Ice Ages

Scientists have been asking about the causes of ice ages for more than a century. The answer (or answers) to that question appears to have at least two main parts, astronomical factors and terrestrial factors. By astronomical factors, scientists mean that the way the earth is oriented in space can determine the amount of heat it receives and, hence, its annual average temperature.

One of the most obvious astronomical factors about which scientists have long been suspicious is the appearance of sunspots. Sunspots are eruptions that occur on the sun's surface during which unusually large amounts of solar energy are released. The number of sunspots that occur each year changes according to a fairly regular pattern, reaching a maximum about every 11 years or so. The increasing and decreasing amounts of energy sent out during sunspot maxima and minima, some scientists have suggested, may contribute in some way to the increase and decrease of ice fields on the earth's surface.

By the beginning of the twentieth century, however, astronomers had identified three factors that almost certainly are major contributors to the amount of solar radiation that reaches the earth's surface and, hence, the earth's average annual temperature. These three factors are the earth's angular tilt, the shape of its orbit around the sun, and its axial precession.

The first of these factors, the planet's angular tilt, is the angle at which its axis is oriented to the plane of its orbit around the sun. This angle slowly changes over time, ranging between 21.5 and 24.5 degrees. At some angles, the earth receives more solar radiation and becomes warmer, and at other angles it receives less solar radiation and becomes cooler.

The second factor, the shape of Earth's orbit around the sun, is important because, over long periods of time, the orbit changes from nearly circular to more elliptical (flatter) in shape. As a result of this variation, the earth receives more or less solar radiation depending on the shape of its orbit. The final factor, axial precession, is a "wobble" in the orientation of Earth's axis to its orbit around the sun. As a result of axial precession, the amount of solar radiation received during various parts of the year changes over very long periods of time.

Between 1912 and 1941, the Yugoslav astronomer Milutin Milankovitch developed a complex mathematical theory that explained how the interaction of these three astronomical factors could contribute to the development of an ice age. His calculations provided rough approximations of the occurrences of ice ages during the earth history.