How often do ice ages happen?

The last ice age led to the rise of the woolly mammoth and the vast expansion of glaciers, but it's just one of many that have chilled Earth throughout the planet's 4.5-billion-year history.

So, how often do ice ages happen, and when is the next freeze expected to begin?

The answer to the first question depends on whether you're talking about big ice ages or the little ice ages that happen within those larger periods. Earth has undergone five big ice ages, some of which lasted for hundreds of millions of years. In fact, Earth is in a big ice age now, which explains why the planet has polar ice caps. 

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Big ice ages account for about 25 percent of Earth's past billion years, said Michael Sandstrom, a doctoral student in paleoclimate at Columbia University in New York City.

The five major ice ages in the paleo record include the Huronian glaciation (2.4 billion to 2.1 billion years ago), the Cryogenian glaciation (720 million to 635 million years ago), the Andean-Saharan glaciation (450 million to 420 million years ago), the Late Paleozoic ice age (335 million to 260 million years ago) and the Quaternary glaciation (2.7 million years ago to present).

These large ice ages can have smaller ice ages (called glacials) and warmer periods (called interglacials) within them. During the beginning of the Quaternary glaciation, from about 2.7 million to 1 million years ago, these cold glacial periods occurred every 41,000 years. However, during the last 800,000 years, huge glacial sheets have appeared less frequently — about every 100,000 years, Sandstrom said.

This is how the 100,000-year cycle works: Ice sheets grow for about 90,000 years and then take about 10,000 years to collapse during warmer periods. Then, the process repeats itself.

Given that the last ice age ended about 11,700 years ago, isn't it time for Earth to get icy again?

"We should be heading into another ice age right now," Sandstrom told Live Science. But two factors related to Earth's orbit that influence the formation of glacials and interglacials are off. "That, coupled with the fact that we pump so much carbon dioxide into the atmosphere [means] we're probably not going to enter a glacial for at least 100,000 years," he said.

What causes a glacial?

A hypothesis put forth by the Serbian astronomer Milutin Milankovitch (also spelled Milanković) explains why Earth cycles in and out of glacials and interglacials.

As the planet circles the sun, three factors affect how much sunlight it gets: its tilt (which ranges from 24.5 degrees to 22.1 degrees on a 41,000-year cycle); its eccentricity (the changing shape of its orbit around the sun, which ranges from a near-circle to an oval-like shape); and its wobble (one full wobble, which looks like a slowly spinning top, happens every 19,000 to 23,000 years), according to Milankovitch.

In 1976, a landmark paper in the journal Science provided evidence that these three orbital parameters explained the planet's glacial cycles, Sandstrom said.

"Milankovitch's theory is that the orbital cycles have been predictable and very consistent throughout time," Sandstrom said. "If you are in an ice age, then you'll have more or less ice depending on these orbital cycles. But if the Earth is too warm, they basically won't do anything, at least in terms of growing ice." 

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One thing that can warm Earth is a gas such as carbon dioxide. Over the past 800,000 years, carbon dioxide levels have fluctuated between about 170 parts per million and 280 ppm (meaning that out of 1 million air molecules, 280 of them are carbon dioxide molecules). That's a difference of only about 100 ppm between glacials and interglacials, Sandstrom said.

But carbon dioxide levels are much higher today when compared with these past fluctuations. In May 2016, Antarctica carbon dioxide levels hit the high level of 400 ppm, according to Climate Central.

Earth has been warm before. For instance, it was much warmer during the dinosaur age. "[But] the scary thing is how much carbon dioxide we've put in [the atmosphere] in such a short period of time," Sandstrom said.

The warming effects of that carbon dioxide will have big consequences, he said, because even a small increase in Earth's average temperature can lead to drastic changes, he said. For instance, Earth was only about 9 degrees Fahrenheit (5 degrees Celsius) colder, on average, during the last ice age than it is today, Sandstrom said.

If global warming causes both Greenland's and Antarctica's ice sheets to melt, the oceans will rise about 196 feet (60 meters) higher than they are now, Sandstrom said.

What leads to big ice ages?

The factors that caused the long ice ages, such as the Quaternary glaciation, are less well-understood than those that led to glacials, Sandstrom noted. But one idea is that a massive drop in carbon dioxide levels can lead to lower temperatures, he said.

For instance, according to the uplift-weathering hypothesis, as plate tectonics pushed up mountain ranges, new rock became exposed. This unprotected rock was easily weathered and broken apart, and would fall into the oceans, taking carbon dioxide with it.

These rocks provided critical components that marine organisms used to build their calcium-carbonate shells. Over time, both the rocks and the shells took carbon dioxide out of the atmosphere, which, along with other forces, helped lower carbon dioxide levels in the atmosphere, Sandstrom said.

Originally published on Live Science.