The latest Ice Age, with its four phases, was by no means the only one in the Earth's long history, according to geologists. During the Paleozoic era, the Coal Age with its luxurious swamp growth seems to have been closed by the most severe ice age the world has ever known. Even tropical lands in India and Africa show evidence of ice sheets. South America may have been ice-capped from the Brazilian jungles southward. Causes Are Mysterious Ice ages arrive, as we have seen, when the climate of a region becomes cooler.
Summer heat fails to melt away winter ice and snow.
Some scientists have tried to explain this by variations in the movement of the Earth around the sun. But no variation has ever been found that fits both the theories of astronomy and the record of what has happened on the Earth. Others have guessed that blankets of dust or of carbon dioxide got into the air and cut off part of the sun's heat. Another theory was advanced in the early 1930s by the British meteorologist Sir George C. Simpson. He suggested that, as in the case of some other stars, the sun's temperature may have varied greatly in past ages.