low in the northern hemisphere high latitudes. Low summer insolation occurs when Earth's orbital tilt is small and the northern hemisphere is in the aphelion position. Ice sheet melting happens when Earth's orbital tilt is large and the northern hemisphere is in the perihelion position. The boundary between the region of positive ice mass balance and negative ice mass balance is the equilibrium line .
Ice sheets contribute to their own growth.
By growing upwards, the ice sheet increases the area of positive ice mass balance. This positive feedback is known as the ice elevation feedback . Ice sheets do not grow in phase with insolation, but exhibit a phase lag . An ice sheet does not pop up suddenly, but requires 1000s of years to accumulate. Thus, the maximum ice volume will occur after the minimum summer insolation.