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Ice ages are times in earth history when earth’s climates were appreciably colder than normal and glaciers covered significantly larger areas of the earth compared to today. Ice ages represent conditions associated with the extreme cold periods and global cooling of an otherwise normally fluctuating global climate system. During an ice age, ice sheets, which normally occur only in high latitude areas close to the north and south poles, extend farther into the lower latitudes. Alpine glaciers, which normally exist at higher elevations, extend to much lower elevations during ice ages.
Ice ages occurred as far back as the Precambrian, and occurred sporadically until the present. Major ice ages occurred during the late Precambrian (800-600 million years ago); late Ordovician and early Silurian (460-430 mya); Pennsylvanian and Permian (350-250 mya); in addition to the Pliocene and Pleistocene (last 3 million years). The most recent Ice Age is the most well-known. The Pleistocene Ice Age, and probably also the ancient ice ages, had several glacial and interglacial episodes. Many consider the current warm global climate episode as being an interglacial episode following the last glacial episode.
Evidence of the Pleistocene Ice Age is found in the many glacial landforms in different parts of the world. For example, in the United States, glacial moraines that can be found as far south as Long Island, NY, and extending along a line that generally passes westward through most of the northern tier of states. North of this line, areas are covered with glacial tills and moraines, and bedrock surfaces are marked by glacial striations. This indicates that areas north of this line, including the northern tier of the United States, almost all of New England, and all of Canada were covered by an ice sheet. Similar Pleistocene glacial features are found on other Northern Hemisphere continents. In the Southern Hemisphere, the only continents close enough to the South Pole to be glaciated were Antarctica and Tierra del Fuego at the southern tip of South America. Ice ages older than the Pleistocene have been recognized based on more subtle evidence. Glacial tillites are interbedded with fossiliferous marine deposits or other deposits that can be dated, thereby establishing the age of glaciation.
Glacial deposits of Permian age are found on several continents. When these continents are viewed in the Pangean position (during the Permian), the glacial deposits cluster around the paleo-south pole.
In the oceans, seawater contains a certain ratio of two oxygen isotopes (O16), and the heavier isotope (O18). During an ice age, when seawater is cooler, the evaporation ratio of these two isotopes is different than during warmer intervals. Therefore the oxygen isotope ratio of seawater, as well as minerals and shells that precipitate from seawater during glacial intervals, reflect this isotopic difference. As such, oxygen isotope ratios of ancient fossils and minerals may serve as a proxy for recognizing ancient ice ages.
There are several possible causes for global cooling. Milankovich cycles are oscillations in global temperatures that are caused by variations in the rotation of the earth and the revolution of the earthsun system. Global temperatures are warmer during times when there is more carbon dioxide (CO2) in the atmosphere (greenhouse conditions), and cooler when CO2 levels are lower (icehouse conditions). Global temperatures are cooler when the atmosphere contains a lot of particulate matter from volcanic eruptions or meteorite impacts. “Glaciation” may also be more prevalent during times in earth history when continents drift into polar locations. It is thought that the Ice Ages may correspond to times when several of these causal factors coincide.
During an ice age, or even during times when global climates are generally cooler, several effects may result. Sea level drops as water in the global water budget shifts from the oceans to glaciers. During the Pleistocene, sea level was approximately 200 meters lower than today and most of the continental shelves were exposed as coastal plains. Because terrestrial and marine temperatures become cooler over the entire globe during an ice age, global climate belts shift toward the equator. Similarly, terrestrial and marine species and ecosystems shift toward the equator during ice ages. Atmospheric and oceanic circulation patterns-especially those that are driven by temperature or density differences – change during an ice age.
- John Imbrie and Katherine P. Imbrie, Ice Ages: Solving the Mystery (Harvard University Press, 2005);
- Douglas Macdougall, Frozen Earth: The Once and Future Story of Ice Ages (University of California Press, 2004);
- Paul Martin, Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America (University of California Press, 2005).