What Turns Ice Ages On...and Off?

Research has shown that the amount of sunlight shining on the polar region determines when ice ages occur. Less sunlight shining on the northern polar region has been linked to the last four ice ages. Interglacial periods (time between glacial epochs) occur when the area receives more sunlight. Why? I knew you were going to ask! The amount of sunlight changes due to the orbit of Earth about the sun and the tilt of Earth (23.5 degrees) on its imaginary axis. (It's important to remember these important factors as you read on.) These orbit and tilt variations are due to interactions between Earth, Sun, the moon, and other planets. Scientists refer to this as astronomical forcing.

It is the tilt of Earth's axial rotation that creates seasons. During the summer (warm) season in the Northern Hemisphere, the North Pole is tilted toward the sun and the sun is high in the sky at noon. This allows the sun's rays to come in more perpendicular to the ground on the Northern Hemisphere. During the winter (cold) season in the Northern Hemisphere, the North Pole is tilted away from the sun. The sun stays close to the horizon, and days are short. In the Southern hemisphere, the sun is high in the sky at noon, and days are longer. This is why the seasons are reverse in the two hemispheres. While those of us living in the Northern Hemisphere enjoy summer, our neighbors in the Southern Hemisphere are experiencing winter.

Precession of the rotational axis. (©1997 Wadsworth Publishing Company/ITP)

Earth wobbles slightly on its axis of rotation, giving rise to a precession, or wobble, with a time period of about 23,000 to 26,000 years. Because Earth "wobbles" (like a spinning top) in space, its tilt changes between 22 and 25 degrees on a cycle of about 41,000 years. That's a long time just to get back to where you started! It's during the periods of less tilt (cooler summers) that scientist believe snow and ice in the high latitudes tend to last (without melting) from year to year. The build-up of this snow and ice from year to year creates massive ice sheets.

Equinox is another term you'll need to be familiar with before you can fully understand present theories on the occurrence of ice ages. An equinox is the time when the sun crosses the equator, making night and day of equal length in all parts of Earth. In the Northern Hemisphere, the vernal (spring) equinox occurs about March 21 and the autumnal (autumn) equinox occurs about September 22. It is during the vernal and autumnal equinoxes that the sun is overhead at the equator throughout the day. In other words, the angle of incidence of the sun's rays to Earth's surface at noon on the equator is 90 degrees. The sun's rays at the equator at noon are perpendicular to Earth's surface.

The tilt of the Earth's rotational axis changes over a period of time.

Changes in the degree of Earth's tilt can cause the seasons to become more or less severe. More tilt would result in colder winters and hotter summers, while less tilt would result in cooler summers and milder winters. Other processes are also important. During ice ages, snow and ice bulid up on the land close to the poles.

Because of this build-up, more of Earth is covered with ice and snow giving it more albedo. In simpler terms, the whiteness of the snow and ice reflects the sun's energy back into space. Thus, more cooling takes place. As the ice sheets continue to grow, scientists also believe less carbon dioxide is in the atmosphere. This adds to the cooling process.

The above image shows how much the Earth's orbit can vary in shape. This process in a slow one, taking roughly 100,000 to cycle.

Earth's orbit (path around the sun) is slightly elliptical (oval-shaped). This means that Earth is slightly closer to the sun during some parts of its year-long orbit and further away at other times. The closest approach of Earth to the sun is called perihelion, and it now occurs in January, making Northern Hemisphere winters slightly warmer. Eleven thousand years ago, perihelion occurred in July, making the summer slightly warmer in the Arctic.

Periodic changes in the "roundness" of Earth's orbit varies in cycles of 100,000 to 400,000 years. This affects how important "timing of the perihelion" is to the strength of the seasons. The growth and retreat of ice sheets are thought to be controlled by the combination of:

  • the tilt cycle which takes 41,000 years from onset to completion
  • the year precession cycle which is 26,000 years, and
  • the perhaps smaller effect of Earth's orbit eccentricities (non-circular and elliptical) cycle which can take 100,000 to 400,000 years for completion.

This seems reasonable as each of these cycles has a definite effect on the severity of the summer and winter seasons.

Ice cores obtained by boring deep into Earth's ice sheets provide scientists with information of Earth's paleoclimate. Two ice cores through the Greenland ice sheet have provided a continuous record of atmospheric conditions over Greenland from abut 100,000 years ago to present time. The ice cores show remarkable temperature variability (changes) over the past 100,000 years with rapid warming and cooling of the northern hemisphere, sometimes within decades (tens of years).