Milankovitch’s theory: changes in the Earth’s orbit and tilt alter the amount of energy reaching the Earth. When there is less energy reaching Earth during the Northern Hemispheres' Summer ice will last longer increasing the Albedo effect. This is because most of the Earth land in is in the Northern Hemisphere.
Eccentricity This is the change in the shape of the earth's orbit around the sun. Over time the Earth will be closer to, or further away from the Sun altering the amount of energy we receive. Occurs over 97,000 years.