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What are ice ages? -

Ice ages are extended periods of time, spanning millions of years, in which significant portions of the Earth are covered in ice sheets and glaciers.

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How many ice ages were there? -

As far as scientists can gather, there have been five major ice ages since the Earth was formed. Within these extended ice ages, there are smaller cold snaps called glaciations, or glaciation periods, that can be separated by uncharacteristically warm periods within an ice age, known as interglaciations.

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What can cause an ice age? -

All in all, ice ages are seen as a natural part of the Earth's cycle, and can be caused by a number of factors. Everything from the Earth's distance from the Sun, movements in the tectonic plates, and the levels of carbon dioxide in the atmosphere can contribute to an ice age.

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The slow carbon cycle -

The Earth's slow carbon cycle takes hundreds of millions of years. Carbon moves from the atmosphere, where it takes the form of greenhouse gases, through the oceans, where it slowly sinks to the sea floor and is pressurized into rock, until it eventually returns back to the atmosphere through volcanic eruptions. The rock stage in this cycle, when carbon is sequestered within rocks and therefore does not contribute to the warming of the planet, is often cited as a common cause for ice ages and glaciations.

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The position of the Earth -

Everyone knows that the Earth's relative position to the Sun causes the seasons and weather to change on a yearly basis, but the Earth's rotation, tilt, and wobble can have much larger effects on the world over a longer period of time. Earth's tilt sways from 22° to 24° every 40,000 years; the Earth also wobbles on its axis, making a full rotation in this manner every 23,000 years. When these two factors align in just the right way, Earth can remain far further from the Sun than what we are used to, ushering in a prolonged period of cold and glaciation.

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One side of two extremes -

Naturally, as ice ages are a normal part of the Earth's lifetime, affected by changes in the Earth's atmosphere and its position relative to the Sun, there is of course an opposite extreme. These are known as greenhouse periods, and scientists have found evidence of ancient species of alligators swimming in the waters of the North Pole, and even palm trees growing on the sunny beaches of Antarctica.

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The Huronian ice age -

The first ice age in recorded history is known as the Huronian ice age, and took place about 2.4 billion years ago, during the Proterozoic era. The Huronian ice age, which is actually a collection of shorter glaciation periods stretched that continued until 2.2 billion years ago, was likely caused by the Great Oxidation Event, a period in history when the thin atmosphere and shallow oceans of the Earth experienced a massive influx of oxygen.

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Snowball Earth -

The Huronian ice age likely caused a rare phenomenon known as a snowball Earth, where the entire globe was covered in snow and ice, from the poles to the equator. This is only thought to have happened a handful of times in the history of the planet.

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The Cryogenian ice age -

The Cryogenian ice age began about 850 million years ago and ended 200 million years later. The Cryogenian period included two of the most severe and significant glaciations in history, the Sturtian and the Marinoan glaciations. It is thought that both of these events caused either a snowball Earth effect, or a "slushball Earth" effect, which indicates a thin band of unfrozen water wrapping around the equator.

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The Andean Saharan ice age -

It might seem strange to see the Andes mountain region and the Sahara desert mentioned in the same geologic event, and even stranger to see the Sahara associated with an ice age, but 460 million years ago the areas now known as South America and West Africa were located at the poles of the globe, far from the equator.

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The Karoo ice age -

The Karoo ice age that started 360 million years ago and went on until 255 million years ago was caused by massive shifts in the Earth's tectonic plates. When the ancient continents of Laurasia and Gondwana combined to form Pangaea, the warm water passage of the Rheic and Iapetus oceans was severed, causing the world's two major oceans to rapidly cool down, eventually triggering an ice age.

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Warm ice ages -

Even though the world might be covered in ice and snow during an ice age, that doesn't necessarily mean it's always unbearably cold. In some cases, like the Karoo ice age, after the initial freeze temperatures can rise to relatively normal again. However, the thick glaciers and layers of ice prevent the Sun's heat from warming the Earth's surface, and instead deflects the heat right back into the atmosphere.

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Quaternary ice age -

The Quaternary ice age began 2.6 million years ago, and, if you can believe it, is still going on today. By definition, an ice age is a period of widespread glaciation on Earth, and the entire continent of Antarctica, along with the ice sheets and glaciers of the Arctic Circle and North America, provide more than enough glaciation to warrant the term "ice age." For the past 12,000  years, however, we have been in an interglacial period, or a warm stretch that can occur within an ice age.

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The Ice Age -

While there have been five major ice ages, there is one particular period of time that is commonly referred to as the Ice Age. Interestingly, the Ice Age wasn't a full ice age at all, but simply the most recent glaciation period of the Quaternary ice age, which began around 120,000 years ago and ended nearly 12,000 years ago.

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Tectonic movements -

The Ice Age was caused largely by a relatively tiny strip of land known as the Isthmus of Panama. Some 120,000 years ago, the Isthmus of Panama emerged from the ocean, cutting off the warm, tropical currents between the Atlantic and Pacific oceans. This trapped warm water moved northwards, evaporated, and came back down to Earth as massive amounts of snow in the Northern Hemisphere that accumulated and eventually turned into massive ice sheets.

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The Last Glacial Maximum -

The Ice Age reached its peak around 20,000 years ago, in what is called the Last Glacial Maximum, or LGM. At the time of the LGM, global ocean levels were an astonishing 400 feet (122 m) lower than they are today.

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Ice age flora -

Contrary to popular belief, the world of the Ice Age wasn't a barren wasteland devoid of anything green. In fact, there were many species of hardy vegetation that thrived in the cold temperatures and plentiful sunshine. Grasslands and forests of pine and birch thrived during the Ice Age.

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Ice age megafauna -

Maybe the most exciting aspect of the Ice Age was the great and magnificent species of megafauna that roamed the Earth. In fact, most megafauna thrived during the Ice Age, since most were mammals that had already evolved in cold, mountainous regions.

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Woolly mammoths -

The stars of the Ice Age, woolly mammoths, were perfectly equipped for the cold world they found themselves in. Their warm, furry hides and their useful fat storages kept them energized and comfortable through the unending winter. In fact, it wasn't until the Earth began to warm again that the change in climate caused many of the herbivore mammoths' main food sources to go extinct, consequently leading to the extinction of the mammoths themselves.

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Smaller animals thrived, too -

It wasn't just the giants and the beasts who survived the Ice Age, though. Much smaller, energy-efficient animals like mice and thrushes also faired well, spending large parts of their lives warm in underground igloos and subsisting off of minimal vegetation.

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Why did the megafauna die out? -

The fate of the woolly mammoth was the fate of many megafauna. The change in vegetation caused by the world's warming caused mass extinctions amongst the herbivores, which in turn left many of the large omnivores and carnivores with nothing to eat themselves. This, paired with the constant threat posed by the rapidly developing humans, caused most of the world's megafauna to die out relatively quickly after the Ice Age.

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Human development -

Perhaps the Ice Age's most resilient species of all were the humans. Necessity begat innovation in many hunter-gatherer societies who learned how to craft clothing out of animal skins, construct large, insulated buildings out of mammoth tusk and other materials, and developed a heightened capacity for cooperation in order to survive in the harsh environment.

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Age of migration -

The Ice Age also saw new chances for exploration open up for these primitive humans. It was around 16,500 years ago, just after the LGM, that the first people crossed the Bering land bridge, which was only made possible by the drastically low ocean levels. This was the beginning of human life in the Americas.

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The end of Homo erectus -

While Homo sapiens adapted and even thrived during the Ice Age, the same cannot be said for their cousins, Homo erectus. Already weakened as a species by frequent clashes with modern humans, the changing climate and H. erectus' inability to adapt put the final nail in their coffin. This species of archaic human went extinct a little over 100,000 years ago, at the beginning of the Ice Age.

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Earthworm extinction -

Another curious extinction event caused by the Ice Age was the complete and total eradication of earthworms from the North American continent. At the beginning of our current interglacial period, around 12,000 years ago, as glaciers began to loosen and slowly grind down the continent, they tore up the land beneath them. As a result, every last earthworm in North America fell victim to the slow destruction of the ice sheets. It wasn't until the 1600s that European colonists inadvertently reintroduced earthworms to the continent.

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Remnants of the ice ages -

The effects of these massive melting events can be seen all over the world. Even glacier movements from the very first ice age, the Huronian ice age, can be seen with the naked eye in Canada. As the ice sheets of old slowly began to loosen, they would tear down the mountainsides, leaving deep gouges in the rocks known as glacial striations.

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If these glaciers could talk -

Glaciers and their melting have shaped the world in incredible ways. Some of the world's most famous bodies of water, from Scotland's Loch Ness to North America's Great Lakes, were formed by melting glaciers filling up previously dry basins.

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Louis Agassiz -

How do we know all this about ice ages, anyway? Well, it all started with the Swiss-American naturalist Louis Agassiz. It was Agassiz who hypothesized the past occurrence of a global ice age, and is credited with the founding of the field of glaciology.

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The next ice age -

When the next ice age, big or small, might hit Earth is a point of contention within the scientific community. Studying the Earth and the Sun's natural cycles, some say it is possible that we will enter a mini ice age as early as the year 2030. Most scientists believe, however, that the incredible amount of greenhouse gases caused by human civilization has completely thrown the natural cycle out of sync, and could delay the next ice age for as long as 500,000 years.

Sources: (Mental Floss) (History) (Facts Legend)

Sources: How extreme weather is affecting our world

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