The Milankovitch Theory

The Milankovitch theory is an explanation of long term climate change.  Milankovitch built his theory from previous work done by J.A. Adhemar and James Croll.  In 1842 Adhemar explained glacial climate using only precession (Davis, 2002).  In 1864 Croll wrote about orbital change and the ice of the Ice Ages using both the eccentricity cycle and the precession cycle.  Later in 1875 he took into account obliquity (tilt of the axis) cycle (Davis, 2002).
Milankovitch, being a mathematician, took Croll's work and set out to develop a mathematical theory of climate change.  His theory states "that as the Earth travels through space around the sun cyclical variations in three elements of Earth-sun geometry combine to produce variations in the amount of solar energy that reaches Earth (Kaufman, 2002).  These three elements that have cyclic variations are eccentricity, obliquity, and precession.
Eccentricity is a term used to describe the shape of Earth's orbit around the sun.  The variation of Earth's orbit around the sun ranges from an almost exact circle (eccentricity = 0.0005) to a slightly elongated shape (eccentricity = 0.0607) (Thomas, 2002).  The time frame for the cycle is approximately 98,000 years (Davis, 2002).  The impact of the variation is a change in the amount of solar energy from perihelion (around January 3) to aphelion (around July 4).  Currently the Earth's eccentricity is 0.016 and there is about a 6.4 percent increase in insolation from July to January (Berger, 2001).  Thomas (2002) states,
"The eccentricity influences seasonal differences:  when the Earth is closest to the
sun, it gets more solar radiation.  If the occurs during the winter, the winter is less
severe.  If a hemisphere has its summer while closest to the sun, summers are
relatively warm."

 Diagrams depicting variation in eccentricity.  Images by Robert Simmon, NASA GSFC. Taken from http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch.html.

Obliquity is the variation of the tilt of the earth's axis away from the orbital plane.  The tilt varies between 22.1o and 24.5o and the average is 23.5o (Kaufman, 2002).  The obliquity changes on a cycle taking approximately 40,000 years.  As this tilt changes, the seasons become more exaggerated.  Kaufman (2002) states "the more tilt means more severe seasons - warmer summers and colder winters; less tilt means less severe seasons - cooler summers and milder winters."  For an increase of 1o in obliquity, the total energy received by the summer hemisphere increases by approximately 1% (Berger, 2001).  The cool summers are thought to allow for the yearly build up of snow and ice in high latitudes, possibly leading to the development of an ice sheet (Kaufman, 2002).  Obliquity change alos causes equatorward motion of the tropical circles and the poleward motion of the polar circles.  Currently this motion is 1.4 km/century as the obliquity decreases -46.85 inches/century (Berger, 2001).

 Diagram depicting variation in obliquity.  Image by Robert Simmon, NASA GSFC.  Taken from http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch.html.

Precession is the change in orientation of the Earth's rotational axis.  The precession cycle takes about 19,000 - 23,000 years.  Precession is caused by two factors:  a wobble of the Earth's axis and a turning around of the elliptical orbit of the Earth itself (Thomas, 2002).  Obliquity affected the tilt of the Earth's axis, precession affects the direction of the Earth's axis.  The change in the axis location changes the dates of perihelion (closest distance from sun) and aphelion (farthest distance from sun), and this increases the seasonal contrast in one hemisphere while decreasing it in the other hemisphere ( Kaufman, 2002).  currently, the Earth is closest to the sun in the northern hemisphere winter, which makes the winters there less severe (Thomas, 2002).  Another consequence of precession is a shift in the celestial poles.  5000 years ago the North Star was Thuban in the constellation Draco.  Currently the North Star is Polaris in the constellation Ursa Minor.

 Diagram depicting variation in precession. Image by Robert Simmon, NASA GSFC.  Taken from http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch.html.

"Using these three orbital variations, Milankovitch was able to formulate a comprehensive mathematical model that calculated latitudinal differences in insolation and the corresponding surface temperature for 600,000 years prior to the year 1800"(Kaufman, 2002).  These variations are correlated in the figure below.

 Graph showing the calculated values for 300,000 years of orbital variation by Berger and Loutre, 1991.  Taken from http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch.html.

His theory concludes that ice caps at the poles increased and decreased in size as a reflection of the insolation at high latitudes.  Specifically, he used calculations from the 65°N to develop his model.
During the 1940s and 1950s the theory fell into disrepute due to radiocarbon dating indicating a lag in cooling versus insolation and to a scale problem with high frequency glacial advances (Davis, 2002).  The theory was revived several times throughout the late 60s to present.  Look to the recent developments section for more information.