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Ice Ages

Ice Ages

Earth has cooled dramatically over the last 50 million years. Ice sheets, flora, and fauna all record those changes. About 55 million years ago, palm-like trees and crocodile-like reptiles lived north of the Arctic Circle and beech trees grew in Antarctica. By 35 million years ago, glacier ice was starting to spread in Antarctica. Today, the continent is more than 90 percent ice-covered.

The first major glaciations in Greenland began between 7 and 3 million years ago. The first continental ice sheets in the Northern Hemisphere appeared around 2.7 million years ago. The following discussion focuses on the Pleistocene, the most recent epoch of glacial and interglacial cycles of the Northern Hemisphere.

Pleistocene Glacial Cycles

About 18,000 years ago, when the most recent glacial age was near its peak, vast expanses of the northern continents were covered by glacial ice. In North America, two ice sheets covered most of Canada: the Laurentide Ice Sheet (LIS) in the east and midwest, and the Cordilleran Ice Sheet in the west. The LIS blanketed New England and formed great lobes that flowed down what are now the Great Lakes basins in the upper midwestern United States. Meltwater flowed from the margin of the ice sheet down the St. Lawrence and Mississippi Rivers. The ice carried sediments plucked from the ground beneath the ice. Most of that material, called glacial till, was deposited near the ice but some was carried by floating icebergs and by melt-water, to eventually be deposited on the ocean floor. Today, those sediments and many other geologic records are used to reconstruct past glacial cycles.

Geologists call the recent epoch of Northern Hemisphere glacial and interglacial cycles the Pleistocene. During the Pleistocene, about 2.75 million to 10,000 years ago, the great northern ice sheets grew and shrank about 50 times. The most recent of these cycles were first recognized in layers of sediments the ice sheets deposited on the land surface as they retreated. Four LIS cycles were identified in this way.

The first evidence for older Pleistocene glacial cycles came from sediment samples brought up from the sea floor.* The sediments contain the shells of tiny ocean-dwelling organisms called foraminifera that sink to the sea floor when they die. The foraminifera shells record a key indicator for the ice sheets and their temperature cycles. Oxygen isotopic composition of the calcium carbonate shells can be related to the temperature of the sea water in which they grew, as discussed below.

Isotope Applications.

Isotopes are forms of the same element that have different numbers of neutrons and slightly different weights and chemical behaviors. In the case of water (H2O), hydrogen has two stable isotopes and oxygen has three. This is useful to paleoclimatologists because ocean water or ice is a mixture of water molecules with different combinations of light and heavy isotopes, and thus with different weights.

Water molecules containing the lighter isotopes evaporate more easily than the water made of the heavy isotopes, whereas heavier water precipitates more readily than lighter water. Thus, when water evaporates from the ocean, lighter isotopes are preferentially removed from the ocean. The process by which light and heavy isotopes are separated is called fractionation.

When water vapor travels over the land surface, forms clouds, and precipitates as snow onto an ice sheet, the lighter oxygen isotopes are moved from the ocean into the ice sheet, leaving the ocean relatively enriched in the heavier isotopes. When the ice sheets shrink, the lighter water returns to the ocean. Foraminifera use calcium, carbon, and oxygen from the ocean water to build their shells; hence, as the oxygen isotopic ratios of the ocean water change, the ratios in the shells record those changes.

When the sea-floor sediment cores are analyzed, large cycles in the ratio between heavy and light oxygen isotopes are found. The oxygen isotopic record shows that over the last 0.9 million years, large ice sheets grew and decayed over approximately 100,000-year cycles. Before that time, there were two shorter cycles, one of about 41,000 years and one of 23,000 years. The oxygen isotopic record also shows that ice sheets grow slowly but shrink rapidly.

Climate Mechanisms

Climatologists often classify the processes responsible for climate change as either forcings, which are large-scale processes that drive the climate system, or feedbacks, which are interactions among components of the system. The most important forcing during the Pleistocene is cyclic changes in Earth's orbit around the Sun. Orbital cycles are caused by the complicated gravitational interactions among the planets in our solar system. The result is predictable changes in the strength of the insolation (i.e., the solar energy arriving at Earth's surface). Earth's orbit changes slowly, and the insolation cycles are tens of thousands of years long, with periods that are very similar to the ice-sheet growth-and-decay cycles found in the oxygen isotopic record.

In general, when summertime insolation is low in the northern hemisphere, ice sheets grow, and when it peaks, ice sheets retreat. The last northern summer insolation maximum was about 12,000 years ago, a time when the great northern ice sheets were shrinking.

Earth today is experiencing an interglacial climate. In North America, the ice-sheet retreat began about 15,000 years ago and, except for Greenland, the great Northern Hemisphere ice sheets are now gone. Mountain glaciers are still retreating. However, if the current cycle is like the last three or four, the climate should be cooling and Earth should be entering a time of glacier regrowth.

A short regrowth did happen, from about 1400 to 1900 C.E., a time called the Little Ice Age (LIA). The LIA had a profound impact on human welfare and culture. Climate cooling caused crops to fail and the resulting turmoil brought the end of feudal societies around the world. It is interesting to note that while Viking settlements in Greenland failed during the LIA, native communities adapted and survived.

In the late 1800s, global climate warmed again, ending the LIA. The warming continues to this day. The end of the LIA might have been normal climate variability or it might be linked to industrialization, the use of fossil fuels, and the associated increase in atmospheric carbon dioxide (CO2), a greenhouse gas .

Searching for Answers

Why did the glacial cycle change 0.9 million years ago? Paleoclimatologists and glaciologists are still looking for the answer to that question, using a combination of fieldwork and computer simulations of the interaction between ice sheets and climate. Earth's climate had been gradually cooling for about the last 50 million years. One possibility is that once Earth's climate grew cool enough, the ice sheets could grow large enough to start forcing climate to change, instead of simply responding to it. Another idea is that, over time, the ice sheets modified the land surface enough to affect the way they moved, and thus the timescale over which they could grow and decay.

Why did Earth start cooling 50 million years ago? Again, paleoclimatologists are not in agreement. Two possible answers lie in plate tectonics, which changes the arrangement of the continents and shapes of the oceans. Over the last 65 million years, Antarctica became an isolated continent over the South Pole, the Atlantic Ocean widened, and North and South America became connected. These changes isolated Antarctica, allowing it to become very cold. They also gave rise to the Atlantic Ocean's Gulf Stream, a current that very effectively moves warm tropical water north, where it cools and sinks, to return south again. Computer models show that the establishment of the Gulf Stream would have increased atmospheric moisture over Greenland and northern Europe, increasing snowfall and helping ice sheets to grow. Other theories have also been suggested, such as changes in cosmic ray intensity. Long-term cooling and warming likely results from a combination of several processes.

see also Climate and the Ocean; Ice Cores and Ancient Climatic Conditions; Glaciers and Ice Sheets; Glaciers, Ice Sheets, and Climate Change; Global Warming and Glaciers; Global Warming and the Ocean; Isotopes: Applications in Natural Waters; Ocean Currents; Plate Tectonics.

Christina Hulbe

Bibliography

Andersen, B. B., and H. W. Borns. The Ice-Age World. Oslo, Norway: Scandinavian University Press, 1994.

Pielou, E. C. After the Ice Age: The Return of Life to Glaciated North America. Chicago, IL: University of Chicago Press, 1991.

Internet Resources

Adams, Jonathan. North America During the Last 150,000 Years. Oak Ridge National Laboratory, Quaternary Environments Network. <http://www.esd.ornl.gov/projects/qen/nercNORTHAMERICA.html>.

THE SALT OSCILLATR

Climate can be modified on a variety of timescales. Ice cores from Greenland record millennial-scale oscillations in temperature that cannot be explained by Earth orbital cycles. Wally Broeker, a geochemist and paleoclimatologist at Columbia University suggested that changes in Atlantic Ocean circulation might explain the oscillations via a theoretical mechanism called the salt oscillator.

The Atlantic Ocean's equator-to-pole circulation pattern depends on warm, salty water arriving in the north and cooling, so that it becomes more dense and sinks, to flow back south through the deep ocean. But if the salinity of the surface water were to decrease, the rate of sinking, and thus the ocean circulation, would slow down. That might happen if the southern edge of the Greenland ice sheet started to melt, sending fresh water to the ocean surface. When the circulation rate decreases, the northern Atlantic cools. That cooling would reduce the rate of melting, and thus the rate of fresh-water supply to the North Atlantic. Over time, the surface water would become saltier and denser, and the circulation rate would increase, bringing warmer water and warmer weather back to Greenland and Northern Europe.

* See "Ocean-Floor Sediments" for a photograph of a sediment core sample.

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Ice Ages

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 warmingan 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.

See also Earth (planet); Glacial landforms; Glaciation; Historical geology; Polar axis and tilt; Polar ice

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Ice Ages

Ice ages

Ice ages were periods in Earth's history when glaciers and vast ice sheets covered large portions of Earth's surface. 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 low enough to allow fields of ice to grow and cover large regions of Earth.

The most recent ice age

Over the last 2.5 million years, about two dozen ice ages have occurred. That means that Earth's average annual temperature greatly shifted upwards and downwards about two dozen times during that time. In each case, a period of significant cooling was followed by a period of significant warmingcalled an interglacial periodafter which cooling took place once more.

Scientists know a great deal about the cycle of cooling and warming that has taken place on Earth over the last 125,000 years, the period of the last ice age cycle. They have been able to specify 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 move up and down. No one is quite certain whether the last ice age has ended or whether we are still living in it.

Evidence for the ice ages

A great deal of what scientists know about the ice ages they have learned from the study of mountain glaciers. When a glacier moves downward out of its mountain source, it carves out a distinctive shape on the surrounding land. The "footprints" left by ice-age glaciers are comparable to those formed by mountain glaciers.

Ice Age Refuges

The series of ice ages that occurred between 10,000 and 2,500,000 years ago had a dramatic effect on the climate and life-forms in the tropics. During each glacial period, the tropics became both cooler and drier, turning some areas of tropical rain forest into dry seasonal forest or savanna. However, some areas of forest escaped the dry periods and acted as refuges (protective shelters) for forest plants and animals. During subsequent interglacials, when humid conditions returned to the tropics, the forests expanded and were repopulated by plants and animals from the species-rich refuges.

Ice age refuges correspond to present-day areas of tropical forest that typically receive much rainfall and often contain unusually large numbers of species. The location and extent of the forest refuges have been mapped in both Africa and South America. In the African rain forests, there are three main centers located in Upper Guinea, Cameroon and Gabon, and the eastern rim of the Zaire basin. In the Amazon Basin, more than 20 refuges have been identified for different groups of animals and plants in Peru, Colombia, Venezuela, and Brazil.

The transport of materials from one part of Earth's surface to another part is also evidence for ice ages. Rocks and fossils normally found only in one region of Earth may be picked up and moved by ice sheets and deposited elsewhere. Also, 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.

Causes of the ice ages

Although scientists do not know exactly what causes ice ages or periods of glaciation, they have offered many theories. These theories point to either astronomical (space-based) factors or terrestrial (Earthbased) factors.

Astronomical factors. One of the most obvious astronomical factors 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 11 years. Some scientists have suggested that the increasing and decreasing amounts of energy sent out during sunspot activity may contribute in some way to the increase and decrease of ice fields on Earth's surface.

Other scientists have pointed to the changes in the geometry of Earth's orbit around the Sun as factors that have led to ice ages. Those

changes have resulted in Earth receiving more or less solar radiation, becoming consequently warmer or cooler. In the 1930s, Serbian mathematician Milutin Milankovitch (18791958) proposed a theory to explain such changes. The Milankovitch theory states that three periodic changes in Earth's orbit around the Sun affect the amount of sunlight reaching Earth at different latitudes, leading to ice ages. First, Earth's axis wobbles like a gyroscope, tracing a complete circle every 23,000 years or so. Second, while wobbling, the axis tilts between 22 and 24.5 degrees every 41,000 years. Third, Earth's elliptical orbit pulses, moving outward or inward every 100,000 and 433,000 years.

Terrestrial factors. Changes that take place on Earth itself may also have contributed to the evolution of ice ages. For example, volcanic eruptions can contribute to significant temperature variations. Dust particles thrown into the air during an eruption can reflect sunlight back into space, reducing heat that would otherwise have reached Earth's surface.

A similar factor affecting Earth's annual average temperature might be the impact of meteorites on Earth's surface. If very large meteorites had struck Earth at times in the past, such collisions would have released huge volumes of dust into the atmosphere. The presence of this dust could have also reduced Earth's annual average temperature for an extended period of time.

Whatever the cause of ice ages, it is clear that they can develop as the result of relatively small changes in Earth's average annual temperature. It appears that annual variations of only a few degrees can result in the formation of extensive ice sheets that cover thousands of square miles of Earth's surface.

[See also Geologic time ]

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ice ages

ice ages Periods when ice has accumulated at the poles and the continents have been glaciated repeatedly. Exactly why glaciation occurred is not clear. There are suggestions of a middle Precambrian glaciation about 2300 Ma in N. America, S. Africa, and Australia. More information exists to suggest that the Earth was glaciated between 950 and 615 Ma, and there are at least two glacial horizons in Africa, Australia, and Europe. There is good evidence for a glaciation at the end of the Ordovician in N. Africa, but glacial deposits described from elsewhere at this period are problematical, so the extent of the glaciation is not known. The Permo–Carboniferous glaciation of S. America, S. Africa, India, and Australia was widespread and is well documented. There is no evidence for further glaciation until the Quaternary. Suggestions have been made for other ice ages during the Palaeozoic but evidence for them is sparse. The Pleistocene ice age is the best documented, but there is undoubted evidence of earlier glaciations in the geologic record.

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ice ages

ice ages Periods when ice has accumulated at the poles and the continents have been glaciated repeatedly. Exactly why glaciation occurred is not clear. There are suggestions of a middle Precambrian glaciation about 2300 Ma ago in North America, South Africa, and Australia. More information exists to suggest that the Earth was glaciated between 950 and 615 Ma ago, and there are at least two glacial horizons in Africa, Australia, and Europe. There is good evidence for a glaciation at the end of the Ordovician in North Africa, but glacial deposits described from elsewhere at this period are problematical, so the extent of the glaciation is not known. The Permo-Carboniferous glaciation of South America, South Africa, India, and Australia was widespread and is well documented. There is no evidence for further glaciation until the Quaternary. Suggestions have been made for other ice ages during the Palaeozoic but evidence for them is sparse. The Pleistocene ice age is the best documented.

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ice age

ice age A period in the earth's history during which ice advanced towards the equator and a general lowering of temperatures occurred. The last major ice age, that of the Pleistocene period (sometimes known as the Ice Age), ended about 10 000 years ago. At least four major ice advances (glacials) occurred during the Pleistocene period; these were separated by interglacials during which the ice retreated and temperatures rose. At present it is not known if the earth is between ice ages or is in an interglacial of the Pleistocene Ice Age. It has been established that ice ages also occurred during the Precambrian (over 500 million years ago) and during the Permo-Carboniferous (about 250 million years ago).

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Ice Ages

Ice Ages Periods in the Earth's history when ice-sheets and glaciers advance to cover areas previously not affected by ice. There is evidence of at least six Ice Ages having occurred throughout the Earth's history, the earliest dating back to 2.3 billion years ago. The best known is the most recent Ice Age, which began c.2 million years ago and lasted until the retreat of the ice to its present extent c.10,000 years ago. The present time may be a warmer period known as an interglacial. The last Ice Age produced many of the landforms seen in northern continents and affected sea level on a global scale.

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ice age

ice age • n. a glacial episode during a past geological period.See glacial period. ∎  (the Ice Age) the series of glacial episodes during the Pleistocene period.

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Ice Age

Ice Age: see Pleistocene epoch.

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