Ice Ages

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Ice Ages

Introduction

Ice ages are periods of time when vast areas of Earth's surface are covered with ice sheets and glaciers. Ice sheets (continent-sized glaciers) and glaciers are slow-moving rivers of ice formed from snow that has compacted over thousands to millions of years. The term ice age is often used to refer to long, cold intervals in Earth's history, during which there are large differences in temperature from the equator to the poles and glaciers advance and retreat over time scales of hundreds to millions of years. The Ice Age, when both words are capitalized, is commonly used to refer to the last major glaciation that occurred in North America and Eurasia ending about 10,000 years ago during the late Pleistocene epoch. Our modern climate represents a very short, warm period between glacial advances with ice sheets still present in both the Northern and Southern Hemispheres, and thus Earth is presently in an interglacial period.

Historical Background and Scientific Foundations

Four major periods of glacial advance and retreat, or ice ages, have occurred during the last billion years of Earth's history. The first occurred during the late Proterozoic eon (between 800 and 600 million years ago) and the second during the Pennsylvanian and Permian periods (between 350 and 250 million years ago). Somewhat less-extensive glaciations occurred during portions of the Ordovician and Silurian periods (between 460 and 430 million years ago). The current ice age began with the growth of the Antarctic ice sheet approximately 40 million years ago and intensified during the Quaternary period (the last four million years) with the expansion of ice sheets in the Northern Hemisphere.

Within each ice age, cycles of advancing and retreating ice sheets occur that are thought to be driven by regular, predictable variations in Earth's orbit and orientation relative to the sun. Each cycle consists of a long, generally cold phase, called a glacial period, during which the ice sheets slowly reach their maximum extent over tens of thousands of years followed by a relatively short, warm phase, called an interglacial period, during which the ice sheets rapidly retreat. Recent cycles of glacial/interglacial periods have occurred every 100,000 years.

During glacial periods, global sea levels drop due to the loss of water from the oceans in the formation of large glaciers and ice sheets. Lower sea levels cause the continental shelves to be exposed and land bridges are formed between landmasses, allowing animals to migrate long distances. During interglacial periods, melting ice-water returns to the oceans, causing sea levels to rise once again.

The factors considered important in causing long-term changes in Earth's climate, including the ice ages or glacial and interglacial periods, are several. They include: changes in the positions of the continents, variations in the energy output of the sun, changes in the concentrations of atmospheric gases and particulates such as carbon dioxide (CO₂) and methane (CH₄), changes in volcanic activity, the influence of the biosphere, and variations in Earth's orbit known as Milankovitch cycles.

In 1941, Serbian mathematician Milutin Milankovitch (1879–1958) identified three principal cycles that influence Earth's climatic variability. The longest of these cycles (100,000 years) concerns the planet's orbit around the sun. The second cycle (42,000 years) concerns the tilt of Earth on its axis. The third and shortest cycle (22,000 years) concerns the wobble of Earth on its axis. Only when continental drift brings large land surfaces to the poles can Milankovitch cycles cause ice ages. At that point, when the cycles are in the right configuration, mild summers and harsh winters allow large amounts of snow to fall at the poles until large ice sheets

are formed. However, the Milankovitch cycles are unable to explain these large-scale changes alone. Greenhouse gases are likely to play a crucial role. Computer modeling of Milankovitch cycles are unable to simulate the onset of an ice age without the concomitant reduction of atmospheric CO₂ in the Southern Hemisphere.

Impacts and Issues

Earth is now experiencing a warm phase, or interglacial period, that has lasted more than 10,000 years, longer than many of the previous warm intervals. If the pattern of glacial cycles still holds true, Earth should be due for the beginning of the next glacial period. Over the last century, however, average global temperatures have started to rise rather than fall. Scientists have recently concluded that much of the recent warming is due to the release of greenhouse gases from anthropogenic (human-generated) activities. There is a realistic possibility that continued human-caused global warming could disrupt or override the natural climate cycle of the current ice age.

William Ruddiman, an environmental scientist at the University of Virginia, proposed a theory in 2003 called the Early Anthropocene Hypothesis, which suggests that humans began to have a significant impact on Earth's climate as far back as 8,000 years ago. By charting the levels of CO₂ and methane in ice cores taken from the Greenland and Antarctic ice sheets, Ruddiman discovered that atmospheric concentrations of CO₂ and methane started rising long before the eighteenth century's advent of coal-burning factories and power plants, as was commonly assumed. At about 8,000 years ago, atmospheric greenhouse-gas concentrations stopped following the periodic pattern of rises and falls that were driven by natural variations in Earth's orbit.

For example, methane should have started declining 8,000 years ago and gone into rapid decline by 5,000 years ago if it were controlled by Milankovitch cycles. Methane is normally produced in large volume by swamps that form during warm, wet periods as opposed to dry, cold periods. However, methane concentrations have shown a slow, but emphatic rise since that time. It was wet agriculture, such as the creation of flooded rice paddies in eastern Asia, that has caused the production of methane gas to increase. Ruddiman gives a similar story for CO₂ increases and the destruction of forests for agricultural land starting around 8,000 years ago. In his overdue-glaciation hypothesis, Ruddiman states that the next glacial period of our present ice age would probably have started several thousand years ago, but that next phase has been forestalled by the activities of early farmers and now our present industrial era.

In 2004, a group of scientists in Europe challenged Ruddiman's hypothesis with information from ice cores taken at Dome C, Antarctica, which provides a climate record over the past 740,000 years. They determined that the glacial-to-interglacial conditions about 430,000 years ago resemble the transition into our present interglacial period in terms of greenhouse-gas levels and the magnitude of change in temperature. That interglacial period lasted for an exceptionally long 28,000 years. Our present interglacial period has lasted for 12,000 years. Thus, their results may imply that without human intervention, a climate similar to the one we have experienced could extend well into the future.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

CONTINENTAL DRIFT: A theory that explains the relative positions and shapes of the continents, and other geologic phenomena, by lateral movement of the continents. This was the precursor to plate tectonic theory.

GLACIAL: Related to glaciers, a multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.1 km² in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of freshwater on Earth and is second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

INTERGLACIAL PERIOD: Sometimes called simply an interglacial: geological time period between glacial periods, which are periods when ice masses grow in the polar regions and at high elevations. The world is warmer during interglacials. The world is presently experiencing an interglacial that began about 11,000 years ago.

A popular idea in the media is that global warming caused by human activities will cause another “ice age” (an abrupt return to a glacial period) by dramatically altering the North Atlantic ocean circulation. This would supposedly result in cooling over Europe, leading to the slow growth of glaciers and the onset of the glacial period. Andrew Weaver and Claude Hillaire-Marcel refute this idea in their 2004 article “Global Warming and the Next Ice Age” by explaining that global warming is unlikely to dramatically change ocean circulation and that elevated levels of atmospheric CO₂ cause high summer temperatures that preclude glacier formation and growth.

BIBLIOGRAPHY

Books

Flannery, Tim. The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth. New York: Atlantic Monthly Press, 2005.

Periodicals

EPICA Community Members. “Eight Glacial Cycles from an Antarctic Ice Core.” Nature 429 (2004): 623–628.

Ruddiman, William F. “The Anthropogenic Greenhouse Era Began Thousands of Years Ago.” Climatic Change 61 (2003): 261–293.

Shackleton, Nicolas J. “The 100,000-Year Ice-Age Cycle Identified and Found to Lag Temperature, Carbon Dioxide, and Orbital Eccentricity.” Science 289 (2000): 1897–1902.

Weaver, Andrew J., and Claude Hillaire-Marcel. “Global Warming and the Next Ice Age.” Science 304 (2004): 400–402.

Web Sites

“Ice Age in Depth.” Denver Museum of Nature and Science, December 5, 2007. <http://www.dmnh.org/main/minisites/iceage/ia_indepth/depth1.html> (accessed December 9, 2007).

“Ice Ages.” Illinois State Museum, November 27, 2007. <http://www.museum.state.il.us/exhibits/ice_ages> (accessed December 9, 2007).

Michele Chapman

Climate Change: In Context