When you think about an "ice age", perhaps the popular children's animation franchise comes to mind, and well, you wouldn't be wrong. The Ice Age films take place during the Paleolithic ice age and its depiction of life during the glacial period is pretty realistic, for the most part.
An ice age is marked by very cold global temperatures and glacial expansion that can last hundreds of millions of years. Throughout the history of the Earth, scientists have agreed that there were at least five significant ice ages, the first of which is dated to around two billion years ago, and the latest is believed to have begun around three million years ago. They are followed by warmer interglacial periods, where temperatures were similar to those of today's time. That's right! We are living through a warm interglacial period that began nearly 11,000 years ago. At its peak, global temperatures dropped by approximately 9° F and local temperatures dropped by almost 40° F. Some 20 thousand years ago, in cities like Chicago, Stockholm, and Glasgow, ice was more than 1 km thick, which really highlights the drastic differences in climate. Since much of the Earth's water is frozen in glaciers and other ice, the sea level generally dropped by 400 feet.
The cause of ice ages is complex and there are several different factors. However, it does have a lot to do with the moving continents. Continental drift leads to the slow movement and spreading of the earth's continents, as they collide and drift apart. Specifically, when the continents collide and mountains form, it allows for the accumulation of more snow, and more snow means higher albedo, which is the proportion of light reflected from a surface, which subsequently cools the Earth. As the ice weathers down rocks, minerals from the rocks mix with the CO2 in the atmosphere, thus reducing the greenhouse effect and cooling down the earth. Continental drift also plays a really big role in the movement of the earth's oceans and the flow of the water, which thus affects regional and global climate. However, as the continents continue to move, they can also drift away from each other and weathering eventually wears down the mountains, and the climate generally returns to its previous state. Because continental drift takes place extremely slowly, all of these other processes also occur on a long-term scale.
Paleorecords hint that during the last ice age, the circulation of the ocean's waters was less stable than it has been during the last 10,000 years. This is clearly a result of differences in sea ice coverage, precipitation, other climate patterns, etc. Furthermore, according to an examination of the carbon isotopes in sequences of earth that were laid down deep into the previously existing oceans from 550 million to 750 million years ago, there is evidence that the entirety of the earth, including the land and the oceans, was frozen over- at least twice. The earth was in this state for millions of years, but what happened to life? Harvard geologist and geochemist, Paul Hoffman and Daniel Schrag analyzed these rock sequences and concluded that the ocean was biologically dead and frozen, though the interior of the planet functioned fine.