The Milankovitch Cycles

Earth's orbit does not remain fixed; it varies over time. Major contributing factors to ice ages are these natural variations that occur between the Earth-Sun relationship. There are three natural cycles that have an effect on Earth's climate. The cycles have to do with Earth's axial tilt, its elliptical orbit around the Sun, and its "wobble," or precession (the direction the North Pole points).

These orbital parameters have cyclic occurrences (periodicities) that range from 22,000 to 100,000 years. Geologically speaking, these time frames are relatively short. Because of this, these cycles have more of an influence on the advance and retreat of ice sheets and glaciers during an ice age than they do in the timing of an ice age (which can be separated by millions of years).

These three natural variations in Earth's orbit are commonly referred to as the Milankovitch cycles, named after Milutin Milankov-itch, a Serbian astrophysicist, in 1920. His theory states that variations in Earth's orbit through time cause changes in the amount and intensity of incoming solar radiation (insolation) that reaches Earth's surface. These differences in insolation directly affect the behavior of ice sheets and glaciers. Depending on where the cycle's periodicities are in relation to each other, this partly determines the behavior of the world's ice sheets, glaciers, and climate.

Earth's axis is tilted, a characteristic it has had since it formed. Some scientists believe Earth was struck by a massive object in space while it was being formed, knocking it off the perpendicular; others speculate the

Thousands of Years Ago

O Infobase Publishing

Thousands of Years Ago

O Infobase Publishing

The variation in the tilt of the Earth's axis over the past 750,000 years: The red line represents the tilt in degrees, which varies between 22 and 24.5. The gold line represents today's value for comparison.

Thousands of Years Ago

G Infobase Publishing

Thousands of Years Ago

G Infobase Publishing

The graph shows the variation in the eccentricity of the Earth's orbit over the past 750,000 years, from circular (0.0) to elliptical (0.05). The gold line represents today's value for comparison.

tilt may have something to do with past ice ages and the distribution and weight of massive ice sheets. It is the tilt that gives Earth its seasons. Currently that tilt is approximately 23.5 degrees. During Earth's history, the tilt has varied from 21.6 to 24.5 degrees. Over the past 750,000 years, Earth's tilt has changed on a 41,000-year cycle, as shown in the figure above.

When Earth's tilt changes, the seasonal distribution of insolation at the higher (polar) latitudes and the length of long, cold winter periods at the poles also change. Changes in tilt do not have much effect on the equatorial regions. The higher the degree of tilt, the more pronounced are the seasons. The more extreme the winters are at the poles (longer and colder), the more likely ice sheets are to grow.

Earth's eccentricity is the second cycle that influences insolation. Earth's orbit around the Sun is not a perfect circle; instead, it is an ellipse, which varies from 1 to 5 percent. With a periodicity rate of approximately 100,000 years, it affects the amount of radiation Earth's surface receives at aphelion and perihelion. Aphelion is the point on its orbit where Earth is farthest from the Sun; perihelion is the point where it is the closest. This cycle changes the seasonal contrasts in the Northern and Southern Hemispheres. When the orbit is very elliptical (oval shaped), one hemisphere will have hot summers and cold winters and the other hemisphere will have warm summers and cool winters. When the orbit is more circular, both hemispheres will have similar seasons.

Precession, in the third cycle, has a periodicity of about 22,000 years. Twice each year the Sun is positioned exactly over the equa-

Thousands of Years Ago

© Infobase Publishing

Thousands of Years Ago

© Infobase Publishing

The precession of the equinox over the past 750,000 years: The "longitude of the perihelion" is the precession from the vernal (spring) equinox.

tor—on the two days referred to as the "equinoxes" (one in the spring and one in the fall). Presently, the equinoxes are on March 21 and September 21, but they will not always be because Earth's axis of rotation wobbles like a top does when it begins to wind down. As it "wobbles," the timing of the equinoxes changes. When the equinoxes change, so do the aphelion and perihelion. This affects Earth's climate because it affects the seasonal balance of insolation.

Currently, perihelion is in January for the Northern Hemisphere winter, which makes winters milder. During its 22,000-year cycle, the precession can cause significant changes in Earth's climate and in the growth and recession of ice sheets.

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