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Ice Age..... When the Earth was covered in ice...

If you are a movie fan, you must have heard or seen the popular animated movie 'Ice Age'. It's a story about a mammoth, a sabre toothed tiger and a sloth trying to survive the advent of the ice age.

Sure, most of us enjoyed the movie very much, but did anyone of you query what an ice age is?

An ice age is a period of long-term downturn in the temperature of the Earth's climate. This results in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers.

Ice age is often used to mean a period when ice sheets exist in the northern and southern hemispheres; by this definition, we are still in an ice age (because the Greenland and Antarctic ice sheets still exist).More colloquially (used in informal speech), when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and Eurasian continents. In this sense, the last ice age ended about 10,000 years ago.

This article will use the term ice age in the former, glaciological, sense; and use the term glacial periods for colder periods during ice ages and interglacial for the warmer periods.

Many glacial periods (colder periods) have occurred during the last few million years, initially at a 40,000-year frequency, but more recently at 100,000-year frequencies. These are the best studied. There have been four major ice ages in the distant past.

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Origin of ice age theory

According to the Wikipedia encyclopaedia, the idea that, in the past, glaciers had been far more extensive was folk knowledge in some Alpine regions of Europe. No single person invented the idea.

At this early stage of knowledge, what was being studied were the glacial periods within the past few hundred thousand years, during the current ice age. The existence of ancient ice ages was as yet unsuspected.

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There have been at least four major ice ages in the Earth's past. The earliest hypothesised (assumed) ice age is believed to have occurred around 2.7 to 2.3 billion years ago, during the early Proterozoic Age.

The earliest well-documented ice age, and probably the most severe of the last one billion years, occurred from 800 to 600 million years ago (the Cryogenian period) and it has been suggested that it produced a Snowball Earth in which permanent sea ice extended to or very near the equator. There were extensive polar ice caps at intervals from 350 to 260 million years ago, during the Carboniferous and early Permian Periods, associated with the Karoo Ice Age.

The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene (starting around three million years ago) with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period ended about 10,000 years ago.

The timing of ice ages throughout geologic history is in part controlled by the position of the continental plates on the surface of the Earth. When landmasses are concentrated near the polar regions, there is an increased chance for snow and ice to accumulate.

Small changes in solar energy can tip the balance between summers in which the winter snow mass completely melts, and summers in which the winter snow persists until the following winter. Due to the positions of Greenland, Antarctica, and the northern portions of Europe, Asia, and North America in the polar regions, the Earth today is considered prone to ice age glaciations.

Evidence for ice ages comes in various forms, including rock scratchings, glacial moraines (mass of stones carried and deposited by a glacier), drumlins (usually oval ridges formed under the ice sheet of a glacial period), valley cutting, and the deposition of till or tillites (a type of clay) and glacial erratic (unevenness). Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. It took some time for the current theory to be worked out.

In between ice ages, there are multi-million year periods of more temperate, almost tropical, climate, but also within the ice ages (or at least within the last one), temperate and severe periods occur.

The colder periods are called 'glacial periods', the warmer periods 'interglacials', such as the Eemian interglacial era. We are in an interglacial period now, the last retreat ending about 10,000 years ago.

But, what causes ice ages? The cause of ice ages remains controversial for both the large-scale ice age periods and the smaller ebb and flow of glacial/interglacial periods within an ice age. The general consensus (widespread agreement) is that it is a combination of upto three different factors: atmospheric composition (particularly the fraction of CO2-Carbondioxide and methane), changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy), and the arrangement of the continents.

The first of these three factors is probably responsible for much of the change, especially for the first ice age. The "Snowball Earth" hypothesis(suggested explanation of something) maintains that the severe freezing in the late Proterozoic was both caused and ended by changes in CO2 levels in the atmosphere. However, the other two factors do matter.

An abundance of land within the Arctic and Antarctic Circles appears to be a necessity for an ice age, probably because the landmasses provide space on which snow and ice can accumulate during cooler times and thus trigger positive feedback processes like albedo (a measure of the reflecting power of an object, expressed as the proportion of incident light it reflects) changes.

The Earth's orbit does not have a great effect on the long-term causation of ice ages, but does seem to dictate the pattern of multiple freezings and thawings that take place within the current ice age.

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