Milankovitch Cycles, Wikipedia
Explains how variations in the orbit of the earth and variations in the tilt of the earth's axis of rotation both affect climate. The theory attributes many of the past ice ages and centuries long warm spells to changes in earth's orbit and rotation.
According to this theory, present orbit and rotation conditions indicate a gradual and slight cooling over the coming milleniums. In general, earth's present warm interglacial climate will continue, if other factors do not intervene.
Summary by Alan Detwiler, self sufficiency advocate. Bio at http://www.smashwords.com/profile/view/alandetwiler