Ice Ages

Primary Causes of Ice Ages

An ice age occurs when a huge ice sheet forms covering both the land and the sea. The sheet can be several kilometres thick depending on the severity of the ice age and/or the distance from the equator. An ice age is followed by an interglacial period when the ice melts and the climate becomes milder. The last interglacial period lasted from 128,000 to 115,000 years ago (13,000 years long) before the last ice age began. From 115,000 years ago the world became steadily cooler reaching its coldest (GLACIAL MAXIMUM – GM) about 20,000 years ago. During the GM the ice sheets covered half of North America and much of northern Europe and Asia. The lower temperatures reduce surface evaporation of water from oceans and lakes resulting in lower rainfall. As a result in Australia, northern Tasmania became a desert and the Brazilian rainforests had shrunk to tiny refuges in moist valleys and enclaves. In an ice age, huge volumes of water shift from the oceans to the polar ice sheets, which lowers the sea level by as much as 120 metres.

Sequence of pictures showing how glaciers form (American Scientist, 2002)

Frequency of Ice Ages

The climate changes associated with ice ages repeat at frequencies of about 100,000, 40,000 and 20,000 years. The reason why the climate oscillates at these frequencies lies in the orbit of the earth around the sun. The first parameter is the eccentricity of the earth’s orbit. Every 100,000 years the earth’s orbit changes from being circular (equal distance from the sun at all times of the year) to being elliptical (further away at one time of the year and closer at another time of the same year). The second parameter is the tilt of the earth’s axis. On average the earth is tilted to the sun at an angle of 23.5 degrees. However, the angle varies from 22.2 degrees to 24.5 degrees at a frequency of about 40,000 years. It is caused by an oscillation or wobble of the earth as it spins on its axis, similar to a spinning top wobbling as it slows down. The third parameter is the position of the earth on its elliptical orbit during the northern hemisphere summer, which changes every 40,000 years.

Ice Albedo Feedback

The reflectivity of the earth’s surface is called its albedo. Heat and light from the sun will either be absorbed by the earth or reflected back into space. If the albedo is high, more radiation is reflected and the earth cools. If the albedo is low the earth’s surface absorbs radiation and the world will warm. As the earth cools during one of earth’s orbital cycles, snowfields and ice sheets expand increasing the earth’s albedo. The increase in albedo increases surface reflection of heat and light resulting in further cooling of the earth. This in turn increases the albedo and the earth quickly begins to enter and ice age.

Atmospheric Carbon Dioxide Feedback

Atmospheric carbon dioxide acts like a blanket reflecting heat and light back to the earth keeping it warm. For reasons not fully understood, during glacial times the concentration of carbon dioxide diminishes. Hence a cooling that arises from other causes lowers carbon dioxide levels thus allowing more heat to be reflected into space causing further cooling.

Conclusions

When the earth’s oscillations combine with the two feedback mechanisms an ice age can be generated. An ice age finishes when the earth in its path around the sun, receives more olar energy by moving closer to the sun or changing its orientation on its axis.