Frozen Earth: the Once and Future Story of Ice Ages

Natural History, April, 2005 by Laurence A. Marschall

Frozen Earth: The Once and Future Story of Ice Ages

by Doug Macdougall University of California Press, 2004; $24.95

It may seem counterintuitive, but it's no secret to geologists that we are living in an ice age. The simple fact is that throughout most of the 4.5 billion years of history on our planet, the climate has seldom been as frigid as it has been of late. By "of late" I don't mean the past century or so, which has been characterized by warming trends, but the past several million years, when planetary temperatures took a nosedive. The result has been a succession of massive ice sheets that bulldozed their way into what were once temperate, or even tropical, lands.

Of course, even ice ages have occasional respites--warm periods during which the ice retreats. We are living in one now, a kind of global Indian summer. It is so temperate these days that it is hard to imagine the ice-locked world of 18,000 years ago, when glaciers sometimes two miles thick covered North America as far south as central Pennsylvania.

Signs of the most recent glaciation are all around us, though. Huge glacial erratics, boulders unlike most of the other rocks in their surroundings, stand in mute testimony to their cross-country transport by advancing ice. Meandering ridges of gravel, so-called glacial moraines, trace glacial retreat, where streams of rushing meltwater slowed down enough to deposit their burden of sand and pebbles. But it was not until the 1800s that close observers of the countryside were able to apply a kind of forensic geology to the scars left behind by flowing ice, and thereby reconstruct the chronology of glaciations past. Doug Macdougall, an earth scientist at the Scripps Institution of Oceanography in La Jolla, California, tells an enlightening tale of how that knowledge came to be, and what the science of geology has discovered about the causes and consequences of the ice ages in recent years.

A central figure in Macdougall's story is the Swiss-born naturalist and geologist Louis Agassiz. Having hiked over Alpine glaciers as a youth, Agassiz had a keen eye for the effects of ice. He convinced the scientific community that much of the lowland landscape had been carved by the same kind of glacial action he had seen at high altitudes. Later in life he emigrated to the United States and accepted a professorship at Harvard, where he founded the Museum of Comparative Zoology. There he became something of a national celebrity. The poets Henry Wadsworth Longfellow and Oliver Wendell Holmes Sr. celebrated Agassiz's ideas in verse. At his funeral in 1873 the U.S. vice president and the governor of Massachusetts were among the mourners.

Agassiz contented himself with showing that there had been ice ages, leaving the question of why to less charismatic but no less brilliant investigators. James Croll, a self-taught geologist who lived in the second half of the nineteenth century, showed how the ebb and flow of ice over long periods of time is governed by changes in the shape of Earth's orbit and in the inclination of the planet's axis caused by the tug of the other bodies in the solar system. Croll's work was refined by a Serbian mathematician and engineer, Milutin Milankovitch, in the 1920s, who matched the predicted orbital variations with a growing body of data showing how Earth's climate had changed.

Since then the available historical data has expanded at an accelerated pace, both in volume and in detail. In 1998 an ice core was extracted from the Antarctic ice sheet to a depth of nearly 12,000 feet, providing a virtually unbroken record of global climate changes (as recorded in the annual ice layers), spanning more than 400,000 years. Yet current theory cannot account for the complex patterns in the data. For example, climate has sometimes flipped within a single decade--too rapidly to have been caused by slow changes in Earth's orbit. And during the period now known as the "little ice age," which peaked from the seventeenth until the nineteenth centuries, an unexplained absence of sunspots was accompanied by unusually hard winters all over Europe.

Just as surely as sunshine follows rain, the present warming period will give way to a return of the ice, despite the inadvertent influence of our fossil-fuel economy. But no one yet, not even a scientist as knowledgeable as Macdougall, can say whether the new ice curtain will fall in a century or 10,000 years in the future.