Ice Ages
The ice ages were periods in Earth's history during which significant portions of the earth's surface were covered by glaciers and extensive fields of ice. Scientists often 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 the 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 the earth's surface. The seven ice eras have covered an average of about 50 million years each.
The ice era that scientists understand best (because it occurred most recently) began about 65 million years ago. Throughout that long period, the earth experienced periods of alternate cooling and warming. Those periods during which the annual temperature was significantly less than average are known as ice epochs. There is evidence for the occurrence of six ice epochs during this last of the great ice eras.
During the 2.4 million-year lifetime of the last ice epoch, about two dozen ice ages occurred. That means that the earth's average annual temperature fluctuated upwards and downwards to a very significant extent about two dozen times during the 2.4 million-year period. In each case, a period of significant cooling was followed by a period of significant warming—an interglacial period after which cooling once more took place.
Scientists know a great deal about the cycle of cooling and warming that has taken place on the earth over the last 125,000 years, the period of the last ice age cycle. They have been able to specify with some degree of precision the centuries and decades during which ice sheets began to expand and diminish. For example, the most severe temperatures during the last ice age were recorded about 50,000 years ago. Temperatures then warmed before plunging again about 18,000 years ago.
Clear historical records are available for one of the most severe recent cooling periods, a period now known as the Little Ice Age. This period ran from about the fifteenth to the nineteenth century and caused widespread crop failure and loss of human life throughout Europe. Since the end of the Little Ice Age, temperatures have continued to fluctuate with about a dozen unusually cool periods in the last century, interspersed between periods of warmer weather. Scientists are not certain as to whether the last ice age has ended, or continues to the present.
A great deal of what scientists know about the ice ages they have learned from the study of mountain glaciers. For example, when a glacier moves downward out of its mountain source, it carves out a distinctive shape on the surrounding land. The "footprints" left by continental glaciers formed during the ice ages are comparable to those formed by mountain glaciers.
The transport of materials from one part of the earth's surface to another part is also evidence for the formation of continental glaciers. Rocks and fossils normally found only in one region of the earth may be picked up, moved by ice sheets, and deposited elsewhere. The "track" left by the moving glacier provides evidence of the ice sheets movement. In many cases, the moving ice may actually leave scratches on the rock over which it moves, providing further evidence for changes that took place during an ice age.
Scientists have been asking what the causes of ice ages are 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, which 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 eleven 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 the 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. Because of this variation, the earth receives solar radiation in varying amounts depending on the shape of its orbit. The final factor, axial precession, is a "wobble" in the orientation of the 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.
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 assert 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 the 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 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 the 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 the 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 average annual temperature might be affected by the impact of meteorites on the earth's surface. If very large meteorites had struck the 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 the 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 the 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.
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