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BOOK REVIEW

Time Travel on Ice

J. A. Rial

The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Richard B. Alley. viii + 229 pp. Princeton University Press, 2000. $24.95.

Books in which scientists write about their professional experience and describe in lay terms the stuff that makes them excited about science rarely disappoint. Richard Alley's The Two-Mile Time Machine is no exception. It describes a fascinating journey into the geologic past and the history of the Earth's climate.

Alley modestly calls his book a "progress report" on abrupt climate changes. Yet its contents could not be more relevant to the current debates about global change, control of carbon dioxide emissions, future global warming and environmental protection. The book suggests that we must heed the warning of the past in order to make wise decisions when managing the future of our environment.

As recorded in layers of ice piled up over millennia on the great ice sheets of Greenland and Antarctica, the history of climate is wild. Enormous changes in global climate have at times taken place within the span of just a few years and without demonstrable cause. The lack of a precise explanation for this extreme variability is the reason that climate change is hotly debated in scientific circles. Advancing through the chapters of The Two-Mile Time Machine, the careful reader learns to interpret the clues found in graphs of real geologic data and can try to anticipate the author's solution to the enigma of global climate change.

On the huge ice caps of Greenland and Antarctica, the snow rarely melts, even in summer. So year after year the snow piles up in layers, compacts and turns to ice, adding to a frozen mass that is now some two miles thick. Like a page in a book, each annual stratum contains a faithful record of some aspects of the environment in which it was deposited: temperature, humidity, air chemistry and so forth. In Greenland this two-mile-thick layer cake of ice contains details of the history of the polar and global environments over the last 100 millennia. But the only way to read this peculiar history book is to drill through its icy thickness and collect samples from all depths.

Alley and his colleagues from the United States, Europe and Asia climbed to the top of Greenland's ice cap and drilled through. Using a hollow drill bit, they pulled out cylindrical ice cores a few inches in diameter and a few feet long, with an accumulated length of more than two miles. These ice cores are the time machine referred to in the title. They contain visible layers that can be dated, just like tree rings. Inside the layers are trapped ancient bubbles of air, from which the chemical composition of the atmosphere through time can be determined. The chemistry and isotopic content of the oxygen and hydrogen in the water that formed the ice indicate how much temperature changed over the years as a result of global climate change. From the isotopic composition of the water, scientists can read evidence of, among other changes, long epochs of intense cold, interrupted every 90,000 years or so by short intervals of mild temperatures lasting some 10,000 years. These are the 100,000-year cycles of the great ice ages of the Pleistocene.

In Greenland the ice about two miles below the surface is about 110,000 years old. Thus, between the near-surface ice (which is just a few decades old) and the deepest reach of the drill has been fully recorded the history of the last ice age, which started about 100,000 years ago and reached its coldest point about 20,000 years ago. Then, all of a sudden (in geologic terms), and without apparent cause, the huge ice sheets covering much of North America and northern Eurasia melted away, and over a period of several thousand years the climate changed from that of an ice-house to that of the warm interlude we presently enjoy.

The story that can be read in the ice cores is astonishingly detailed. The weather during the Medieval Warm Period, when the Vikings settled Greenland, was much warmer than today. This was followed by the so-called Little Ice Age (from about 1300 to 1850 a.d.), during which the middle latitudes were colder than today. The ice cores also reveal very rapid changes in global climate. They tell of rain and snow, wind and fire, changing solar activity, huge volcanic eruptions, abundances and shortages of atmospheric carbon dioxide, and periodic and quasiperiodic events?some likely induced by secular changes in the Earth's orbital parameters, some by still unknown processes.

Alley's book focuses not on the long-term changes that may have caused the ice ages, but on more recent, newly discovered "flickering" climate changes that the drilling through Greenland's ice cap revealed. He emphasizes that many of the important climate changes he and his colleagues have been able to read from the ice cores are sudden, abrupt and enormous. The Earth's climate has at times changed drastically in just a few years from livable to glacially inhospitable or unbearably hot and saunalike. If such changes were to occur today, we would face a climate catastrophe of proportions unprecedented in the history of humankind.

Alley ends his entertaining book by polishing his crystal ball, envisioning what the future climate will be and what we might do about it. He warns that one possible consequence of the current, human-induced global warming could be a new ice age: If enough of Greenland's ice melts and floods the North Atlantic, that fresh, cool water would stop the vast warm ocean currents (such as the Gulf Stream) that bring heat to the North Atlantic from the tropics. Preventing the warm current from reaching high latitudes would translate into an advance of the ice.

This and other predictions of doom are based on computer models, which are not infallible. Nonetheless, climate models are superb, powerful tools for understanding climate, whether they are composed of a few simple differential equations running on a personal computer or a large number of them running on a supercomputer. They are the only way we can play the "what if" game with nature.

Climate modeling seems to point clearly to the fact that the climate is delicately, capriciously sensitive to disturbances. A climatic disturbance created by nature, or by humans dumping increasing amounts of carbon dioxide into the atmosphere, could well trigger climate change across the planet.

Some scientists warn that global climate is like a drunk: If left alone, a drunk will stay put, but a tap on the shoulder may cause him to panic, stagger, fall and break his neck. The simile makes the important point that climate is highly sensitive to disturbances and highly unpredictable. With our notorious disregard for the environment, we humans could well become the tap on the drunk's shoulder. Alley warns that even a minor "push" can make the climate flip, in a few years, from benign to unbearable. Such is the message we must heed from the ice.?J. A. Rial, Geological Sciences, University of North Carolina at Chapel Hill


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