Astronomical cycles

Ice caps have come and gone over the Earth’s surface for many millions of years. What has caused the cyclical warming and cooling of our planet.

In the 1920s and 1930s, Milutin Milankovitch, a geophysicist, showed that the amount of heat received from the sun varied, and that this was related to cyclical changes in the Earth’s orbit. Further, this variation was linked to whether the Earth was in a glacial (colder) or interglacial (warmer) period.

The changes in Earth’s orbit are due to changes in Earth’s eccentricity, axial tilt and precession:

• Eccentricity refers to whether the Earth’s orbit is more or less circular – it changes between the two over a period of 100,000 years.
• Earth does not spin absolutely upright, but is tilted on its axis – we call this axial tilt. The degree of tilt changes over about 41,000 years.
• Precession means that the Earth wobbles on its axis, over a period of about 23,000 years.

Together, these three cycles affect the amount of solar radiation reaching the Earth, influencing the climate and bringing about the advance and retreat of the polar icecaps.

How does this work?

It happens when northern hemisphere summers are coolest (the Earth is farthest from the Sun due to precession and eccentricity) and winters are warmest (as a result of minimum axial tilt). This means that snow can accumulate across large areas of the northern land masses.

Snow has a lower albedo than vegetation and bare soil, so it reflects much more solar radiation back into space. This cools the climate, so that more snow will fall and also stay on the ground for longer. This is a positive feedback loop that results in growth of glaciers and icecaps in both the northern and southern hemispheres – and potentially the onset of a global glacial period.