The Milankovitch Cycles

In the early 20th century, a Yugoslavian astronomer and mathematician provided an answer to what causes ice ages. Milutin Milankovitch recognized that minor changes in Earth’s orbit around the sun and in the tilt of Earth’s axis causes slight but important variations in the amount of solar energy that reaches any given latitude on Earth’s surface. Three changes are involved.

 

Climate Variations

By reconstructing and dating the history of climatic variations over hundreds of thousands of years, scientists have shown that fluctuations of climate on glacial-interglacial time scales match the predictable cyclic changes in Earth’s orbit and axial tilt. This persuasive evidence supports the theory that these astronomical factors control the timing of glacial-interglacial cycles.

 

Word of Warning:

Climatic variations that result from these movements cause little or no variation in the total amount of solar energy reaching Earth’s surface. Instead, their impact is felt because they change the degree of contrast between seasons. Somewhat milder winters in the middle to high latitudes mean greater snowfall totals, while cooler summers would bring a reduction in snowmelt.

 

Shape of Earth’s Orbit

The first orbital variation is that of eccentricity. It describes the extent to which Earth’s orbit departs from a perfect circle. This variation changes over a period of 100,000 years. About 50,000 years ago, Earth’s orbit was more circular (lower eccentricity) than it has been for the last 10,000 years.

This affects the amount of solar energy received by Earth at the point in its orbit when it is closest to the sun (perihelion) and where it is farthest from the sun (aphelion).

 

Tilt of Earth’s Axis

The second variation is obliquity, which describes the angle by which Earth’s axis tilts from the perpendicular to Earth’s orbital plane. It determines the severity of seasons. Today, the tilt is about 23.5°. It shifts about 1.5° to either side during a span of about 41,000 years.

Season when Earth is Closest to Sun

The third orbital variation describes changes in the direction of Earth’s axis (but with the same tilt) and determines where Earth is located in its orbit during winter and summer in a particular hemisphere. It involves precession of the equinoxes. An equinox occurs whenever the sun is directly above the equator, and the tilt of Earth’s axis causes this to happen twice a year as Earth orbits the sun. The two equinoxes occur at positions on Earth’s orbit that shift or precess slowly over an interval ranging from 16,000 to 26,000 years. This in turn affects the time of year at which Earth is at perihelion, that is, closest to the sun. Currently, perihelion is about January 2, which means that winters in the northern hemisphere are a bit warmer than average and summers in the southern hemisphere a bit cooler. About 9,300 B.C., perihelion was on June 21, and these temperature relationships were reversed.

 

Glossary

Here are some terms and their definitions that are found throughout this submodule.

 

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