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Ice age to warming - and back?

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The Little Ice Age and "the 8,200-year event" are not exactly household terms. Once only a handful of climate scientists puzzled over these episodes of abrupt climate change. Now, the topic is getting close scrutiny from the Pentagon, the halls of Congress, and even Hollywood - where a disaster movie set for release in May depicts a sudden deep freeze.

One reason for all the interest? While policymakers have worried long and hard about global warming, which might raise Earth's temperature 1.4 to 5.8 degrees C by century's end, a growing body of evidence suggests natural forces could just as easily plunge Earth's average temperatures downward. In the past, the planet's climate has changed 10 degrees in as little as 10 years.

That may not sound like much. But the last time the planet was 10 degrees colder, it was still in an ice age. "There's the very real potential of the climate system changing dramatically and rapidly" in ways that lie outside modern human experience, says Mark Eakin, who heads the paleoclimatology program of the National Oceanic and Atmospheric Administration (NOAA).

The possibility of a sudden freeze doesn't mean mankind can relax efforts to curb global warming, many scientists warn. Indeed, given the complexity of Earth's climate, human activities that spew greenhouse gases into the atmosphere may increase the potential for an abrupt cooling.

For example: Regional and global climates have undergone quick and dramatic changes even after what would appear to be only gentle prodding by natural influences, Dr. Eakin says. In many cases, that prodding has been far less severe than the changes humans have wrought via industrial emissions of carbon dioxide.

"In the absence of better knowledge, we have to assume that humans are making abrupt climate change more likely - not because humans are worse than nature, it's just because we're changing the system," says Richard Alley, a Penn State University paleoclimatologist. Dr. Alley led a 2002 National Research Council panel that examined abrupt climate change and laid out recommendations for research priorities and possible adaptation strategies.

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