These are exciting times in climate science. Discoveries that grew out of a line of research that began 50 years ago as a small geophysical field experiment are making their way onto the public stage. A big secret about how climate behaves was buried a mile deep in the polar ice on Greenland, and scientists went there and found it. What the earth was keeping from us was this: When change comes, it can be big and fast.

More than 110,000 years of snowfalls are piled up in a continuous layered sequence two miles deep atop Greenland. Scientists figured out how to core through this ice sheet as if it were an apple, and what they pulled up in the mid-1990s is the clearest record of climate that anyone has ever seen. Through some ingenious chemistry and other clever laboratory work, they are able to read this record like a book—to count individual years, to tell the temperature of the air when the snow fell, to estimate the amount of snow and determine the composition of the atmosphere at the time.

These cores tell a story about the last ice age that is nothing like the ponderous, slumbering epoch of cold stability that everyone has grown up hearing about. The whole record that we expected to be smooth as a knife blade is punctuated with enormous lurching changes between warm and cold, wet and dry. This is the signature of abrupt climate change—a result so surprising and contrary to some basic assumptions about climate that it is just now working its way through the earth sciences and beyond.

While this revolution in thinking is under way, climate and the science of its behavior are becoming topics of increasing public awareness. Signs of change are widespread, especially at high northern latitudes, where temperatures are rising faster than elsewhere on the planet and glaciers and sea ice are melting rapidly. Around the world, extreme weather events seem to be more common, and many people are on edge about what this portends.

Abrupt climate change is finding its way into the popular culture. Many people who may have been only vaguely aware that there were such beings as paleoclimatologists saw one portrayed as the hero of a blockbuster Hollywood motion picture in 2004. Who would have thought? Of course, 20th Century Fox won't win any awards for its rendering of the science of abrupt climate change in The Day After Tomorrow. Don't expect a single ice storm that fractures New York City, an outbreak of tornadoes that tear apart the skyscrapers of downtown Los Angeles, or a long-term ice age any time soon. Yet, though it doesn't happen in a day or two—that's the difference between weather and climate—it happens faster than you think. And one basic idea, unfortunately, Hollywood got just right: Climate change can be dangerous, even catastrophic.

While theater patrons were munching their popcorn, the Pentagon was mulling over a private think tank study it had commissioned that painted a particularly gloomy scenario of abrupt climate change as a threat to national security during the new century. "Military confrontation may be triggered by a desperate need for natural resources such as energy, food and water," its authors warned. This was not meant to be a prediction of the future, but rather a device to encourage strategists to begin thinking in different terms about climate.

But there is nothing imaginary about abrupt climate change or, for that matter, about this story of its discovery. It is not a hypothesis or a computer simulation. It is a solid theory supported by a careful reading of the remarkable direct evidence, the hard data that scientists pried from the earth itself—or, more exactly, the ice, sea, and land. In fact, other climate archives around the world not only confirm the Greenland record, they are yielding a very different picture than the previously supposed stability of the past 10,000 years, the period that saw the rise of human civilizations.

On top of this new view of a more changeable climate is the unnerving discovery that it is basically unpredictable. The climate is a chaotic system, like a stock market. All kinds of things are going on for different reasons, working on different timescales, and one day they line up together and the market comes tumbling down. If you educate yourself on the subject, and hedge your bets, you will probably be a more successful investor than if you don't—but you can never entirely eliminate the possibility that one day, to everyone's surprise, the market will crash. And even though we stand to learn a lot more about climate, this uncertainty may never go away.

Uncertainty poses a real problem for scientists trying to convey the idea that abrupt climate change might be a big risk for modern society. It poses an even bigger problem for policy makers trying to generate support for measures that might reduce those risks. When it comes to spending money or political capital, it is hard to attract a lot of interest in a patently unsure thing.

Of course, scientists familiar with the workings of the atmosphere have worried about the impact of industrial pollution for a long time. The brilliant meteorologist Carl G. Rossby, who ushered in the era of modern weather forecasting, warned in the 1950s about the potential of industrial gas pollution. "Nature can be vengeful," he said. Well, Rossby was taking a certain poetic license with that expression, suggesting that some immoral behavior was about to be punished; indeed the effects of abrupt climate change certainly may feel like vengefulness. For what it's worth, though, Rossby may have been right about the risks posed by changing the composition of the atmosphere. Climate scientists now realize that, just as a moving hand is more likely to throw a switch than a still one, anything that changes the system runs the risk of provoking abrupt climate change.

Climate science is young, and researchers speak of abrupt change with a real sense of discovery. Gerard Bond at Lamont-Doherty Earth Observatory, after analyzing seafloor cores that revealed abrupt change, saw the big changes coming in 1993. "It's like just before plate tectonics revolutionized geology," he told Richard A. Kerr of the journal Science. "Everything then was in a state of confusion. A few key pieces of evidence came to light, and then it looked simple. There could be a new theory coming out of this for how the earth's climate system operates."

This book does not present a grand new theory of how Earth's climate system operates, although it tells a remarkable story of researchers whose work has dramatically advanced their science toward that goal. Abrupt climate change is a brilliant new insight into the way the world works. In the end, there is still uncertainty. It is the nature of the climate system. Scientists don't have all of the answers, of course; science doesn't work that way. But they have more of them, and they are fascinating, and there is less confusion.

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