The main climatic changes of the Quaternary are linked to the orbital position of Earth in relation to the sun. Astronomer Milutin Milankovitch proposed the orbital theory that is established by a variety of evidence, starting with tree ring variation. The precession of the equinox, the obliquity of the Earth's orbit around the sun, and the eccentricity of the Earth's orbit all contribute to the level of insolation received by Earth and are therefore the cause of geologic shifts in climate.
Before the Quaternary was the warm so-called Tertiary period that brought tropical conditions to the mid-latitudes, and extensive temperate forests grew in the Northern Hemisphere. The north and south polar areas were at least 36 degrees F (20 degrees C) warmer than today, perhaps around 41 degrees F (5 degrees C) average annual temperature. Orbital causes for this part of Earth's history have not been specified, but undoubtedly were important factors.
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