Ice Ages

With recent discussions in the press concerning Global Warming, El Nino, and other climatic changes it is often forgotten that until around 10,000 years ago, large continent spanning ice sheets covered much of the Earth. Ice sheets thousands of feet thick once stretched as far south as Kansas and Illinois in the United States. The cause of the ice ages is still being debated, but the evidence is clear, the Earth has experienced several glacial periods in its history.

There have been at least three known periods of continental scale glaciation. The earliest evidence suggests that large glaciers once covered the land during Late Ordovician time (458 to 438 Ma), then again during the Late Carboniferous and Early Permian (about 300 to 280 Ma). The evidence for these ice ages is found in large glacial deposits, but due to their extreme age it is difficult to map their maximum coverage or determine their cause. The last ice age began around 2 to 3 million years ago and the debate is still fought over whether this latest ice age has ended or not.

An Ice Age is not just a period of colder temperatures and large ice sheets. In reality, the Ice Age is punctuated by alternating periods of colder and warmer climates. During the cooler periods the ice sheets advance to cover parts of the globe, and during the warmer periods the ice sheets retreat. It is estimated that the Earth was only 5 degrees Celsius cooler during the last ice age, and that the warmer interglacial periods were slightly warmer than average global temperature are today. (This changes the perspective on the global warming debate - are the increasing global temperatures a result of human activity or the natural warming process of the earth in an interglacial period. An intriguing question, but not the focus of this article.)

There is ample evidence for multiple glaciations in North America and Europe over the past 2 million years (the Quaternary Period). This evidence is found both on land and at sea. On land, glacial deposits in the forms of glacial till, outwash, and other features can be used to map the furthest extent of the glaciation. Unfortunately, as each new glaciation occurred, the newly advancing ice sheets destroyed much of the evidence of previous glaciations. Only in limited areas is there evidence of earlier glaciations. At sea, the evidence is more complete. Geologists have drilled into the ocean sediments and have looked at the presence and abundance of different species of microscopic plankton. Many of these plankton species are sensitive to temperature changes and their relative abundance is an indication of climatic change. Other evidence is found in the ratio of Oxygen isotopes. Like many elements, Oxygen exists in different isotopes, ¹⁸O, ¹⁷O, and ¹⁶O. Generally as ocean surface water temperatures are lowered, the water becomes enriched in the denser ¹⁸O isotope. During warmer periods, with warmer surface temperatures, the ¹⁸O isotope is removed from the water. By measuring the ratios of the ¹⁸O to ¹⁶O isotopes scientists can estimate the oceans temperature, and thereby global temperatures. (This is a simplification, and there is a lot of variation among the research of the different scientists working on the Oxygen isotope ratios.)

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