According to the theory of the Milankovitch weather cycles, ice ages are cyclical, caused by changes in the earth's orbit. The theory was developed by Serbian geophysicist Milutin Milankovitch (1879–1958) in the 1930s, and it postulates that the amount of available sunlight in the northern hemisphere is affected by the earth's orientation in space. Because the earth is a globe in motion, sunlight strikes the earth differently depending on the following factors: The eccentricity of its orbit, which returns to the same point every 100,000 years; the tilt of the axis of its rotation, a 41,000-year cycle; and the precession of the equinoxes, a 23,000-year cycle. Milankovitch proposed that decreased sunlight prevents ice and snow from melting in the summer in the northern latitudes. This, in turn, cools the atmosphere, because ice reflects 90 percent of solar radiation back into space, and over a long period of...