Ice ages

Several periods of earth history were marked by major glacial episodes. Possible causes of these episodes include the movement of Earth's continental areas into higher latitudes, changes in atmospheric composition and motion, and changes in Earth's orbit. Effects of the ice ages included substantial changes in sea level and coastal topography, alteration of lake and river drainage, and the shifting of plants and animals accompanying the changing climates.

Periods of Widespread Glaciation

There have been several periods in earth history in which large glaciers have covered substantial portions of the earth's surface. These “ice ages” were created during times in which more snow fell during the winter than was lost by ablation (melting, evaporation, and loss of ice chunks in water) during the summer. Approximately 2 billion years ago, during the Proterozoic eon, large ice sheets covered substantial portions of North America, Finland, India, and southern Africa as indicated by glacially deposited sediments termed tillites. Such glacial deposits have also been found in rocks of late Ordovician and early Silurian age in northern Africa, approximately 440 to 420 million years before the present. Glacial episodes intensified on the Gondwanaland continent during the Carboniferous (Mississippian and Pennsylvanian periods of North American classification) and Permian periods, as tillites and glacially scoured areas indicate the presence of vast ice sheets over large areas within the southern portions of South America, Africa, India, and Australia. Antarctica was probably almost completely covered with ice sheets at this time, and there are indications that the ice sheets expanded and retreated at intervals within these periods, spanning some 360 to 245 million years before the present.

Although these earlier glacial episodes may represent substantial cooling periods for portions of the earth's surface, the term “ice age” has almost become synonymous with the last great glacial episode at the end of the Cenozoic era. There is evidence that within South America, this glaciation began somewhere between 7 and 4.6 million years ago. In most areas, the major glacial episodes spanned the latter part of the Pliocene and the Pleistocene epochs, a period between 3 million and 10,000 years before the present. During this interval, ice sheets with thicknesses of 3 kilometers or more accumulated over much of the higher latitudes of the Northern Hemisphere.

Traditionally, these late Cenozoic ice ages have been separated into four subdivisions. In the European Alps, these included, from oldest to youngest, the Gunz, Mindel, Riss, and Würm glacials. These periods were believed to be separated by warmer interglacial periods termed the Gunz-Mindel, Mindel-Riss, and Riss-Würm. In North America, a fourfold subdivision was also utilized, which included (from oldest to youngest) the Nebraskan, Kansan, Illinoian, and Wisconsin glacials. These were divided by the Aftonian, Yarmouth, and Sangamon interglacials, respectively. This fourfold subdivision seemed to be represented in other global regions as well. By the late 1970's, however, studies on deep-sea sediments indicated that the Alpine glacial sequence encompassed at least eight glacial cycles rather than four or five. Because severity of glaciation depends on both latitude and altitude as well as on local climatic factors, it has become apparent that such a simplistic classification for the last great ice age is untenable. Interdisciplinary studies are better establishing these glacial episodes and more precisely dating the glacial-interglacial cycles.

Causes of Ice Ages

The causes of the ice ages may be quite varied. One possible explanation seems to be related to the distribution of continental areas as a result of plate tectonics. According to this theory, Earth is divided into a series of rigid plates that shift their position relative to one another because of the movement of underlying ductile material. Calculations of the position of Gondwanaland during the Carboniferous-Permian glaciation indicate that the southern portion of the supercontinent was situated over the South Pole. Such a position stimulated the growth of large ice sheets. The position of the continents during the last great ice age may also have caused the growth of ice sheets, as the most prominent glacial areas were at higher latitudes. Plate tectonics cannot, however, entirely account for late Cenozoic glaciation, as several glacial-interglacial periods have been recorded during a relatively brief period in which the earth's plates could not have been repeatedly repositioned in “colder” and “warmer” latitudes. Therefore, other explanations need to be sought to determine the specific causes of the last ice age.

Another explanation for the initiation of ice ages is that at certain intervals, the sun's luminosity decreases. Proposed mechanisms have been the movement of the sun through a dense interstellar cloud of gases and particles or fluctuations in the energy output of the sun. Other suggestions have concerned the earth's albedo, or the amount of solar energy reflected back into space. Twice during the nineteenth century, eruption of volcanoes in Indonesia resulted in colder winter temperatures worldwide as the result of a blockage of the sun's radiation by particles of floating volcanic ash. Short intervals of declining temperatures following these volcanic eruptions led to the hypothesis that severe climate changes could have been caused by more severe volcanism. It has also been suggested that the emergence of land areas during a relative drop in sea level also increases albedo because of the greater reflectivity of terrestrial surfaces. Such episodes may lead to a decrease in the relative temperature at the earth's surface. The greenhouse effect, in which an increase in carbon dioxide content in the atmosphere leads to greater temperatures, may also indirectly result in increased albedo. The higher temperatures may cause greater evaporation rates, and therefore more clouds would form. In turn, the tops of the clouds would reflect the sun's energy, possibly leading to a drop in temperature and initiating glaciation. A decrease in carbon dioxide content, however, could lead directly to the ice ages, because a decrease in this heat-trapping gas would cause a concomitant decrease in temperature. Another hypothesis suggests that mountain building along continental margins, where the earth's lithospheric plates collide, may have resulted in the formation of substantial mountain glaciers at the higher altitudes. The increased albedo resulting from the presence of these highly reflective ice surfaces may have resulted in cooler temperatures and more ice buildup, setting off a chain reaction leading to larger continental glaciers and major glacial episodes. There is no direct evidence, however, that volcanic eruptions, interstellar clouds and solar phenomena, emergence of landmasses, creation of mountain chains, or carbon dioxide fluctuations have led to glacial episodes.

Two hypotheses seem to fit the evidence better as pertains to the initiation of glaciation. Approximately 3.5 million years ago, the isthmus of Panama emerged, apparently resulting in the strengthening of the Gulf Stream's northward flow. This strengthening may have fed more moisture to the northern high latitudes, thereby creating more snowfall and thus a buildup in glaciation. One of the most widely accepted theories concerning the origin of the ice ages was proposed by a Yugoslavian mathematician, Milutin Milankovitch. During the 1920's and 1930's, Milankovitch studied the possible effects of variations in Earth's orbit upon the timing of glacial episodes. These effects include the angle of the ecliptic (axial tilt), the precession of the equinoxes, and the eccentricity of Earth's orbit. At present, Earth's axial tilt is approximately 23.5 degrees from the perpendicular to the plane of Earth's orbit around the sun. According to Milankovitch's calculations, Earth's axial tilt would vary from approximately 22.1 to 24.5 degrees every 41,000 years. As the angle of the ecliptic is the primary factor producing the seasons, such cyclicity may lead to significant climate change.

The second possible cause of climate change involves a variance in the eccentricity of Earth's orbit. A complete cycle between times of maximum orbital eccentricity occurs at approximately 93,000-year intervals, which closely corresponds to the twenty cold-warm cycles recorded in deep-sea cores from the last ice age. The final cycle theorized by Milankovitch involved the precession of the equinoxes: The movement of Earth's axis would approximate the wobbling of a spinning top, with a periodicity of 21,000 years, which would cause a slow shift in the position of the solstices and equinoxes through time. As the earth's climate may be affected by each of these three cycles, the combination of their effects may at times create significant climate changes. As evidence from deep-sea cores seems to support the theory that Milankovitch's cycles correspond to glacial periodicity during the last ice age, this theory has become especially popular for explaining the origin of the major glacial periods.

Whatever caused the initiation of the ice ages, once large glaciers began forming, their presence may have stimulated further ice buildup. With more ice, the earth's albedo would increase, with resulting lower temperatures. Also, a drop in sea level accompanying glacial buildup would likely have led to variation in oceanic and atmospheric circulation patterns, along with a relative rise in altitude of the landmasses. Such a cycle may have been self-perpetuating, with more ice buildup creating more severe glaciation.

Effects of Glaciation

The effects of widespread glaciation were varied and profound. One result was a relative decrease in sea level, as more moisture became locked within the glacial ice. Estimations vary as to the amount of sea-level drop, although many scientists believe that it was 75 meters or more. This drop resulted in many changes in shoreline topography. Land bridges were formed between the British Isles and Europe, as well as between Asia and North America across the Bering Strait. The ice ages also greatly affected regional drainage patterns. Prior to Pleistocene glaciation, the northern portions of the Missouri and Ohio Rivers drained toward the northeast. With incursion by the great ice sheet, called the Laurentide Ice Sheet, drainage patterns changed, eventually resulting in the modern southward drainage patterns. Because of this ice sheet's great weight, the continental crust beneath the glacial areas was depressed as much as 300 meters. As the glaciers melted and receded, the Great Lakes were formed within these basins.

Even outside glacial areas, large lakes were formed because of the increased rainfall in certain areas. One of the largest of these pluvial lakes was Lake Bonneville, the remnant of which constitutes the Great Salt Lake of Utah. Glaciers also created large lakes by damming watercourses. In the Pacific Northwest, glacial Lake Missoula was a glacially dammed lake covering an area almost 8,000 square kilometers in extent. The disastrous collapse of the dam occurred during glacial retreat, with the ensuing catastrophic flooding creating the Channeled Scablands of eastern Washington. Another effect of glaciation is downcutting and subsequent erosion by rivers as a result of the lowering of sea level. Once the glaciers melt, sea level again rises, and rivers deposit large amounts of sediment in their floodplains.

Even in areas not covered by glacial ice, climates were affected. Studies indicate that glacial periods in the higher latitudes corresponded to drier (interpluvial) periods in temperate and tropical regions. Pluvial periods were essentially equivalent to the interglacials of higher latitudes.

Another possible effect of changing climates within and between glacial periods may be the selective extinction of animal species. At the end of the last great ice age, especially in North America, many types of large mammals became extinct. These extinctions have been theorized as resulting from the direct or indirect influence of climate change or, alternatively, as the result of the invasion of North America by Paleo-Indians across the Bering Strait land bridge. A seemingly less disastrous change that occurred during the Pleistocene—but one that is just as crucial for understanding the climate changes during the past glacial-interglacial periods—involves the displacement of plants and animals. The distributions through time of a wide variety of Pleistocene invertebrates, vertebrates, and plants have been documented. During periods of glaciation, forests diminished, and desert areas, steppes, and grasslands increased in size. The peculiar presence of seemingly cold-and warm-climate species of mammals within the same deposits have led some scientists to speculate that some areas in front of the glaciers had a moderate climate. The winters were warmer (allowing the warm-climate species to live within the area), and the summers were cooler (enabling the cold-climate species also to become established in the same region). The fossil distributions of plants and animals during the past ice age indicate that they were often quite different from those observed at present.

Principal Terms

albedo: the amount of solar energy reflected by the earth's surface back into space; an increase in albedo is believed to lead to lower temperatures and stimulates the expansion of glaciers

glaciation: a major formation of land ice and the period in which it occurs

glacier: an accumulation of ice that flows viscously as a result of its own weight; it forms when snowfall accumulates and recrystallizes into a granular snow (firn, or névé), which becomes compacted and converted into solid, interlocking glacial ice

interglacial period: the interval of milder climate between two major glacial episodes

Pleistocene: the epoch of earth history characterized by the presence of large ice sheets in the higher latitudes of the Northern Hemisphere, approximately 2 million to 10,000 years before the present

scabland: a region characterized by rocky, elevated tracts of land with little soil cover and by postglacial dry stream channels

Bibliography

Anderson, Bjørn G., and Harold W. Borns, Jr. The Ice Age World. Oslo: Scandinavian University Press, 1994. This well-illustrated text details the Quaternary history of North America and northern Europe over the last 2.5 million years. Contains an extended glossary, references list, and index. For the general reader as well as the serious student.

Bloom, Arthur G. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. 3rd ed. Long Grove, Ill.: Waveland Press, 2004. This college-level text covers the basics of geomorphology. Includes three chapters on glaciers and glaciology. Index and bibliography.

Chernicoff, Stanley, and Donna Whitney. Geology: An Introduction to Physical Geology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2006. This is a good overview of the scientific understanding of the geology of the earth and surface processes. Includes sections on glaciers, glaciology, and glacial deposits. Includes a link to a Web site that provides regular updates on geologic events around the globe.

Dolgoff, Anatole. Physical Geology. Boston: Houghton Mifflin Harcourt, 1999. This is a comprehensive guide to the study of the earth. Extremely well illustrated and includes a glossary and an index. Although this is an introductory text for college students, it is written in a style that makes it understandable to the interested layperson. Contains a section on the development of glaciers and the types of glacial deposits.

Fagan, Brian M., ed. The Complete Ice Age: How Climate Change Shapes the World. New York: Thames & Hudson, 2009. Discusses the development and cycle of the ice age. Discusses the effects of the ice age on human evolution and includes information on global warming and interglacials. Written in a manner accessible to high school students.

‗‗‗‗‗‗‗. Cro-Magnon: How the Ice Age Gave Birth to the First Modern Humans. New York: Bloomsbury Press, 2011. A literary account of human evolution during the last global ice age. This book provides many scientific details and bibliographies to encourage further reading of specialized scientific literature. Discussing the interaction between Neanderthal and Cro-Magnon, this book is an excellent opportunity for the layperson to enter an ever-growing field of study in anthropology and archaeology.

Lowe, J. J., and M. J. C. Walker. Reconstructing Quaternary Environments. 2d ed. Upper Saddle River, N.J.: Prentice-Hall, 2005. This volume outlines the techniques for analyzing the environments and ecology of the Quaternary period. Included are discussions of landforms and characteristic sediments of the Quaternary, as well as dating techniques and analyses of plants and animals. Although the text would most easily be read by students with introductory training in geology and biology, all terms are thoroughly defined, and it could be comprehended by a general audience.

MacDougall, Doug. Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press, 2006. The author discusses current global warming as a snapshot within the current ice age cycle. Presents a history of the study of ice ages, including the work of many famous scientists.

Nilsson, T. The Pleistocene: Geology and Life in the Quaternary Ice Age. Stuttgart: Ferdinand Enke Verlag, 1983. One of the most thorough accounts of the ice ages available. The bulk of the text concerns continental reviews as to the extent and characteristics of Pleistocene glaciation, as well as the mammal fauna and human fossils found. Emphasis is on the European record, although chapters are devoted to each of the other continents. Although all terms are thoroughly defined, the text is primarily designed for upper-level students of biology, paleontology, and geology.

Sutcliffe, A. J. On the Track of Ice Age Mammals. Cambridge, Mass.: Harvard University Press, 1985. This volume is written for a general audience with no formal training in geology or paleontology. Well written, it avoids unnecessary scientific jargon and is profusely illustrated. Several chapters give a detailed summary of the chronology and general features of the ice ages. Other chapters cover the general characteristics of Pleistocene mammals, modes of preservation, and regional accounts of Pleistocene mammalian faunas.