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2.5.2. Orbital Variations

In the mid-19th century, Croll (1867a, 1867b) proposed an astronomical theory linking the Pleistocene (2 Ma to 10 Ka) ice ages with periodic changes in the Earth's orbit around the Sun. Croll's ideas were later refined and elaborated by Milankovitch (1941). The Milankovitch theory is the name given to the astronomical theory of climate variations. Since these ideas were put forward, much evidence has been found to support the theory. A review of the mechanics of empirical climate reconstruction is given in chapter 3, whilst chapter 5 covers the global climate changes associated with the ice ages. In this section, the forcing mechanisms involved with the Milankovitch theory are discussed.

The original Milankovitch theory identifies three types of orbital variation which could act as climate forcing mechanisms, obliquity or tilt of the Earth's axis, precession of the equinoxes and eccentricity of the Earth orbit around the Sun. Each variation has its specific time period.