Perhaps the most striking result of our understanding of glaciers has been the recognition that several times in the past, large areas of the earth have been covered by glaciers. Detailed studies of glacial deposits indicates numerous advances and retreats of glaciers during each of these glaciations. Explanations for these glaciations differ widely, and no single explanation is completely satisfactory. Nevertheless, the effect of these glacial episodes has been profound and varied. 

We know that the last ice age affected sea level, lowering it by as much as 400 feet at the height of the glacial advances. Conversely, if all the water tied up in present glaciers were to melt, sea level would rise by over 200 feet. Visualize the effect of such a variation on the low peninsula of Florida. 

Changes in climate associated with the advances and retreats of ice have affected civilizations, changed the course of rivers, and controlled settlement patterns in much of the northern hemisphere. Soils transported by glaciers make the Midwest one of the premier breadbaskets of the world, whereas denudation (especially in Eastern Canada) has exposed the rich mineral resources of Quebec, Ontario, and the Upper Peninsula of Michigan. 

Rebound of the earth's crust due to the unloading effect as the ice melted, has caused areas once covered by ice, such as Scandinavia, to rise rapidly; whereas peripheral areas such as Holland are slowly settling below sea level. Thus, glaciers have exerted a profound effect even in areas not directly affected. Many of the coastal erosional problems such as those faced by Florida today are due to the world-wide melting of glaciers and resulting sea level rise. In this context, emission of industrial waste products in the atmosphere may well be warming climate, affecting glaciers and sea level.