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.
Eccentricity half
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. This shows eccentricity half.
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.
Eccentricity zero
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. This shows eccentricity zero.
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.
Axial tilt
Earth does not spin absolutely upright, but is tilted on its axis – this is called axial tilt. The degree of tilt changes over about 41,000 years.
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.
Natural climate cycles versus anthropogenic climate change
The cyclical warming and cooling of our planet happens over millennia. This differs from anthropogenic climate change – the rapid, human-caused warming of the planet, primarily due to greenhouse gas emissions.
Nature of science
Science is based on and derived from observations of the world around us from which interpretations are made. Scientists depend on empirical evidence to produce scientific knowledge. Any scientific explanation must be consistent with empirical evidence, and new evidence brings the revision of scientific knowledge.
Useful links
Find out more about Milutin Milankovitch in this series of articles on the Earth Observatory website.
Unlocking the Ice House lesson plan.
