Earth & Space Science: Session 6
A Closer Look: Ice Ages
What are ice ages?
Ice ages are recurring periods in the Earth's history, usually thousands or tens of thousands of years in length, when the entire Earth experiences colder climatic conditions. During these periods, enormous continental glaciers called ice sheets cover large areas of the Earth’s surface. Ice ages are separated by warmer periods called interglacial periods. Several ice ages have occurred throughout our planet's history. The last ice age peaked about 18,000 years ago, after which the Earth again began to warm.
What causes an ice age?
Ice age intervals seem to correspond with long periods of worldwide climatic cooling. It should be noted that there are numerous factors that affect the Earth's climate. There are two main natural factors, however, that are thought to influence long-term changes in the Earth's climate.
- Changes in the positions of the continents are linked to periods of multiple glaciations. The presence of large landmasses at the poles seems to trigger the development of extensive ice sheets. For example, throughout the Ice age that occurred during the Pennsylvanian and Permian geologic eras (250 –350 million years ago), the southern portion of the super-continent Pangaea was at the South Pole. Scientists believe this may have caused extensive glaciation over what are now Africa, South America, India, Antarctica, and Australia.
- Major uplift at continental plate boundaries can cause profound changes in oceanic and atmospheric circulation patterns. Changing circulation patterns cause climate change. Some scientists hypothesize that climatic changes caused by uplift are critical to the development of ice ages.
- Large or numerous volcanic eruptions, also the result of plate tectonics, release huge amounts of gases (carbon dioxide and water vapor) that act to trap heat within the atmosphere, causing global warming (sometimes referred to as the greenhouse effect). In contrast, particles of ash can impact the global climate by reducing the amount of solar radiation reaching Earth's surface, causing global cooling.
Named after the Serbian mathematician Milutin Milankovich, the Milankovitch cycles describe variations in Earth’s position as it orbits the Sun that affect the amount of sunlight it receives. One cycle involves the shape of Earth’s orbit, which varies from being more elliptical to more circular. Another cycle involves Earth’s “tilt” on its axis, which is not perpendicular to the plane of its orbit around the Sun. Presently, Earth tilts 23.5¾ from the perpendicular, but there is evidence that in the past this angle varied from 22¾ to 24¾. A third variation involves Earth’s “wobble.” Like a top wobbling on its axis, Earth wobbles as it orbits. The results are that sometimes the North Pole points to the North Star, and sometimes it points to the star Vega. These cycles vary from tens of thousands to hundreds of thousands of years and can be traced back through the Earth's geologic record.
When did the ice ages occur?
Several ice ages have occurred throughout our planet's history. Major periods of glaciations occurred during the late Proterozoic Era (between 600 and 800 million years ago), during the Pennsylvanian and Permian Eras (between about 250 and 350 million years ago), and the late Neocene to Quaternary Eras (the last 4 million years). Somewhat less extensive glaciations occurred during parts of the Ordovician and Silurian Eras (between about 430 and 460 million years ago). The most recent ice age began about 1.8 million years ago during the Pleistocene Era. During this time, giant ice sheets advanced and retreated many times in North America and Europe.
Recent cycles of advancing and retreating ice sheets have occurred approximately every 100,000 years. Each cycle consists of a long, generally cold period during which the ice sheets slowly reach their maximum size, and a relatively short, warm period during which the ice sheets rapidly retreat.
We are now in a warm period that has lasted more than 10,000 years, which is longer than many of the previous warm intervals. If the pattern of glacial cycles holds true, scientists believe the Earth is soon due for another cold period. In the 1800s, global temperatures began decreasing during a period known as the Little Ice Age. Currently, patterns indicate that the Earth is nearing the end of an interglacial period, meaning that another ice age is predicted in a few thousand years.
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