Astronomical Causes
What can cause such large changes in the climate of the Earth? Although the average temperature during the peak of the ice age is only 4 degrees C lower than the temperature at the present time, it can have profund consequences - please see the following diagram by clicking here ! . You would see that a large portion of the landmass in the Nothern Hemisphere had significantly lower temperatures than today's, and that temperatures in parts of the Pacific Ocean were actually higher!
To explain events such as the Glacial Cycles, it is natural to look for astronomical explanations, both because are known to affect the climate on Earth (the seasons, for example) and because of their regularity and predictability.
Astronomical Cause #1
If the Earth rotated with its axis perpendicular to the plane of the orbit around the Sun, likely there would be no seasons, since the amount of solar energy falling on a given point on Earth would be the same throughout the year (except for daylight/night cycles). There could be changes of weather due to changes in winds and cloud cover, but it wouldn't follow the cycle that we call seasons.
In reality, the Earth spins around an axis which is tilted about 23 degrees from the normal to the plane of the orbit around the Sun. This axis traces a cone in space. As the Earth moves around the Sun, this axis has a different inclination with respect to the Sun. During winter in the Northern Hemisphere, the North Pole is tilted away from the Sun, and less energy is absorbed (in the Northern Hemisphere); during summer, the situation is reversed, and more sunlight reaches the Northern Hemisphere.
The inclination of this axis is not constant in time, but changes from 21.5 to 24.5 degrees over time; a complete cycle is made in 23,000 years. This motion of precession is believed to be responsible for part of the minicycles during an ice age cycle; however, it doesn't explain the main 100,000 year period and the peaking of the ice cover followed by the rapid warm-up, as we have seen above.
Astronomical Cause #2
The orbit of the Earth around the Sun is an ellipse. The shape of this ellipse changes a bit over time, becoming more or less elongated. The period of the change of the shape of the orbit is about 100,000 years, which coincides nicely with the observed period of glacial cycles. However, the change of the orbit's shape can only be a partial explanation for glaciation on Earth. The reason is that there is little change in the shape of the orbit, and that the inclination of the Earth's axis of rotation has a much stronger effect on weather on Earth than the slight change in the distance between Earth and Sun.
We have to look for other causes driving the main glacial cycle on Earth. This will be done in the next section when we examine the role of the ocean.
This page was produced by Gianfranco Vidali.