The Milankovitch Cycle
The amount of solar energy reaching various parts of the earth's surface is controlled by cyclic variations in the earth's orbit around the sun and in the earth's rotation. The shape of the earth's orbit varies from a more elongated to a less elongated ellipse on a 105,000 year cycle. The time of year at which the earth is closest to the sun changes on a 21,000 year cycle. The tilt of the earth's axis varies on a 41,000 year cycle and its rotation wobbles around the axis on a 23,000 year cycle. These four cycles combine to produce a 100,000 year cycle in the distribution of solar energy between the northern and southern hemispheres and between high and low latitudes. The climatic affect of this cycle, known as the Milankovitch cycle, is variation in the degree of contrast between summer and winter temperatures.
At one extreme of the Milankovitch cycle high northern latitudes have cool summers and mild winters, while at the other extreme they experience warm summers and cold winters. Continental glaciation occurs during the first extreme because summer temperatures are not warm enough to melt the previous winter's snow and ice. As the second extreme is approached summers become warm enough to melt each winter's snow and ice and continental glaciers begin to recede. The current Milankovitch cycle reached this point about 18,000 years ago and the last glaciation gave way to the current interglacial period.
