Stories from Physics for 11-14 14-16
Whilst it has been erroneously used by climate deniers to argue against anthropogenic climate change, the eccentricity of the Earth’s orbit does cause minor changes to the radiation incident on the Earth. However, orbital variation cannot account for the rapid and large-magnitude changes in global temperature linked to human activity.
In the 1920s, the Serbian geophysicist Milutin Milanković hypothesised that small changes in the eccentricity of the Earth’s orbit, its axial tilt and precession would cause cyclical changes to the intensity of solar radiation reaching the surface of the Earth.
First, Milanković noted that the eccentricity of the Earth’s orbit varies between 0.005 and 0.058 (it is currently 0.017) with the major component of the variation having a period of 404,000 years. This change in the shape of the Earth’s orbit arises because of the gravitational forces exerted by other bodies in the solar system, primarily Jupiter and Saturn. The changing eccentricity of the Earth’s orbit changes the lengths of the seasons and consequently the radiation incident of the surface of the planet. Currently, the Earth’s orbit is becoming less eccentric and so season length is equalising.
Second, the Earth precesses on its axis due to the torque exerted by the Sun and the Moon. The Earth’s axis of rotation currently aligns with the star Polaris. But around 3000 BC, the Egyptians observed that the night sky seemed to revolve around Alpha Draconis. The major component of the axial precession has a period of 25,700 years and currently we are roughly in the middle of a cycle of change between 21.8° and 24.4°. Changes to axial tilt alter the amount of solar radiation incident on the poles.
Third, the Earth’s elliptical orbit itself precesses in space relative to fixed stars with a period of 112,000 years. This precession changes the relative length of the seasons on Earth.