During the lead-up to a snowball Earth episode, the Earth gradually cools because the amount of carbon dioxide in the Earth's atmosphere decreases. Relatively fast weathering of silicate rocks on large tropical landmasses causes this decrease that locks up carbon (NASA, 2001). As the earth cools, the oceans begin to freeze. As snow falls on land, or ice forms on the oceans, the albedo increases (more radiation is reflected from the surface) causing greater cooling (Maykut, 1998). This is called ice-albedo feedback, and is an important dynamic in yearly retreat and advance of ice sheets (Hoffman, 1999).
Mikhail Budyko's model shows that if the Earth's climate were to cool, and ice were to form at lower latitudes, the global albedo would rise at an accelerating rate because there is more surface area per degree of latitude as one approaches the Equator (Hoffman, 1999). Furthermore, once ice is formed beyond a critical latitude (around 30 degrees north and south, equivalent to half the Earth's surface area), the positive feedback becomes so strong that temperatures of the surface plummet (- 50 o C), yielding a completely frozen planet (Hoffman, 1999).