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Fall/Winter 2004 Vol. 4 No. 3

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Making Sense of a Big Screen Ice Age

By Richard B. Alley

Photo courtesy Richard B. Alley

The environment appears to be somewhat less stable in Hollywood than out here in the real world. We barely had time to recover from an impossibly big earthquake shaking our TV sets in the movie "10.5," before we had to face an impossibly large and rapid climate change freezing over our movie theaters and New York City last summer in "The Day After Tomorrow."

Fortunately, the prospect of a new ice age "the day after tomorrow" is fiction of the humorously entertaining sort. But what is the average person to make of the science hiding behind movie fiction?

Much is clear. Climate has changed, and will change. There are many humans in the world, more all the time, and we're affecting a lot of things including the composition of the air. A car produces nearly 300 pounds of carbon dioxide a week, some of which is absorbed by the oceans or plants, but a lot stays in the air. More atmospheric carbon dioxide leads to a warmer planet, based on physical principles that have been well-understood for more than a century. Measurements show that carbon dioxide is rising in the atmosphere, and that the planet is warming, whether you look at melting glaciers, satellite measurements, or thermometers in the air, on the ground, or under the oceans.

Ice cores, tree rings, and other "histories" of climate show that at certain times in the past, large and rapid jumps occurred locally across much of the globe, more than 15 degrees Fahrenheit in as little as a decade or less. The last of these really big abrupt changes was more than 8,000 years ago, and most were during the last ice age. Ecosystems were forced to move to other locations -- had there been more civilizations then, people likely would have moved, too. Smaller, regional abrupt climate changes have persisted into the current warm period, often as quick-starting and persistent droughts that may have contributed to the fall of civilizations such as the Mayan empire in the ninth century.

Abrupt changes occur when the Earth's system is forced just enough to cross a threshold. Lean a little in a canoe and you're still dry. Lean too far, and it flips over. Usually, the climate has "leaned" slowly in response to changes in sunlight or orbits or carbon dioxide. But occasionally, it has flipped to a new state and stayed there for centuries or longer. Without even knowing where all the flipping points are in the climate, we can't control or even skillfully navigate those changes.

Much attention has been focused on a possible change in the ocean circulation in the North Atlantic -- a shutdown of the cycle in which cold Arctic water sinks and flows toward the equator and then brings warmth gained there back to the poles. This shutdown, which has been implicated in past abrupt climate changes, would likely cause cold, dry, and windy conditions across much of the northern hemisphere. Although not impossible, a large change soon seems quite unlikely, and would not in any case trigger the next ice age or flash-freeze New York City. Smaller impacts of North Atlantic changes remain possible, and the slight chance of a big change draws attention. Shifts in droughts or floods, in such natural variability as El Niño -- the warming of ocean currents that affects rain patterns across much of the world -- and in the stability of ice sheets are also of interest, and might impact humans and ecosystems in many ways.

Faster and less-expected changes are harder to deal with, potentially causing societal disruption, economic hardship, and ecological stress. Emerging knowledge of abrupt climate change may prove useful to reduce or avoid these dangers. In deciding what to do about climate change, we weigh costs and benefits for us and future generations. Planning for smooth global warming over centuries is in some sense highly optimistic; all histories of climate show faster and less-predictable bumps and wiggles.

I chaired the National Research Council's Committee on Abrupt Climate Change, which authored a 2002 report that recommends the study of possible abrupt climate changes and impacts they might have. The report also recommends that we look for low-cost or no-cost ways -- we called them "no-regrets" strategies -- that could minimize the possibility of abrupt changes, and maximize the ability of economies and ecosystems to bend rather than break, in case an abrupt change occurs.

Climate change and warming are highly likely to continue, but this knowledge provides only a starting place for our ability to predict the future. Confident prediction of climate in your backyard decades from now is still a dream. Those of us who study the climate don't expect to rival movie heroes, but we hope to help.

Richard B. Alley is the Evan Pugh Professor of Geosciences at Pennsylvania State University. Alley conducts research on the paleoclimatic records, dynamics, and sedimentary deposits of large ice sheets, as a means of understanding the climate system and its history, and projecting future changes in climate and sea level.

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Copyright 2004 by the National Academy of Sciences