Tilting at ice ages

Here’s a story to cool you off on a hot summer day. One of the major mysteries of ice ages may have been solved by a Harvard climatologist.

Most scholars believe that much of North America, Europe and northern Asia were buried under thousands of feet of ice when Earth was furthest away from the sun during the northern hemisphere summer. Our planet completes an oval or elliptical orbit around the sun once a year. Presently, we swing away from our natural heater during the north’s summer, and approach it in winter. Ten thousand years ago, the situation was reversed. Earth moved closest to the sun during summer in the northern hemisphere and moved away in winter. This happened because Earth’s axis, on which it rotates once every 24 hours, wobbles like a gigantic top. Scientists call this wobbling “precession.”

Our spin axis not only wobbles, it tilts at an angle of about 23.5 degrees from the vertical. While the 24-hour rotation brings us days and nights, the tilt gives us four seasons. In the winter, the northern hemisphere tilts away from the sun, providing less radiation than in the summer when the axis tilts the other way.

“Typically, people believe that ice ages ended when Earth began to orbit closer to the sun during northern hemisphere summers,” explains Peter Huybers, an environmental fellow in the Department of Earth and Planetary Sciences at Harvard. “At such times, the intensity of solar radiation that reaches our atmosphere increases. This increase is thought to enhance melting, eventually ending an ice age.”

It’s a neat story, but Huybers doesn’t buy it. When Earth is closest to the sun, it travels faster than when it’s farther away, a fact discovered by the German astronomer Johannes Kepler in the 1600s. This increased speed counterbalances the more intense sunshine because it makes the summer melting period shorter. As a result, Huybers says, “precession appears to exert little effect on the melting of ice sheets.”

What does pace the buildup and collapse of ice sheets is changes in the tilt of the Earth’s axis, the imaginary line that runs through the north and south poles. Increasing the planet’s tilt increases the intensity of summer sun without making summer shorter. That increases melting. Therefore, ice ages come and go mostly because of changes in the tilt of Earth’s orbit around the sun. It’s this so-called obliquity, not the wobble of the planet, that is responsible for the dramatic shifts in Earth’s ice cover.

Wobble comes up short

Astronomers have calculated how this orbit has changed during the past tens of millions of years and found a rhythmic cycle of tilting that lasts 41,000 years. Precession, on the other hand, swings around with a period of only about 20,000 years. “Between two and one million years ago,” Huybers points out, “ice age cycles occurred at intervals of about 40,000 years, matching the obliquity period, but not the 20,000 year precession variability.”

To further support his theory, Huybers did another test. He used temperature records from across North America to calculate the potential for melting ice. “When sunlight is considered over the whole of summer, it is a much better predictor of potential melting than any previous approaches used to calculate how much melting goes on,” he explains. “There turns out to be an excellent correlation between this potential melting and the amount of ice known to cover Earth during the last ice ages, two to one million years ago.” Huybers published his calculation and ideas in the June 23 edition of Science, the journal of the American Association for the Advancement of Science.

Of course, this is not the last word on what causes ice ages. Questions still linger about what specifically triggered the onset of glaciation in the northern hemisphere during the last ice age. And what changes will occur in ice sheets that cover Antarctica and Greenland during the present surge of global warming? Those are questions to ponder on a warm summer day with a glass of ice water in your hand.