Sandra Efstration, age 11, of Somerville, Massachusetts, for her question:

Can the world climate really change?

Our earth has celebrated at least four billion birthdays. It is a very changeable planet, at least on its restless surface of land and sea, and in the atmosphere above. Through the ages, its features have been remodeled and its climate changed many, many times.

There is ample evidence to prove that large areas of North America once were cov¬ered by vast glaciers a mile or more thick. They were, of course, children of a long Ice Age during which the warmth of summer was unable to melt all the icy snows of the wintry seasons. For this to happen, the climate of Northern America and other areas of the globe must have been colder than it is in modern times. Scientists say that such Ice Age glaciers could form if our summer seasons were just a few degrees cooler, just cool enough to allow some of the winter snow to accumulate on the ground from season to season.

A change of this sort would be a major change in the global climate, affecting plant and animal life on the land and perhaps also in the seas. Geological evidence in the rocky ground and the silty deposits around the seas prove that the earth has suffered through several Ice Ages, all caused by changes in the world climate. Other evidence in the rocks records more pleasant chapters of the past. A lot of this evidence is brought to light by streams that have cut down through lower and lower levels of older rocks that once paved the surface.

Earth scientists can figure the dates of these buried old layers back through millions of years. In the Dakotas, they found alligator fossils embedded on rocks de¬posited some 50 million years ago. Farther north in Alaska, the fossilized remains of magnolias and elms were embedded in rocks of a similar era. These animals and plants can survive only in warm and semi tropical climates. Obviously our wintry north once enjoyed a warm climate through most of the year. There is evidence that other major areas of the globe also enjoyed a summery climate at this time.

The earth's long, changeable history falls naturally into a series of eras and periods. Each chapter brought a change, sometimes a major change, in the world climate.

There were hot and cold spells, dry and moist spells, often lasting millions of years. The Mesozoic Era opened about 180 million years ago with a widespread arid climate. This was followed by a semi arid spell and then a mild, moist spell. During the more than 100 million years of this era, the dinosaurs advanced from small reptile ancestors to their monstrous heyday and then departed. We do not as yet know what wiped them out. But plant life changed with the changing climates and some experts suggest that their favorite plant food was choked out and replaced by grasses that they could not chew.

The Ice Ages, without a doubt, are the most dramatic changes in world climate. In terms of geological time, they arrive suddenly and soon depart. The growing glaciers advance year by year, crushing plant life and driving animals from their native haunts. When a spring like climate returns, the melting ice covers vast areas with soggy swamps. The seeds of warm weather plants thrive in these areas and as the moisture drains away, herds of assorted animals return to feed on the greenery. But at present we cannot say for certain what causes these and other changes in the earth's climate.

For more detailed information go to: http://www.globalwarming.org/gw101