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.
Earth's movements
As the Earth spins around its axis and orbits around the Sun, several quasi-periodic variations occur. Although the curves have a large number of sinusiodal components, a few components are dominant. Milankovitch studied changes in the eccentricity, obliquity, and precession of Earth's movements. Such changes in movement and orientation change the amount and location of solar radiation reaching the Earth. Changes near the north polar area are considered important due to the large amount of land, which reacts to such changes more quickly than the oceans do.
Orbital shape
The eccentricity, or shape of the Earth's orbit, varies from being nearly circular (low eccentricity of 0.005) to being mildly elliptical (high eccentricity of 0.058) and has a mean eccentricity of 0.028. The major component of these variations occurs on a period of 413,000 years (eccentricity variation of ±0.012). A number of other terms vary between 95,000 and 136,000 years, and loosely combine into a 100,000 year cycle (variation of -0.03 to +0.02). The present eccentricity is 0.017.
Currently the difference between closest approach to the Sun (perihelion) and furthest distance (aphelion) is only 3% (5 million km). This difference amounts to about a 6% increase in incoming solar radiation. Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is highly elliptical, the amount of solar radiation at perihelion would be about 25% greater than at aphelion.
Axial tilt
The Earth's spin wobbles, causing a slow 2.4° change in the tilt of the axis (obliquity). This precession of the axis follows a cycle of approximately 40,000 years. When the tilt increases to 24.5 degrees, the winters become colder and summers are warmer than at 22.1 degrees, when with less tilt the winters are milder and summers are cooler.
Presently the Earth is tilted at 23.5 degrees from its orbital plane.
Cooler summers are suspected of encouraging the start of an ice age due to their melting less of the previous winter's ice and snow.
Axial orientation
Precession of the equinoxes is the change in the direction of the Earth's axis of rotation relative to the Sun at the time of perihelion and aphelion.
When the axis is aligned so it points toward the Sun during perihelion, one polar hemisphere will have a greater difference between the seasons while the other hemisphere will have milder seasons. The hemisphere which is in summer at perihelion will receive much of the corresponding increase in solar radiation, but that same hemisphere will be in winter at aphelion and have a colder winter. The other hemisphere will have a relatively warmer winter and cooler summer.
When the Earth's axis instead points toward the Sun during spring and autumn, the northern and southern hemispheres will have similar contrasts in the seasons.
At present southern summer occurs during perihelion and its winter during aphelion. Thus the Southern Hemisphere seasons should tend to be somewhat more extreme than the Northern Hemisphere seasons.
Orbital inclination
The inclination of Earth's orbit drifts up and down relative to its present orbit with a cycle having a period of about 70,000 years. Milankovitch did not study this three-dimensional movement.
More recent researchers noted this drift and that the orbit also moves relative to the orbits of the other planets. The invariable plane, the plane that represents the angular momentum of the solar system, is approximately the orbital plane of Jupiter. The inclination of the Earth's orbit has a 100,000 year cycle relative to the invariable plane. This 100,000 cycle closely matches the 100,000 pattern of ice ages.
It has been proposed that a disk of dust and other debris is in the invariable plane, and this affects the Earth's climate through several possible means. The Earth presently moves through this plane around January 9 and July 9, when there is an increase in radar-detected meteors and meteor-related mesospheric clouds.Milankovitch Cycles and Glaciation
