How the Ice Age Worked

In 1785, James Hutton, the father of geology, voiced the idea that the present holds the key to the past. This adage meant that though a major glaciation hadn't covered the world for tens of thousands of years, it had left behind clues to its character and activity. What did the rounded hills known as drumlins have to do with the ice age? Where did these erratic boulders come from?

Scientists like Louis Agassiz were familiar with glaciers, or snow that compacts so tightly that the bottom layer turns to ice. When the boulders in the Jura Mountains in Switzerland were traced back to the Alps, 50 miles (80 kilometers) away, glaciers explained these geologic anomalies that covered Europe and North America. What started as anomalies ended up as insights into what the ice age was like.

The ways in which some rocks were polished smooth and why some showed different layers allowed geologists to measure how thick the glaciers and ice sheets were. Using grooves on the sides of mountains and layers in the rocks, Agassiz and other scientists were able to determine that the glaciers and ice sheets present during the ice age were about 1 mile (1.6 km) thick.

This evidence of glacial activity showed just how much ice there was -- about one-third of the world was under thick ice, for a grand total of 17 million cubic miles (71 million cubic km) of glacial ice. Antarctica, which already had an ice sheet, had 10 percent more ice than it does now.

What really set the ice age apart was the amount of ice in the Northern Hemisphere. In North America, ice covered Canada south through the central United States, extending from New York to Washington State. In Europe, ice covered Scandinavia, Ireland, Germany and western Russia. In North America alone, glaciers covered 10 million square miles (26 million square km), or about 13 times the area that they cover today. To form these massive ice sheets, the water was sucked out of the oceans, causing sea levels to drop about 350 feet to 400 feet (107 meters to 122 meters).

The glaciers weren't static. In fact, they've often been described as bulldozers. They advanced and receded in an undulating motion, leaving behind piles of rocks and other glacial till (natural debris that glaciers leave behind). And even though the ice wasn't everywhere, the glaciers affected the rest of the continent. The outskirts of the glaciers turned into arctic deserts, and a wind of dust called loess covered the land, created by the grinding motion of the moving glaciers.

The glaciers also preserved the fossils of plants and animals who lived through this chilly time. The global temperatures during the last major ice age were about 10 degrees Fahrenheit (5.6 degrees Celsius) lower than they are today. It may not sound like much, but when we look at the adaptations that animals of the time made, we know it must have been cold. Based on fossil evidence, we know that woolly mammoths, bison, wild horses, musk oxen, caribou, lions, antelope and the short-faced bear all roamed the land. They adapted to the cold temperatures by storing up fat reserves and growing specialized coats. The musk ox, for example, has shaggy hair two feet (0.6 m) long and underwool that is the most effective insulator of any animal fur.

How did it get so cold that animals needed hair a few feet long? How do ice ages start anyway? We'll tackle some of the theories on the next page.

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. 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. 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. 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. 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.

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. 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.

In an age where global warming is on everyone's mind, could we still really be in an ice age or headed for another? After all, most of the ice melted and the woolly mammoths are gone, but there are some remnants in the ice sheets that cover Antarctica and Greenland and that border Alaska and the Yukon Territory. That mixed evidence makes sense if you remember that ice ages are made up of both icy periods (known as glacials), and intervening warm periods (interglacials). We're currently in an interglacial period. Historically, interglacial periods last approximately 10,000 years, which is about how long this one has lasted.

There was a "Little Ice Age" in the Northern Hemisphere from A.D. 1300 to A.D. 1850 [source: NASA]. In this period, Holland's canals routinely froze over. But this ice age is considered "little" because it lacked a global punch. There's no precise definition of an ice age, which makes it difficult to know when another glacial period has officially started. One scientist has suggested that to qualify as an ice age, ice sheets must cover 400,000 square miles (1,035,995 square km) and appear on nonpolar continents.

As we lack both a basic definition and the exact reason for why the Earth sometimes creates the conditions for an ice age, it'll be hard to predict when the next one will take place. Some scientists think that humans might have affected the Earth enough to stave off an ice age by burning fossil fuels [source: University of South Hampton]. Still, you might want to hang on to that warm winter coat.