This is an absolutely fascinating account of the various ice ages that have periodically taken over the earth. From the ancient "Snowball Earth" (sometimes called "Slushball Earth," 550 to 850 million years ago) in which the entire planet was more or less frozen from pole to pole, to the "Younger Dryas," a cold spell beginning 12,800 years ago and lasting for about 1,200 years, to "Little Ice Age" in Europe (700 to 150 years ago) to the "year without a summer," in 1816, UCSD Professor of Earth Sciences Doug MacDougall chronicles the ebb and flow of glacial advance and retreat in a most interesting and informative manner. Much of this is a historical account of how scientists discovered the past ice ages through geology and the study of cores taken from the Antarctic, the Arctic, from the sea floor, and from still standing glacial ice packs. MacDougall explains how these cores are read to reveal climate changes in the past based on evidence from isotopes, pollen, and bubbles of trapped atmospheric gases. It is really amazing how much information can come from such minute bits of evidence.
In the early chapters MacDougall recalls the first scientists who became aware of the earth's climate in previous ages--Louis Agassiz, James Croll, Milutin Milankovitch and others. MacDougall recalls their efforts to get their ideas accepted by the geological establishment. It is fascinating to see how gradually it was realized that great rocks had arrived at various places, having been carried there by ancient glaciers. A particularly interesting story is how the Channeled Scablands of the Columbia Plateau in Washington were created when the glacial Lake Missoula sudden broke through the melting ice and drove an immense wall of water clear to the Pacific Ocean.
Part of his concentration is on the glacial and interglacial periods that have characterized the environment during the rise of the genus homo and especially the last 150,000 years or so during which homo sapiens have evolved. Chapter Ten, "Ice Ages, Climate, and Evolution" is devoted to how the advance and retreat of the ice affected the evolution of hominids and other animals and plants.
For those of us who might be worried about global warning it is perhaps refreshing to be warned that we are still living in an ice age. MacDougall writes, "We are in a warm period, one of the many interglacial intervals that have occurred throughout the Pleistocene Ice Age [now three million years old]." (p. 233)
Near the end of the book MacDougall looks at today's climate and takes into account the warming due to human activities, in particular the burning of fossil fuels. But he is not alarmed. He notes that the atmosphere on the planet Venus (surface temperature 485 degrees Centigrade, more than hot enough to melt lead) is almost all CO2 while that on earth is less than four-tenths of one percent CO2. No runaway greenhouse effect seems likely here any time soon. MacDougall explains the carbon dioxide cycle on earth and assures us that most CO2 is locked up in limestone. (p. 238)
While he seems unconcerned about global warming (taking the very long view) he does remark that "It is difficult to comprehend how further additions [of CO2] to the atmosphere at current rates could fail to raise global temperatures and possibly influence the course of the [current] Pleistocene Ice Age." (p. 241) Still he ends the book with this (possibly understated) possibility: "...mankind may inadvertently bring the Pleistocene Ice Age to a premature close, ushering in another long period of ice-free existence for our planet."
Which brings me to the question, if the earth does become more or less permanently as hot and muggy as a Jurassic swamp, what will it mean for human beings? There will be less land available since the melting of the glaciers would raise sea level some 60 meters, drowning New York and London, not to mention much of Florida and a host of South Pacific islands, Bangladesh, and other low-lying lands. Will we live most of our lives indoors in air-conditioned buildings? Will the verdant plants and fantastic diversity of creatures characteristic of tropical jungles be all around us? Will dragon flies be as big as birds?
On the other hand, suppose, as MacDougall intimates, that the forces affecting the earth's climate dwarf our puny doings. In that case the present interglacial will come to a close and the immediate future will be cold as the ice once again advances from the poles. As MacDougall points out, the air will be dryer with all that water locked up in ice, and great parts of the planet will be desert and the winds will blow the sand around the globe (as happened before during such periods as evidenced by the core samplings). I am reminded of the poet Robert Frost's dilemma, "Fire or Ice?" Probably our lives will not end in such extremes, but our way of life may change dramatically, and many will endure great hardships.
On the third hand, is it possible, maybe even probable, that we human beings will be able to affect the climate in such a way as to stave off the extremes? One of the cold-amplifying effects of great sheets of ice over the earth is to reflect sunlight and further cool the planet. Might not humans spread dark matter over the ice, melting it? Or in the case of too much CO2 or other greenhouse gases in the atmosphere, might humans somehow find a way to sink such gases and cool things down?
This is a distinguished work, very well written, beautifully edited, full of fascinating information about how cold the earth has been in the past (and how cold it might become again) and why.
