Milankovitch cycles - Definition and Overview

This article or section should be merged with Orbital forcing

Milankovitch cycles is the name given to the collective effect of changes in the Earth's movements upon its climate. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of the Quaternary glaciation over the last few million years. The Earth's axis completes one full cycle of precession approximately every 26,000 years. At the same time the elliptical orbit rotates, more slowly, leading to a 22,000 years cycle in the equinoxes. In addition, the Earth's tilt relative to the Sun changes between 21.5 degrees to 24.5 degrees and back again on a 41,000 year cycle. The Earth's axis today is tilted 23.5 degrees relative to the normal to the plane of the ecliptic.

The Milankovitch Theory of Climate Change is not perfectly worked out: in particular the largest response is at the 100,000 year timescale but the forcing is apparently small at this scale - see Ice age for more discussion. Various feedbacks (from CO₂, or from ice sheet dynamics) are invoked to explain this discrepancy.

Milankovitch-like theories were advanced by Joseph Adhemar, James Croll, Milutin Milankovic and others, but verification was difficult due to the absence of reliably dated evidence and doubts as to exactly which periods were important. Not till the advent of deep-ocean cores, and the seminal paper by Hayes, Imbrie and Shackleton "Variations in the earths orbit: pacemaker of the ice ages" in Science, 1976, did the theory attain its present state.

The above image shows the change in the Earth's axis during one 41,000-year cycle of axial tilt. The duration of the precession cycle is about half the duration of the axial tilt cycle, resulting in the spiral trace shown in green.

See also

Milutin Milankovic, James Croll, Ice age

External links

1. http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm - "Milankovitch Cycles and Glaciation"
2. http://www.agu.org/revgeophys/overpe00/node6.html
3. http://muller.lbl.gov/pages/IceAgeBook/IceAgeTheories.html - some history of the adoption of the Milankovitch hypothesis (and an alternative)
4. http://muller.lbl.gov/papers/sciencespectra.htm - more detail on orbital obliquity also matching climate patterns
5. http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html - Graph of variation in insolation. Note 20,000 year, 100,000 year, and 400,000 year cycles are clearly visible