In the past million years, the Earth experienced a major ice age about every 100,000 years. Scientists have several theories to explain this glacial cycle, but new research suggests the primary driving force is all in how the planet leans.

The Earth’s rotation axis is not perpendicular to the plane in which it orbits the Sun. It's offset by 23.5 degrees. This tilt, or obliquity, explains why we have seasons and why places above the Arctic Circle have 24-hour darkness in winter and constant sunlight in the summer.

But the angle is not constant – it is currently decreasing from a maximum of 24 degrees towards a minimum of 22.5 degrees. This variation goes in a 40,000-year cycle.

Peter Huybers of Woods Hole Oceanographic Institution and Carl Wunsch of the Massachusetts Institute of Technology have compared the timing of the tilt variations with that of the last seven ice ages. They found that the ends of those periods – called glacial terminations – corresponded to times of greatest tilt.

"The apparent reason for this is that the annual average sunlight in the higher latitudes is greater when the tilt is at maximum," Huybers told LiveScience in a telephone interview.

More sunlight seasonally hitting polar regions would help to melt the ice sheets. This tilt effect seems to explain why ice ages came more quickly – every 40,000 years, just like the tilt variations -- between two and one million years ago.

"Obliquity clearly was important at one point," Huybers said.

Colder planet

The researchers speculate that the glacier period has become longer in the last million years because the Earth has gotten slightly colder – the upshot being that every once in a while the planet misses a chance to thaw out.

The glacial cycles can be measured indirectly in the ratio of heavy to light oxygen in ocean sediments. Simply put, the more ice there is on Earth, the less light oxygen there is in the ocean. The oxygen ratio is recorded in the fossils of small organisms – called foraminifera, or forams for short – that make shells out of the available oxygen in the ocean.

"These ‘bugs’ have been around for a long time – living all across the ocean," Huybers said. "When they die, they fall to the seafloor and become part of the sediment."

Drilled out sediment cores from the seafloor show variations with depth in the ratio of heavy to light oxygen – an indication of changes in the amount of ice over time. This record of climate change goes back tens of millions of years.

By improving the dating of these sediments, Huybers and Wunsch have showed that rapid decreases in the oxygen ratio – corresponding to an abrupt melting of ice – occurred when the Earth had its largest tilt.

Other orbital oddities

The significance of this relationship calls into question other explanations for the frequency of ice ages.

One popular theory has been that the noncircular shape, or eccentricity, of Earth’s orbit around the Sun could be driving the glacial cycle, since the variations in the eccentricity have a 100,000-year period. Curiously different, but interesting.

Variation in Orbit	Period
Tilt	40,000 yr
Wobble	20,000 yr
Eccentricity	100,000 yr

By itself, though, the eccentricity is too small of an effect. According to Huybers, changes in the orbit shape cause less than a tenth of a percent difference in the amount of sunlight striking the planet.

But some scientists believe a larger effect could be generated if the eccentricity fluctuations are coupled with the precession, or wobble of the Earth’s axis. It's like what is seen with a spinning top as it slows down.

Earth’s axis is currently pointing at the North Star, Polaris, but it is always rotating around in a conical pattern. In about 10,000 years, it will point toward the star Vega, which will mean that winter in the Northern Hemisphere will begin in June instead of January. After 20,000 years, the axis will again point at Polaris.

Huybers said that the seasonal shift from the precession added to the eccentricity fluctuations could have an important effect on glacier melting, but he and Wunsch found that the combined model could not match the timing in the sediment data.

Skipping beats

The question, then, that Huybers and Wunsch had to answer: How does the 40,000-year tilt cycle make a 100,000-year glacial cycle? A more careful sediment dating has shown is that the time between ice ages may on average be 100,000 years, but the durations are sometimes 80,000 years, sometimes 120,000 years -- both numbers are divisible by 40,000. It appears there was not a mass melting every time the tilt reached its maximum.

"The Earth is skipping obliquity beats," Huybers explained.

The planet only recently started missing melting opportunities. Although the researchers have no corroborating evidence, they hypothesize that the skipping is due to an overall cooling of the planet.

The last major glacial thaw was 10,000 years ago, which means that the Earth is scheduled to head into another ice age. Whether human influences could reverse this, Huybers was hesitant to speculate. Other researchers have found evidence that the process of climate warming can set up conditions that create a global chill.

"What we have here is a great laboratory for seeing how climate changes naturally," he said. "But this is a 100,000-year cycle, whereas global warming is happening a thousand times faster.”