Causes of Glacial Ages

Any theory that attempts to explain the causes of glacial ages must successfully address two basic questions.First,what causes the onset of glacial conditions?For ice sheets to have formed,average temperatures must have been somewhat lower than at present and perhaps substantially lower than throughout much of geologic time. For that reason,a successful explanation would have to account for the gradual cooling that finally leads to glacial conditions.The second question is:What caused the alternation of glacial and interglacial stages that have been documented for the Pleistocene epoch (1.6 million to 10,000 years ago)?Whereas the first question deals with long-term trends in temperature that occur on a scale of millions of years,this second question relates to much shorter-term changes.

Probably the most attractive proposal for explaining why extensive glaciations have occurred only a few times in the geologic past comes from the theory of plate tectonics.Not only does this theory provide geologists with explanations about many previously misunderstood processes and features,it also provides a possible explanation for some previously unexplainable climatic changes,including the onset of glacial conditions.Since glaciers can form only on land,we know that landmasses must exist somewhere in the higher latitudes before an ice age can commence.Many believe that ice ages have only occurred when Earth’s shifting crustal plates carried the continents from tropical latitudes to more poleward positions.

Glacial features in present-day Africa,Australia,South America, and India indicate that these regions experienced an ice age near the end of the Paleozoic era,about 250 million years ago.For many years this puzzled scientists.Was the climate in these relatively tropical latitudes once like it is today in Greenland and Antarctica? Why did glaciers not form in North America and Eurasia?Until the plate tectonics theory was formulated and proven,there was no reasonable explanation.Today scientists realize that the areas containing these ancient glacial features were joined together as a single supercontinent called Pangaea that was located at high latitudes far to the south of their present positions.Later,this landmass broke apart and its pieces,each moving on a different plate,drifted toward their present locations.It is now believed that during the geologic past,plate movements accounted for many dramatic climatic changes as landmasses shifted in relation to one another and moved to different latitudinal positions.Changes in oceanic circulation also must have occurred,altering the transport of heat and moisture and,consequently,the climate as well.Since the rate of plate movement is very slow,on the order of a few centimeters per year,appreciable changes in the positions of the continents occur only over great spans of geologic time.

Since climatic changes brought about by moving plates are extremely gradual,the plate tectonics theory cannot explain the alternation of glacial and interglacial climates that occurred during the Pleistocene epoch.Therefore,we must look to some other triggering mechanism that may cause climatic change on a scale of thousands rather than millions of years.Today many scientists believe or strongly suspect that the climatic oscillations that characterized the Pleistocene may be linked to variations in Earth’s orbit.This hypothesis was first developed and advocated by the Yugoslavian scientist Milutin Milankovitch and is based on the premise that variation in incoming solar radiation is a principal factor in controlling Earth’s climate.Milankovitch formulated a comprehensive mathematical model based on the following elements:variations in the shape (eccentricity)of Earth’s orbit around the Sun;changes in obliquity,that is,changes in the angle that Earth’s axis makes with the plane of Earth’s orbit;and the unsteady wobbling of Earth’s axis, called precession.

Using these three factors Milankovitch calculated variations in the seasonal timing of the receipt of solar energy and the corresponding surface temperature of Earth over a long period of time in an attempt to correlate these changes with the climatic fluctuations of the Pleistocene.In explaining climatic changes that result from these three variables,it should be noted that they cause little or no variation in the total amount of solar energy annually reaching the ground. Instead,their impact is felt because they change the degree of contrast between seasons.Somewhat milder winters in the middle to high latitudes means greater snowfall totals while cooler summers would bring a reduction in snowmelt.?

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?Any theory that attempts to explain the causes of glacial ages must successfully address two basic questions.First,what causes the onset of glacial conditions?For ice sheets to have formed,average temperatures must have been somewhat lower than at present and perhaps substantially lower than throughout much of geologic time. For that reason,a successful explanation would have to account for the gradual cooling that finally leads to glacial conditions.The second question is:What caused the alternation of glacial and interglacial stages that have been documented for the Pleistocene epoch (1.6 million to 10,000 years ago)?Whereas the first question deals with long-term trends in temperature that occur on a scale of millions of years,this second question relates to much shorter-term changes.

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