blackfoot-valley

lengthened because the glacier was now moving more slowly, so the intervals between fillings of the lake became longer. By 13,000 years ago, the glacier could no longer reach far enough south to dam the Clark Fork’s waters, and the story of Lake Missoula came to an end. Or has it? Probably there were similar episodes of glacial lakes form- ing and emptying in the region during some of the previous ice ages that pre- vailed over the last 2 1/4 million years. While much, though not all, of the geologic record of earlier ice ages has been obliterated by subsequent erosion, it is known that the ice age preceding this one produced even greater glaciers. While geologists do not yet know exactly how many ice episodes or “ages” oc- curred during the last 2 1/4 million years, they do know that they were sepa- rated by warmer interglacial periods such as the one we are presently enjoying. Many scientists suspect that typical ice ages last somewhere on the order of 100,000 years separated by interglacial periods of 10,000 years or so. This will give pause to those who fear global warming since our present interglacial period has already lasted that long - and thus might be approaching its end. While the previous ice advances are unlikely to have been neat or regular, we might be due, or even overdue for the onset of another ice age. The possibil- ity is at least worthy of passing comment at a time when so much public alarm is being sounded over the possibility or likelihood of global warming. Much of that current alarm is based on a reading of weather trends since the early 1980s. Scientists are by no means united on these matters; they hold various opinions on both sides of the issue. The question of whether mankind’s addi- tion of carbon dioxide (CO 2 ) to the atmosphere from the burning of fossil fuels is a causative factor is also widely debated. One statement we can definitely make is that two decades, 1980 to the present, is an extremely short period on which to base any long-term projec- tions on something as fickle as weather and climate. During the preceding two decades between 1960 and the early 1980s, many scientists were sounding warnings over global cooling. The earth has in fact been in a “global warming” period ever since the last ice age began to ameliorate about 15,000 years ago. The general trend of temperatures has been slowly upward during most of that timespan. Consider- able variations have occurred over periods of tens to many hundreds of years. Only quite recently was it discovered that major climatic changes have occurred very suddenly, within periods as short as a few years or decades. A prominent example of sudden cooling spanned the period from 10,700 to 10,300 years 24