We live in a glacial age and have done for the last 2.5 million years. Right now we are in an interglacial period which has lasted longer than usual. The periodic cycles of the glaciations within this era correlate with known orbital variations of the earth, which affect the intensity of sunlight reaching the earth. These are known as Milankovitch cycles, after the Serbian engineer who proposed the theory in 1912 - (he was well ahead of his time.)
The cycles are -
1. Rotation of the elliptical orbit of the earth and variations in eccentricity, affecting the distance of earth from the sun 2. Variation of Tilt of the earths axis which affects the angle with which the suns rays hit the earth 3. Precession of the axis of rotation of the earth affecting the angle of the rays for a particular season
The various cycles combine together such that the ice advances and retreats according to their combined effect on intensity of sunlight and therefore temperature of the earth. It is possible we have contributed to the unusually long interglacial period we are in but the ice will come - enjoy this summer....
http://en.wikipedia.org/wiki/Image:Vostok_420ky_4curves_insolation.jpg
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