Sequencing Ice Ages
Traditionally, climate scientists believed that Earth underwent four major periods of glaciation (being covered by ice sheets) with warm interglacials in between.But climate research has now shown that over the past 800,000 years Earth has seen a complex pattern of some twenty major climatic shifts, with temperatures alternating between very warm and intensely cold, featuring warm seas in northern Europe at one extreme and ice sheets covering vast areas of the globe at the other. The cause of these fluctuations lies in the complex relationship between Earth and the Sun. We are all familiar with the fact that Earth orbits the Sun and that it spins around its own axis, which is at an angle to the plane of its orbit. This causes some parts of Earth’s surface to be nearer the Sun for periods of time, accounting for the differences between summer and winter. If all parts of this system were stable. Earth’s climate would remain constant, but this is not so. First, Earth’s or bit is not perfectly circular but is slightly elliptical, causing a variation on a 100,000-year cycle. Secondly, the axis of Earth Changes its tilt (angle by a fraction over a .41,000-year cycle; and thirdly, the planet . has a slight wobble (shaking movement) about its axis as it spins, setting up changes over a cycle of 23,000 years. The combination of all these factors (known as the Milankovitch cycles) creates very small changes in the Sun-Earth relationship that determine the expansion or contraction of the polar ice cap and thus the sequence of fluctuating ice ages.
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