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At least five prolonged ice ages—epochs when glaciers cover entire continents—have occurred throughout the Earth’s 4.6 billion–year history. These five ice ages represented unusual, relatively short episodes in the whole of Earth’s climatic record (spanning a total of 50 to 200 million years, only 1 to 4 percent), and yet they destroyed entire ecosystems, leaving behind tremendous piles of glacial debris.
Ice ages are epochs of time when massive ice sheets and smaller ice masses called glaciers cover extensive areas of the Earth’s surface. During an ice age the planet is cold, dry, and inhospitable. Whereas few forests can be supported, ice-covered areas and deserts are plentiful. Winters are longer and more severe, and ice sheets grow to enormous sizes, accumulating to thicknesses that measure thousands of feet in depth. These ice sheets move slowly from higher elevations to lower regions, driven by gravity and their tremendous weight. During that process, they alter river courses, destroy entire regional ecosystems, flatten landscapes, and, along their margins, deposit great piles of glacial debris.
Evidence of Ice Ages
Proof of ice ages—glaciation over continent-sized regions—comes from several sources. There is the widespread deposition of unique sediments of dirt (called “till”) found under melting glaciers. These sediments contain a wide variety of rock forms that have been accumulated from disparate areas. In addition, glaciation leaves telltale marks in the form of grooved, striated, polished bedrock pavements, faceted stones in the till, and interspersed layers of gravel containing different rock types. Ice ages also are substantiated by erosional forms believed to have been produced by advancing ice sheets, among them sculptured landscapes, such as glacial uplands or U-shaped valleys.
Such evidence suggests at least five prolonged ice ages: (1) during Precambrian era, 1.7–2.3 billion years ago; (2) during the end of the Proterozoic eon, about 670 million years ago; (3) during the middle of the Paleozoic era, about 420 million years ago; (4) during the Carboniferous period, in the late Paleozoic era, beginning 290 million years ago; and (5) during the Pleistocene epoch of the Quaternary period, beginning 1.7 million years ago. In this most recent era, ice sheets developed in the highlands of North America and Europe and dominated the Northern Hemisphere. Massive ice sheets covered all of present Canada, south of the Great Lakes region, as well as Greenland, Scandinavia, and Russia. Each ice age lasted at least a million years, during which great ice sheets migrated back and forth across a huge paleocontinent (ancient or prehistoric continent). The span of these ice ages totaled 50 to 200 million years, which accounts for only 1 to 4 percent of the Earth’s 4.6 billion-year history. Ice ages thus represent unusual, albeit relatively short, episodes in the Earth’s climatic record.
Theories about Causes of Ice Ages
Although scientists have extensively studied glaciation, no single theory is widely accepted as explaining the causes of ice ages. However, several theories converge into two categories. First are terrestrial theories. Among these, in 1941 the Canadian geologist and explorer A. P. Coleman suggested that change in the elevations of continents provided a natural explanation for an ice age. That is, the uplifting of continental blocks creates increases in the height of land areas by mountains rising or sea levels falling, which cooled the land and produced glacial conditions. A second theory is linked to the changing continental positions associated with plate tectonics. The notion of continental drift, set out by the German geophysicist Alfred Wegener in 1922, proposes that continents sliding around on the globe’s surface could be brought into colder climatic conditions, thereby allowing ice sheets to develop, especially because a continent’s climate is mainly determined by its latitude and size. A third theory suggests that extensive volcanic activity, which spews out dust and ash into the atmosphere, reflects radiant solar heat back into space, thereby producing cooler temperatures on the Earth’s surface. Closely related is the ocean-atmospheric hypothesis. The assumption here is that the only adequate supply of water for massive ice accumulations is the ocean. Because creation of ice sheets on land depends on wind and weather patterns, logic suggests that profound changes between the ocean and the atmosphere could contribute to the onset of ice ages. Another hypothesis was put forward in 1998 by the American climatologist, Maureen Raymo. She proposed the idea that Earth’s cooling climate over the last 40 million years was caused by a reduction in atmospheric carbon dioxide due to enhanced chemical weathering in the mountainous regions of the world, particularly the Himalayas, and that the growth of the Himalayas may, in fact, have triggered the start of the ice ages.
The second category of theories to explain ice ages is extraterrestrial. As early as 1875 the Scottish scientist James Croll proposed that astronomical variations in the Earth’s orbit around the sun produced conditions for the onset of ice ages. He believed that disturbances by the moon and sun cause periodic shifts in the Earth’s orbit, thereby affecting the distribution of solar heat received by the Earth and climatic patterns on the surface. Less heat produces colder climates.
Croll’s theory was modified in 1938 by a Yugoslav scientist, Milutin Milankovitch, in what is today the most popularly accepted theory for climate changes during the Pleistocene epoch. Milankovitch believed that the amount of solar radiation is the most important factor in controlling the Earth’s climate and in producing ice ages. The amount of radiation varies, he argued, according to three key factors. (1) The Earth doesn’t rotate perfectly like a wheel about an axis; it spins like a wobbling top. Every twenty-two thousand years, Milankovitch calculated, there is a slight change in its wobble (called “precession of the equinoxes”). (2) Every 100,000 years there is a change in the Earth’s orbit about the sun (which he labeled “eccentricity”). The Earth’s almost-circular orbit becomes more elliptical, taking the Earth farther from the sun. (3) Finally, Milankovitch discovered that every forty-one thousand years there is a change in the tilt of the Earth’s axis, moving either the Northern or Southern Hemisphere farther from the sun (a process known as “obliquity”). These cycles mean that at certain times less sunshine hits the Earth, so there is less melting of snow and ice. Instead of melting, these cold expanses of frozen water grow. The snow and ice last longer and, over many seasons, begin to accumulate. Snow reflects some sunlight back into space, which also contributes to cooling. Temperatures drop, and glaciers begin to advance. These effects are sufficiently substantial to cause cyclical expansion and contraction of the massive ice sheets. By taking climatic effects and solar insulation of these variations and applying them to computer models of ice sheet behavior, scientists have demonstrated that a correlation exists between these cycles and the cyclic growth and decay of Pleistocene ice sheets over the past 600,000 years. The combination of orbital cycles results in lower summer insulation (exposure to the sun’s rays) at fifty-five degrees north latitude. Such cooler summers in high latitudes tend to preserve each winter’s snowfall, which over thousands of years leads to snow from successive winters inducing growth in the northern ice sheets, evidentially causing the onset of a new ice age.
The Earth may be currently in an ice age because within each major glaciation ice caps and mountain glaciers are oscillating between extension and retreat. The last retreat concluded approximately ten thousand years ago and probably represents only an oscillation, not a final ending. The recent trend toward global warming, however, has reduced fears of any imminent return of an ice age.
Bibliography:
- Andersen, B. G., & Borns, H. W. (1994). The ice age world. New York: Scandinavian University Press.
- Erickson, J. (1990). Ice ages: Past and future. Blue Ridge Summit, PA: TAB Books.
- Fagan, B. (2009). The great warming: Climate change and the rise and fall of civilizations. London: Bloomsbury Press.
- Macdougall, D. (2006) Frozen earth: The once and future story of ice ages. Berkeley: University of California Press.
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