Ice Ageswhat They Are And Why They Occur

Ice ages occur when temperatures remain cool for extended lengths of time. When long intervals of cold prevail, the ice does not melt; instead, ice caps and glaciers are able to advance, extending into lower latitudes, farther from the polar regions. Scientists have discovered evidence for this worldwide. For instance, during the last ice age 20,000 years ago, ice sheets advanced from the North Pole south to cover Canada, the New England states, the upper portions of the Midwest, Alaska, Greenland, Iceland, Scandinavia, most of Great Britain and Ireland, and the northwestern portion of Russia. Evidence confirming these events in the form of glacial landforms and deposits still exists today and is studied by climatologists worldwide.

When climatologists look backward in time and piece together evidence of Earth's past climate, they are able to identify the cyclic nature of glacial periods. Currently, they have determined that over the past millions of years, there have been multiple ice age cycles. Each glacial cycle is separated by a warmer period—referred to as an "interglacial period." During the Phanerozoic (the past 545 million years), Earth has passed through eight great climate cycles, each lasting 50 to 90 million years. There were also ice ages 52, 36, 20, 15, 7, 5, and 3 million years ago. During the past million years alone, there were eight to 10 ice ages, each only about 100,000 years long, interspersed with short, warm interglacial periods about 10,000 years long. Earth is currently experiencing an interglacial climate.

Each glacial period varies in intensity; they are not all alike. Ice ages can last thousands to millions of years and involve major ice sheets that cover continents and also include short intervals when glaciers reach their maximum extents, called "glaciations."

Scientists have determined that over Earth's past billion years, there have been several glacial advances and retreats. They are grouped into the following four distinct time intervals:

• Late Proterozoic 800-600 million years ago

• Ordovician and Silurian 460-430 million years ago

• Pennsylvanian and Permian 350-250 million years ago

• Late Neogene to Quaternary 4 million years ago

During these four periods, many glacial advances and retreats occurred. These advances and retreats involved continent-sized ice sheets. Scientists have proposed several physical mechanisms that

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Proterozoic Ordovician & Pennsylvanian & Silurian Permian

Four Geological Times of Glaciations

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Neogene& Quaternary

Major periods of glaciation occurred four times during the geologic past. During these intervals, there were many glacial advances and retreats. Earth is currently in a short and warm "interglacial"period.

control the onset, duration, intensity, and ending of glacial events. Included are a wide range of diverse contributions: changes in Earth's orbit, referred to as the "Milankovitch cycles"; changes in plate tectonics; reductions in carbon dioxide (CO2); continental uplift; volcanic activity; solar output; and surface albedo. It is the combination of any and/or all of these cyclic factors and their influence on Earth's surface that trigger glacial and interglacial episodes. It is important to understand how each of these triggers relates to and impacts climate change and global warming.

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