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Orbital Forcing


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Variations in seasonality of solar radiation reaching the Earth's surface arising from changes in the Earth's eccentricity, axial tilt (obliquity), and precession have contributed significantly to the pacing of the Earth's glacial-interglacial cycles over the last million years.

Obliquity: The axis of the Earth is tilted with respect to the plane of its orbit around the Sun. The inclination of the Earth's axis is subject to variations between 22.1 and 24.5 degrees with a period of 41,000 years. With an increasing tilt, the difference between summer and winter insolation increases leading to stronger seasonal cycles. When obliquity is small the incoming solar radiation is more evenly distributed between the seasons.

Eccentricity: The Earth's orbit around the sun changes on periods of about 100,000 and 400,000 years from nearly circular to slightly elliptical. This cycle is driven by other planets in the solar system. The eccentricity is a measure for the departure from a circular orbit. Currently, the eccentricity is small and the Earth's orbit is only slightly elliptical. Thus, the difference in incoming solar irradiation between the closest approach to the Sun and the furthest distance amounts only to 6.8 percent. However, the eccentricity modulates the amplitude of the precessional cycle of the Earth and thus the amplitude of the seasonality. (The amplitude of eccentricity in the animation is heavily exaggerated for illustration purposes)

Precession: Precession describes a gyroscopic motion of the Earth's axis generated by the Sun and the Moon. The orientation of the Earth's axis with respect to the fixed stars changes with a period of about 23,000 years. Currently, summers in the Southern Hemisphere occur when the Southern Hemisphere is closer to the Sun. Thus, the Southern Hemisphere experiences a greater difference between summer and winter insolation while the Northern Hemisphere experiences milder seasons. This will be reversed in about 11,000 years from now. The effect of precession is modulated by eccentricity as can be seen by the fact that in a perfectly circular orbit (zero eccentricity) there would be no effect of precession on the distribution of incoming solar radiation over the seasons and the two hemispheres.