Ice ages occur when ice sheets cover at least the polar areas of the Earth, with cycles of glacials and interglacials lasting 40,000 and 100,000 years. The last glacial period began around 70,000 BC and ended between 15,000 and 10,000 BC. Scientists are uncertain about the causes of ice ages, but variables include atmospheric composition, Earth’s orbit, albedo, and solar output. We are currently in an ice age that began 40 million years ago. During ice ages, global sea levels are lower, opening up large sections of land.
An ice age is a period of time, usually about 30 million but occasionally up to 300 million years, during which ice sheets cover at least the polar areas of the Earth. Individual ice ages have subglacial ages, called glacials (when cold) or interglacials (when warmer) that operate in cycles of 40,000 and 100,000 years. When the term “ice age” is used colloquially, it often refers to these shorter glacials, periods in which ice sheets extend significantly beyond the poles and into the heart of continents such as North America and Eurasia. In this sense, “the last ice age” refers to what is formally called “the last glacial period”, which began around 70,000 BC and ended between 15,000 and 10,000 BC This is the ice age experienced by the primitive man.
Scientists can’t say exactly what causes ice ages, although there are a number of implicated variables at play. These include atmospheric composition (greenhouse gases), slight changes in Earth’s orbit known as Milankovitch cycles, Earth’s albedo (reflectivity), changes in the location and amount of crust at different points on the Earth’s surface, variations in solar output, large meteor impact, the release of methane clathrates and supervolcanism. Short-term cycles (40,000/100,000 years) are known to be caused by changes in the Earth’s orbit.
As ice covers Antarctica and Greenland, we are in the midst of an ice age, which began 40 million years ago. Ice ages are quite atypical circumstances for the Earth; aside from six ice ages, the Earth’s poles have been largely ice-free. Fossils of trees have been found in terrain that was only a few hundred miles from the poles at the time they lived. Ice ages are as rare as mass extinctions, which occur once every 100 million years or so.
The typical average global temperature when the Earth is not in an ice age is about 22°C (71.6°F). During an ice age, it drops about 10°C to an average of 12°C (53.6°F). At the poles, the temperature is practically always well below freezing.
During ice ages, large amounts of water are locked up in ice sheets, lowering global sea levels. During the most recent Ice Age, global sea level was about 100 feet lower than it is today, opening up large sections of land such as the North Sea and connecting Papua New Guinea to mainland Southeast Asia and Russia to Alaska across the Bering River. Continental Bridge. Due to the ice age, our ancestors could cross the Americas. It would be more than 10,000 years before humans who traveled to the Americas were reunited with their distant cousins from Europe, Asia and Africa.
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