In the last 2.5 billion years, several periods of glaciation have been identified, separated by periods of mild climate similar to that of today. Glaciations seem to form through a combination of several different factors. One of the variables is the amount of incoming solar radiation, and this changes in response to several astronomical and orbital effects, as explained by the Milankovitch cycles that operate on time scales of 100,000 years (eccentricity of Earth's orbit around the Sun), 42,000 years (tilt of Earth's axis from ecliptic), and 11,000 years (wobble, the variation in tilt with time). Another variable is the amount of heat that is retained by the atmosphere and ocean, or the balance between the incoming and outgoing heat. A third variable is the distribution of land masses on the planet. Shifting continents can influence the patterns of ocean circulation and heat distribution, and placing a large continent on one of the poles can cause ice to build up on that continent, increasing the amount of heat reflected back to space and lowering global temperatures in a positive feedback mechanism.
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