Causes of Glacial Ages

What caused the glacial ages? This question has been asked by scientists since the theory of glacial ages was accepted over a century ago. Only in the last couple of decades have climatologists thought they were beginning to provide acceptable answers.

The question is of more than academic importance. Understanding climate changes could provide the key to accurate, long-range climate predictions. Even minor climate changes affect crops and the problem of feeding humanity. If we knew the causes of fluctuations between glacial and interglacial episodes, we would probably know whether another glacial age is imminent or whether we are in a cooling or warming stage in Earth's history.

A number of theories and hypotheses have been proposed to account for glacial ages. Each explains some of our observations, but none alone satisfactorily accounts for all of the data. We shall review some of the more plausible ideas.

Variations in the Earth's Orbit and Inclination to the Sun

The amount of heat from solar radiation received by any particular portion of the Earth is related to the angle of the incoming sun's rays and, to a lesser degree, the distance to the sun. The dimensions of the earth's orbit around the sun change slightly over a period of thousands of years.

The angle of the Earth's poles relative to the plane of Earth's orbit about the sun also changes periodically. Recent analysis of data has added strong support to the theory (now known as the Milankovitch theory) that variations in orbital relationships and "wobble" of Earth's axis are largely responsible for the glacial and interglacial episodes. In the 1920s, Milutin Milankovitch, a Serbian mathematician, calculated that incoming solar radiation varies in cycles of 21,000, 41,000 and 100,000 years. Proof for the Milankovitch cycles came from oceanographic data. Cores of deep-sea sediment have provided a fairly precise record of climatic variations over the past few hundred thousand years. The cycles of cooling and warming determined from the marine sediments are 23,000, 42,000 and 100,000 years, thus closely matching the times predicted by Milankovitch.

The orbital variation theory fails to explain the absence of glaciation over most of geologic time. Thus, one or more of the other mechanisms described below may have contributed to glacial ages.

Changes in the Atmosphere

A set of hypotheses seeks to explain glacial ages by changes in the ability of the atmosphere to filter solar radiation. Much of the solar energy reaching our planet is either reflected back out to space or absorbed by the atmosphere. If the composition of the atmosphere changes, the amount of the sun's heat reaching and retained by Earth's surface also changes.

One hypothesis regards carbon dioxide as responsible for major climate changes. According to this proposal, if there is an increase in the carbon dioxide content in the atmosphere, Earth warms up. This is because of a "greenhouse effect" in which solar energy penetrates the atmosphere and heat is retained or trapped at the earth's surface. Carbon dioxide in the atmosphere reduces the amount of heat that can radiate from Earth back out into space. This results in a warming trend until, for some reason, the carbon dioxide content would coincide with warm episodes between glacial ages, low carbon dioxide content with periods of glaciation.

A major problem in the carbon dioxide hypothesis is explaining a cyclic change in the concentration of carbon dioxide in the atmosphere. One suggestion has been that when vegetation is abundant, there is less carbon dioxide in the atmosphere. The cooling this causes results in an ice age, and much of the Earth's vegetation dies, releasing carbon dioxide to the atmosphere to begin the warming trend. However, there is no evidence of profound changes of vegetation. An additional difficulty comes in explaining why the process is effective for such a small percentage of geologic time.

Another hypothesis involves volcanoes. Worldwide temperature dropped about 2°C following the major volcanic eruption of Krakatoa in Indonesia in 1883. The SO2 and dust carried by high atmospheric winds reduced solar energy penetrating the atmosphere for a few years (the opposite of the "greenhouse effect"). Presumably a closely spaced series of large eruptions could cause enough of a temperature drop to trigger glaciation and begin a glacial age.

Changing of the Positions of Continents

Another hypothesis is that glacial ages occur when landmasses move closer to polar regions. Plate tectonics provides the mechanism for motion of landmasses. But the fluctuations in late Cenozoic climates are not explained by plate tectonics. All evidence indicates that the positions of the continents have not changed significantly with respect to the poles during the late Cenozoic Era. Certainly the continents have not shuffled back and forth during the recent glacial and interglacial ages.

However, the earlier movement of continents from positions closer to the equator into more northerly latitudes might have placed the present northern continents in a position favorable for glaciation.

Changes in Circulation of Sea Water

Our present climates are very much affected by circulation patterns of sea water. Landmasses block the worldwide free circulation of ocean water, thus some oceans are warmer than others.

According to one hypothesis, a glacial episode begins when Atlantic water circulates freely with Arctic Ocean water. At present the warmer waters of the Atlantic Ocean cannot mix freely with water in the Arctic Ocean. For this reason the surface of the Arctic Ocean is frozen for much of the year. Continental glaciation begins, according to this hypothesis, when warmer Atlantic water flows through a shallow channel between Greenland and Canada. This would keep the Arctic Ocean from completely freezing over during the winter. The moisture picked up by winds blowing over the Arctic Ocean would precipitate heavy amounts of snow on the northern continental landmasses. Ice sheets would grow so long as ice-free seas supplied the moisture. But as glaciation continued, sea level would drop due to the loss of water to the ice sheets. Sea level would eventually drop below the floor of the shallow channel between Greenland and Canada, shutting off the supply of warm Atlantic waters to the Arctic Ocean. Because of this, the surface of the Arctic Ocean would freeze, reducing the moist air supplied to the ice sheets. The ice sheets would recede and disappear.

Some scientists question the adequacy of this mechanism to explain large-scale glaciation. Others point out that this hypothesis cannot explain the very warm interglacial climates. Also, snow and ice reflect much solar radiation. Therefore, a frozen Arctic Ocean coupled with glacial ice covering up to one-third of the world's landmass should tend to perpetuate a glacial age by reflecting more solar radiation into space.

Sliding of the West Antarctic Ice Sheet over the Ocean

One intriguing although highly speculative hypothesis regards changes in West Antarctic Ice Sheet as responsible for glacial ages. A large segment of the ice sheet, lubricated by water along its base, slides as a mass onto the ocean surface. Before the large floating slab of ice can be broken up by wave action, the ice reflects a significant amount of solar heat back into space. This results in a worldwide cooling sufficient to trigger a glacial age.

There is some evidence that parts of the Antarctic Ice Sheet have slid rapidly and suddenly in the past. Whether the mechanism is capable of cooling the world's climate so much has yet to be demonstrated.

Summary

Scientists do not fully understand what causes glacial ages and intervening warm episodes. Only recently has the Milankovitch theory become widely accepted. This, as least, seems to explain what controls the cycles of climatic variation. But one or more of the other postulated mechanisms must also contribute significantly.