Denise Johns, age 14, of Shreveport, La., for her question:
JUST WHAT IS AN ICE AGE?
Ice ages are periods of time in the earth's history when a significant, extended cooling in the atmosphere and ocean took place. Scientists tell us that the earth is in such an ice age today, as the climates of Greenland and Antarctica attest.
The present ice age began about 2 million years ago and will probably continue for a few million more.
Evidence of early ice ages exists. Since the time of the earliest recorded life on earth about 3.6 billion years ago the planet's average surface temperature has been about 70 degrees Fahrenheit, with a range of uncertainty of about nine degrees Fahrenheit. For more than 90 percent of that time the earth has been free of ice ages and no large glaciers have existed except in high mountains. About every 150 million years, however, ice ages occur, last a few million years and then pass away.
The term "Ice Age" usually refers to the Pleistocene ice age which began about 1.75 million years ago and ended about 10,000 years ago.
Although the cause of ice ages is still a subject of controversy, an argument based on astronomical observations of the galaxy has gained credibility in recent years.
The earth and its solar system are located asymmetrically within one limb of the Milky Way galaxy. The whole galaxy completes the rotation about once every 300 million years, taking the solar system through denser and thinner regions of interstellar fields.
As with tidal processes, two disturbing phases appear to exist for each full cycle, so that every 150 million years a very slight change takes place in the solar system's galactic environment. These external processes, according to the theory, alter the earth's climate.
In addition, earth based processes are also involved. Because of continental drift, periodic changes take place in the earth's geography that could cause ice ages.
This then is the ice age scenario: a slight external cooling because of the galaxy's rotation, as well as a favorable geographic oceanographic setting. A chain reaction may then be initiated by minor variations in the earth's orbit.
The longest of the ancient ice ages was probably the Permocarboniferous, which began about 300 million years ago and affected all southern hemisphere lands.
Earlier still, about 450 million years ago, another giant ice sheet extended from Brazil to North Africa and all the way across to Yemen and Saudi Arabia. Paleomagnetic measurements indicate that the South Pole then lay in West Africa.
About 600 million years ago, yet another great glacial age occurred.
Apparently ice ages took place throughout the earth's history, although traces of them become harder to ascertain the longer ago they occurred.