Milankovitch cycles are long term cycles during which the amount of solar radiation received by the earth changes as a result of variations to the earth's orbit. The timing of the Milankovitch cycles have been correlated with ice ages, historic periods of cooling during which large parts of the Northern hemisphere were covered in ice.
The Milankovitch cycles are named after the Serbian mathematician that developed the theory. The theory explains periodic variations in solar radiation received by the earth as a result of changes in the velocity and shape of earth's orbit around the sun. There are three key variables: (i) eccentricity of the earth's orbit; (ii) changes in the inclination of the earth's angle to the sun; and (iii) the precession of the equinoxes. The timing of Milankovitch cycles has been correlated to ice ages - cooler periods of the earth's history - through studying ocean and ice cores. Each of the cycles is explained in more detail below.
The shape the earth's orbit around the sun changes from circular to elliptic. At it's maximum, the difference in solar energy received by the earth between the perihelion (point at which the earth is closest to the sun) and aphelion (point at which the earth is furthest from the sun) is 30%, causing changes in the contrast between seasons. This cycle occurs every 98,000 years.
The tilt of the earth's axis varies from 21.39 degrees to 24.36 degrees, with a periodicity of 41,000 years. This causes significant changes to the level of solar radiation received at higher latitudes (i.e. the poles), with longer periods of darkness in polar regions during winter.
The timing of the perihelion and aphelion vary as a result a 'wobble' in the earth's axis as it rotates around the sun. This results in the timing of the solar equinoxes (the date on which the sun is directly overhead at the equator at noon and when night and day are equal in length) to change through the precession. During the Younger Dryas, c. 11,000 years ago, the earth was tilted towards the sun during midsummer, causing winters (pointed away from the sun) to be colder and longer than present, when the earth is tilted away from the sun during summer.
Milankovitch cycles have been correlated to changes in the chemical makeup of ocean cores, indicating a link between the timings of cooler periods with significant expansion of the earth's ice coverage (referred to as an ice age). Milankovitch cycles are thought to be the primary driving force behind the timing of ice ages during the Quarternary period (covering the last c. two million years). The interaction between the climate and Milankovitch cycles are complex. Changes in the earth's angle impacts mainly higher latitudes (the poles), whilst lower latitudes (equatorial regions) are impacted more by the precession of the equinoxes and eccentricity. Therefore, linking any historic climate changes to Milankovitch cycles relies on an understanding of the phase of each cycle and the impacts of the corresponding changes in distribution of solar radiation on the ocean, land and atmospheric earth systems.
