Ice Ages

The ice ages were periods in Earth's history during which significant portions of Earth's surface were covered by glaciers and extensive fields of ice. Scientists sometimes use more specific terms for an ice "age" depending on the length of time it lasts. It appears that over the long expanse of Earth history, seven major periods of severe cooling have occurred. These periods are often known as ice eras and, except for the last of these, are not very well understood.

What is known is that Earth's average annual temperature varies constantly from year to year, from decade to decade, and from century to century. During some periods, that average annual temperature has dropped to low enough levels for fields of ice to grow and cover large regions of Earth's surface. The seven ice eras have covered an average of about 50 million years each.

Terrestrial factors

Astronomical factors provide only a broad general background for changes in the earth's average annual temperature, however. Changes that take place on the earth itself also contribute to the temperature variations that bring about ice ages.

Scientists believe that changes in the composition of the earth's atmosphere can affect the planet's annual average temperature. Some gases, such as carbon dioxide and nitrous oxide, have the ability to capture heat radiated from the earth, warming the atmosphere. This phenomenon is known as the greenhouse effect. But the composition of Earth's atmosphere is known to have changed significantly over long periods of time. Some of these changes are the result of complex interactions of biotic, geologic and geochemical processes. Humans have dramatically increased the concentration of carbon dioxide in the atmosphere over the last century through the burning of fossil fuels (coal, oil, and natural gas). As the concentration of greenhouse gases, like carbon dioxide and nitrous oxide, varies over many decades, so does the atmosphere's ability to capture and retain heat.

Other theories accounting for atmospheric cooling have been put forth. It has been suggested that plate tectonics are a significant factor affecting ice ages. The uplift of large continental blocks resulting from plate movements (for example, the uplift of the Himalayas and the Tibetan Plateau) may cause changes in global circulation patterns. The presence of large land masses at high altitudes seems to correlate with the growth of ice sheets, while the opening and closing of ocean basins due to Drumlins, like this one near West Bend, Wisconsin, are composed of glacial till. Although their formation is not well understood, some geologists believe they are shaped when a glacier advances over its own end moraine. JLM Visuals. Reproduced by permission.

tectonic movement may affect the movement of warm water from low to high latitudes.

Since volcanic eruptions can contribute to significant temperature variations, it has been suggested that such eruptions could contribute to atmospheric cooling, leading to the lowering of Earth's annual temperature. Dust particles thrown into the air during an eruption can reflect sunlight back into space, reducing heat that would otherwise have reached Earth's surface. The eruption of Mount Pinatubo in the Philippine Islands in 1991 is thought to have been responsible for a worldwide cooling that lasted for at least five years. Similarly, the earth's annual average temperature might be affected by the impact of meteorites on Earth's surface. If very large meteorites had struck Earth at times in the past, such collisions would have released huge volumes of dust into the atmosphere. The presence of this dust would have had effects similar to the eruption of Mount Pinatubo, reducing Earth's annual average temperature for an extended period of time and, perhaps, contributing to the development of an ice age.

The ability to absorb heat and the reflectivity of the earth's surface also contribute to changes in the annual average temperature of Earth. Once an ice age begins, sea levels drop as more and more water is tied up in ice sheets and glaciers. More land is exposed, and because land absorbs heat less readily than water, less heat is retained in the earth's atmosphere. Likewise, pale surfaces reflect more heat than dark surfaces, and as the area covered by ice increase, so does the amount of heat reflected back to the upper atmosphere.

Whatever the cause of ice ages, it is clear that they can develop as the result of relatively small changes in the earth's average annual temperature. It appears that annual variations of only a few degrees Celsius can result in the formation of extensive ice sheets that cover thousands of square miles of the earth's surface.