Frozen Earth: The Once and Future Story of Ice Ages Read Online Free PDF Page B

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    Before embarking on this discussion, however, it is worth reviewing a few general aspects of ice ages.First is the meaning of the term itself, because its usage can be confusing.Initially, “ice age” referred to Agassiz’s original concept, a period of cold at some fairly remote time in the past, during which most of Europe was covered with thick glaciers.As already noted, however, it was fairly soon discovered that there had been a whole series of “ice ages” that were really part of the same episode, separated by short warm periods during which forests grew and animals roamed over land that had once been buried deep in ice.Climate zones marched up and down the continents as the temperature changed and glaciers grew and then melted back again.We know this about the ice age of the past few million years because the fossil record of changing animal and plant species is quite well preserved.But there have been ice ages in the much more distant past, too—hundreds of millions and even billions of years ago.About those intervals we know much less, but they too probably had both cold and warm periods.In current usage, the term “ice age” properly refers to an entire cold episode, including its short warm periods.Hence, it’s possible to talk about the “current ice age,” which seems a contradiction in terms on a hot summer’s day.Large-scale advances and retreats of ice during an ice age are usually referred to as glacials and interglacials respectively.Ice ages finally end when the cycles of these glacials and interglacials cease, and permanent ice, if it exists at all, becomes a minor feature of high mountains and polar regions.Our current ice age is often referred to as the Pleistocene Ice Age, taking its name from the subdivision of the geological timescale that more or less coincides with the time during which the Northern Hemisphere has experienced glaciation.Although the current ice age actually begansomewhat before the start of the Pleistocene, I shall use that nomenclature in this book as a convenient way to distinguish it from others in the geological record.
    In addition to understanding the usage of the term “ice age,” there is also the question of what, exactly, a glacier or icefield is, and how it comes about.Glaciers are actually nothing more than huge accumulations of snow.Pressure from the weight of overlying snow transforms those fluffy flakes that so delight children on a winter’s day into the hard ice of a glacier, brittle enough to crack open in deep crevasses and strong enough to pluck solid rock from a mountainside and carry it down a valley.At the very surface of a glacier, the winter snow is as loosely packed as the drifts you shovel from your driveway after a storm (of course, that snow really doesn’t seem to be so loosely packed when you’re at it, but that’s another story ...).But dig down into a glacier, and you’ll find the individual snowflakes—themselves just exquisitely formed crystals of ice—packed together much more tightly, the air between them forced out.Probe even more deeply, and you’ll discover that the snow has recrystallized into a continuous mass of solid ice, as clear as the ice cubes in your refrigerator.
    A bona fide glacier must be permanent.Generally, this implies that sufficient fresh snow must accumulate during the cold months to offset melting during the summer, although on a year-to-year basis, glaciers may expand or contract, depending on local and global climatic conditions.Today, most glaciers around the world are in retreat because of the warming climate, and it appears that the rate of melting is accelerating.This has been documented spectacularly in places such as the Alps, where historical records have been kept and dated sketches and photographs are available to compare with the present extent of ice.Even over periods as short as a few decades, satellite images show that dramatic shrinkage of mountain glaciers has occurred in the