The hypothesis that the basic cause of long-term climatic change, and particularly of the ice ages, lies in quasi-periodic variations in the Earth's orbit around the Sun. The idea was originally proposed by Sir John Herschel in 1830, extended by J. A. Adhémar and James Croll, and revived by Milankovich in 1920. Current analysis suggests that astronomical factors alone are insufficient to account for the major ice ages, but that they probably have a forcing effect, being reinforced by other mechanisms, notably carbon-dioxide concentration.
There are three principal effects: (a) Variation in the eccentricity of the Earth's orbit (between c.0.005 and 0.05), with a primary period of c.100 000 years and secondary periods of c.400 000 and c.125 000 years. This causes a significant variation in the amount of radiation incident on the Earth. (b) Alteration of the inclination of the Earth's axis (known as the obliquity) between 21.6° and 24.4°. This varies with a period of c.41 000 years, and periods of minimum inclination cause a reduction in the average intensity of radiation incident at high latitudes. (c) Variation in the orientation of the Earth's axis in space that causes the axis to sweep out a cone (precession of the equinoxes). This causes a change in the timing of the seasons relative to aphelion and perihelion. When the (northern) winter solstice and aphelion occur together, for example, insolation is then at an absolute minimum in the northern hemisphere. Precession has two periodicities of c.19 000 and c.23 000 years, which combine to give an overall period of c.22 000 years.
Subjects: Meteorology and Climatology.