Ice Ages
Ice Ages
Introduction
Ice ages are periods of time when large parts of Earth’s surface, especially near the poles, are covered with glaciers and ice sheets (continent-sized glaciers). Ice sheets and glaciers are slow-moving bodies of ice formed from snow that has fallen and compacted over thousands to millions of years. The term “ice age” is technically used to refer to long, cold intervals in Earth’s history, also called glaciations, during which glaciers advance and retreat over millions of years. These long, alternating periods of global warmth and cold are interrupted by colder periods termed glacials and warmer periods termed interglacials, which typically last some tens of thousands of years. Glacials and interglacials, in turn, can be interrupted by shorter cold periods a few thousand years long called stadials, which alternate with warmer intervals called interstadials. Somewhat confusingly, long-term ice ages, glacials, and stadials are all often referred to as “ice ages.” The phrase “the Ice Age,” when both words are capitalized, commonly refers to the last major glacial, which ended about 10,000 years ago. Our modern climate represents a relatively warm, climatically stable period between glacial advances.
Many Earth environments can alter radically during ice ages. Hundreds of millions of years ago, the entire planet may have been frozen over almost entirely more than once, an extreme condition termed Snowball Earth. More recently, glaciers have expanded to cover most of what is now Europe, northern Asia, and northern North America. Such geographical figures as lakebeds and hills are shaped by glacial ice in areas that are covered by glaciation. During extreme ice ages, sea levels can drop by many yards/meters, as water is removed from the oceans by evaporation, falls on land as precipitation, and is locked up there as glacial ice.
Historical Background and Scientific Foundations
Four major periods of glacial advance and retreat termed ice ages or glaciations 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, which continues, 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 or simply a glacial, 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 or so. Glacials are interrupted by warmings and coolings of global climate lasting only one or two thousand years, with cool periods called stadials alternating with warm periods called interstadials.
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 icewater 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
WORDS TO KNOW
ANTHROPOGENIC: Made by humans or resulting from 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: Pertaining to glaciers or ice sheets.
ICE CORE: A cylindrical section of ice removed from a glacier or ice sheet in order to study climate patterns of the past.
INTERGLACIAL: Occurring between periods of glacial action.
reduction of atmospheric CO2 in the Southern Hemisphere.
Impacts and Issues
Earth is now experiencing a warm phase, or interglacial, that has lasted more than 10,000 years. Global tempera-
tures have been remarkably stable during this period. Over the last century, moreover, average global temperatures have started to rise rapidly. Scientists have recently concluded that most of this recent warming is due to anthropogenic (human-released) greenhouse gases such as CO2. It is possible 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. However, methane concentrations have shown a slow but significant rise since that time. Ruddiman argues that it was wet agriculture, such as the creation of flooded rice paddies in eastern Asia, that caused the production of methane gas to increase. Ruddiman suggests 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. Ruddimann’s proposals are taken seriously by other climate scientists but are not agreed upon as fact by the majority of climate scientists due to insufficient evidence.
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.
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. Other scientists emphasize that dramatic changes in Atlantic circulation with attendant cooling of Northern Europe cannot yet be ruled out; however, a sudden, hemispheric plunge into ice age conditions is not likely.
See Also Climate Change; Glacial Retreat; Glaciation; Global Warming; Ice Cores
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
Denver Museum of Nature and Science. “Ice Age in Depth.” http://www.dmnh.org/main/minisites/iceage/ia_indepth/depthl.html (accessed April 1, 2008).
Illinois State Museum. “Ice Ages.” http://www.museum.state.il.us/exhibits/ice_ages (accessed April 1, 2008).
Michele Chapman