Over the past 2.6 million years, the Earth has experienced more than 50 glacial periods, interspersed with warmer interglacial periods.

But what causes the periodic expansion of ice caps and glaciers?

Glacial periods are driven by a complex and interconnected set of factors, including Earth's position in the solar system and more regional influences, such as carbon dioxide levels. Scientists still want to try to understand how this system works, as human-caused climate change may have permanently broken the cycle.

It wasn't until a few hundred years ago that scientists began to find signs of the last glacial period. Rocks in unlikely locations, for example, as well as huge amounts of broken rock known as moraines, which scientists suspected had been pushed here by ancient glaciers from far away.

By the end of the 19th century, scientists proposed four glacial periods between 2.6 million and 11,700 years ago. The understanding of the glacial period cycle saw an important breakthrough in the 1840s. Milankovitch, a Serbian astrophysicist at the time, outlined three main ways in which the Earth's orbit changes in response to the Sun. These determine how much solar radiation reaches the Earth.

The first is the eccentric shape of the Earth's orbit around the Sun, which changes from nearly circular to elliptical in one 96,000-year cycle. Maslin explains, "Because Jupiter has 4 percent of the mass of the solar system, the shape of the Earth's orbit will change back and forth in the presence of such a strong gravitational pull."

The second is the tilt of the Earth, which is why we will have four seasons. The tilt of the Earth's rotational axis means that one hemisphere will always be tilted away from the sun (leading to winter) and the other hemisphere will always be tilted toward the sun (leading to summer). This tilt changes in a cycle of 41,000 years, which in turn changes the extremes of the seasons.

Third is the wobble of the Earth's tilt axis, which moves like a big gyroscope. "The Earth's angular momentum is very fast, causing the axis to wobble with it," Maslin says. This oscillation occurs in 20,000-year cycles. Milankovitch identifies cool-summer orbital conditions as a particularly important precursor to a glacial period. The real trigger for a glacial period, Maslin says, is the underlying feedback in the climate system.

Scientists are still sorting out how many different environmental factors can influence the formation and melting of glaciers, but recent studies have hinted that the levels of greenhouse gases in the atmosphere play an important role. Scientists at the Institute for Climate Impact Research in Potsdam, Germany, have demonstrated that the start of the last glacial period was largely due to a decrease in carbon dioxide. Today the concentration of carbon dioxide in the atmosphere has increased dramatically due to man-made emissions, potentially inhibiting the start of the next glacial period by 100,000 years.