Milankovitch dedicated his career to developing a mathematical theory of climate based on the seasonal and latitudinal variations of solar radiation received by the Earth. Now known as the Milankovitch Theory, it 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:

1. Variations in the Earth's orbital eccentricity—the shape of the orbit around the sun.

2. Changes in obliquity—changes in the angle that Earth's axis makes with the plane of Earth's orbit.

3. Precession—the change in the direction of the Earth's axis of rotation, i.e., the axis of rotation behaves like the spin axis of a top that is winding down; hence it traces a circle on the celestial sphere over a period of time.

Together, the periods of these orbital motions have become known as Milankovitch cycles.

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On the Shoulders of Giants
Milutin Milankovitch
Orbital Variations
Milankovitch Theory
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Images
Top: A portrait of Milutin Milankovitch. (Drawing by Hailey King)

Orbital Variations
Changes in orbital eccentricity affect the Earth-sun distance. Currently, a difference of only 3 percent (5 million kilometers) exists between closest approach (perihelion), which occurs on or about January 3, and furthest departure (aphelion), which occurs on or about July 4. This difference in distance amounts to about a 6 percent increase in incoming solar radiation (insolation) from July to January. The shape of the Earth�s orbit changes from being elliptical (high eccentricity) to being nearly circular (low eccentricity) in a cycle that takes between 90,000 and 100,000 years. When the orbit is highly elliptical, the amount of insolation received at perihelion would be on the order of 20 to 30 percent greater than at aphelion, resulting in a substantially different climate from what we experience today.

Obliquity (change in axial tilt)
As the axial tilt increases, the seasonal contrast increases so that winters are colder and summers are warmer in both hemispheres. Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. Because this tilt changes, the seasons as we know them can become exaggerated. More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters. It's the cool summers that are thought to allow snow and ice to last from year-to-year in high latitudes, eventually building up into massive ice sheets. There are positive feedbacks in the climate system as well, because an Earth covered with more snow reflects more of the sun's energy into space, causing additional cooling.
 

On the Shoulders of Giants
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Left: The eccentricity of the Earth's orbit changes slowly over time from nearly zero to 0.07. As the orbit gets more eccentric (oval) the difference between the distance from the Sun to the Earth at perihelion (closest approach) and aphelion (furthest away) becomes greater and greater. Note that the Sun is not at the center of the Earth's orbital ellipse, rather it is at one of focal points.

Note: The eccentricty of the orbit shown in the lower image is a highly exaggerated 0.5. Even the maximum eccentricity of the Earth's orbit—0.07—it would be impossible to show at the resolution of a web page. Even so, at the current eccentricity of .017, the Earth is 5 million kilometers closer to Sun at perihelion than at aphelion. (Images by Robert Simmon, NASA GSFC)

Precession
Changes in axial precession alter the dates of perihelion and aphelion, and therefore increase the seasonal contrast in one hemisphere and decrease the seasonal contrast in the other hemisphere.
 

Left: The change in the tilt of the Earth's axis (obliquity) effects the magnitude of seasonal change. At higher tilts the seasons are more extreme, and at lower tilts they are milder. The current axial tilt is 23.5°. Image by Robert Simmon, NASA GSFC)

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Left: Precession—the change in orientation of the Earth's rotational axis [this can be seen more clearly in an animation (small (290 kB QuickTime) or large (1.2 MB QuickTime))]—alters the orientation of the Earth with respect to perihelion and aphelion. If a hemisphere is pointed towards the sun at perihelion, that hemisphere will be pointing away at aphelion, and the difference in seasons will be more extreme. This seasonal effect is reversed for the opposite hemisphere. Currently, northern summer occurs near aphelion. (Image by Robert Simmon, NASA GSFC)

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Left: Over thousands of years the orientation of the Earth's rotational axis changes, like that of a spinning top as it slows down. (Animation by Robert Simmon, NASA GSFC)

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On the Shoulders of Giants
Milutin Milankovitch
Orbital Variations
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Left: Over thousands of years the orientation of the Earth's rotational axis changes, like that of a spinning top as it slows down. (Animation by Robert Simmon, NASA GSFC)

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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. He then attempted to correlate these changes with the growth and retreat of the Ice Ages. To do this, Milankovitch assumed that radiation changes in some latitudes and seasons are more important to ice sheet growth and decay than those in others. Then, at the suggestion of German Climatologist Vladimir Koppen, he chose summer insolation at 65 degrees North as the most important latitude and season to model, reasoning that great ice sheets grew near this latitude and that cooler summers might reduce summer snowmelt, leading to a positive annual snow budget and ice sheet growth.

But, for about 50 years, Milankovitch's theory was largely ignored. Then, in 1976, a study published in the journal Science examined deep-sea sediment cores and found that Milankovitch's theory did in fact correspond to periods of climate change (Hays et al. 1976). Specifically, the authors were able to extract the record of temperature change going back 450,000 years and found that major variations in climate were closely associated with changes in the geometry (eccentricity, obliquity, and precession) of Earth's orbit. Indeed, ice ages had occurred when the Earth was going through different stages of orbital variation.

Since this study, the National Research Council of the U.S. National Academy of Sciences has embraced the Milankovitch Cycle model.

...orbital variations remain the most thoroughly examined mechanism of climatic change on time scales of tens of thousands of years and are by far the clearest case of a direct effect of changing insolation on the lower atmosphere of Earth (National Research Council, 1982).

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On the Shoulders of Giants
Milutin Milankovitch
Orbital Variations
Milankovitch Theory
Links and References

Left: These graphs shpw calculated values for 300,000 years of orbital variation. The line labeled "0" represents today, while "-200" indicates 200,000 years in the past and "100" indicates 100,000 years from now. Milankovitch noticed that these cycles of orbital mechanics correspond to many indicators of past climate change, such as Ice Ages. (From Berger and Loutre, 1991)

Links and References

About.com: Milankovitch Cycles—Changes in Earth-sun Interaction

Alaska Science Forum: The Earth's Changing Orbit

J.D Hays, John Imbrie, and N.J. Shackleton, "Variations in the Earth's Orbit: Pacemaker of the Ice Ages," Science, 194, no. 4270 (1976), 1121-1132.

Hays, James D., 1996: Encyclopedia of Weather and Climate, Oxford University Press, Stephen H. Schneider, ed. pp 507-508.

Lutgens, Frederick K. and Edward J. Tarbuck, 1998: The Atmosphere, Prentice-Hall, Inc., 434pp.

National Research Council, Solar Variability, Weather, and Climate, Washington, D.C.: National Academy Press, 1982, p. 7.

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On the Shoulders of Giants
Milutin Milankovitch
Orbital Variations
Milankovitch Theory
Links and References