In the Pleistocene Ice Age, ice sheets originated not at the poles, but in the lower continents. The Laurentide ice sheet originated in the Hudson Bay, the Cordilleran ice sheet formed in the Canadian Rockies, the Finoscandinavian ice sheet started in Scandinavia, and the Alpine ice sheet emanated from the Swiss Alps [source: Erickson]. Ice sheets already present in Antarctica and Greenland thickened. But how did these ice sheets start? Scientists aren't sure what causes ice ages, or what combination of events might trigger another one.
It's not necessarily a matter of the world suddenly getting really cold, though. It's more that it doesn't get warm enough in the summer. In the 1920s, a mathematician named Milutin Milankovitch worked out why summers would be cooler by looking at the factors that limit sunlight's reach to Earth. He identified three factors: the tilt in the Earth's axis, the way the Earth wobbles on its axis and how close the Earth gets to the sun. By combining these factors in a mathematical formula, he was able to predict that ice ages would occur every 22,000, 41,000 and 100,000 years [source: Gosnell]. These rhythms became known as Milankovitch cycles.
It seems that once it's cold, then it's likely to stay cold. Ice has albedo, or reflectivity. A higher albedo results in less absorbed sunlight because it's reflected back. This causes temperatures to drop more, so that the growth of one glacier will likely trigger more glaciers [source: Gosnell]. With cooler summers, only a little snow in the winter would be needed to offset the minimal melting.
The movement of the Earth's plates, or plate tectonics, also appears to play a role. The position of a continent affects its climate. Landmasses at high altitudes and latitudes are more likely to be cold and provide the conditions for glaciers to form. The changes caused by plate tectonics also include uplift brought on by plate collision, mountain ranges formed by plates overriding each other and oceanic trenches created by plates moving away from each other [source: Skinner]. These slow movements change the Earth's composition and could bring on the climate conducive to an ice age.
Another theory centers on atmospheric gases. Studies of trapped air from glacial ages have indicated that carbon dioxide and methane gases were at lower levels [source: Skinner]. When these greenhouse gases are abundant, they trap energy and keep it close to Earth, thus keeping the planet warm. When these gases aren't present, that radiant energy escapes. Scientists don't know exactly why the levels of these gases fell, but it does appear to factor in to the magnitude of temperature changes [source: Skinner].
Other atmospheric changes are also likely in play. Studies indicate that there may have been an unusually high amount of dust in the air during the Pleistocene Ice Age, which would have blocked the sun and kept the Earth's temperature cool [source: Skinner]. Similarly, volcanic eruptions emit ash into the air. Even after the dust settles, the chemicals produced by the ashes' interaction with water vapor also scatter the sun's radiation and prevent it from reaching the Earth's surface. It does appear that periods of intense volcanic activity preceded glacial ages.
Low sunspot activity may also be a predictor of an ice age. A sunspot is a cool, dark spot on the sun that contains magnetic energy. It sounds counterintuitive, but although sunspots are cooler than the rest of the sun, their presence keeps it warmer on the Earth because their resulting magnetic field cuts cloud cover. With lower sunspot activity, cloud cover increases, cutting off the sun's rays from the Earth.
An Australian scientist claims that sunspot activity has been low recently. Does that mean we're headed for another cold period? Find out next.