Ice age to warming - and back?

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.

Policymakers are beginning to pay attention. Last week, the Senate Commerce, Science, and Transportation Committee sent to the full Senate a bill that would give NOAA $60 million for research into the causes of abrupt change. The work could help provide more accurate modeling of past and future climate change, perhaps yielding clues that could serve as an early warning to abrupt change.

Meanwhile, a report prepared for the Defense Department bids Pentagon planners to elevate the study of abrupt climate change "beyond a scientific debate to a US national security concern." The study was prepared by the Global Business Network, a corporate strategic planning and consulting firm in Emeryville, Calif.

These actions are fueled by a growing body of evidence over the past five years that Earth's climate has a history of rapid variations - and that if the paleoclimate record is any indication, this history repeats itself. Some periods, like the Little Ice Age, would cause hardships today, but industrial countries probably could adapt, researchers say. The Little Ice Age lasted roughly from 1300 to around 1870 and dropped temperatures in parts of the northern hemisphere by about 1 degree C.

To the casual observer, that drop may seem small. But from a climate and a social standpoint "it was huge," says Lloyd Keigwin Jr., a senior scientist at the Woods Hole Oceanographic Institution (WHOI) in Woods Hole, Mass.

The Little Ice Age - actually three distinct cooling periods - chilled northern Europe and parts of the United States. It sent the Vikings back to Europe from their outposts in Greenland. Farms in Norway were covered with glaciers and crop failures around Europe caused famines and spikes in grain prices.

In 1816, New England experienced its "year without summer," when many crops failed. One researcher argues that the storm that wiped out a large part of the Spanish Armada - and made Sir Francis Drake's job easier - was part of the Little Ice Age pattern.

"The Little Ice Age is the only abrupt climate change that people have experienced in industrial times," says Dr. Keigwin.

Other abrupt changes, like the rapid cooling event that peaked 8,200 years ago, could be far more disruptive.

As scientists have studied the climate record trapped in glacial ice from Antarctica and Greenland, and in mud samples extracted from beneath the ocean floor, their respect for the speed of change has grown. In the mid-1950s, a change of roughly 3 degrees C over more than 1,000 years was deemed abrupt. In 1999, a team led by Jeffrey Severinghaus at the Scripps Institution of Oceanography in La Jolla, Calif., determined that the last ice age ended with a temperature burst that raised the thermostat at Greenland by some 9 degrees C over a mere decade.

"We still don't understand the causes" behind the increase, he says. A range of abrupt regional and global changes poses the same challenge.

One potential source of change may be the North Atlantic, researchers say. There, warm water moves north along the surface, cooling as it travels. By the time the surface water reaches the far northern portions of the North Atlantic basin, it has cooled and grown denser than the underlying layers of ocean, and it begins to sink. The water then travels south along the bottom, driving an aquatic "conveyor belt" that spans the globe.

But researchers suspect that if enough fresh water, perhaps from melting ice, is injected into key spots in the North Atlantic, it can virtually shut down the conveyor. Fresh water is more buoyant than salt water and can form a layer that blocks the circulation. The Northern Atlantic region would then cool. Reduce the fresh water, researchers say, and the reverse can happen, warming the North Atlantic region.

Yet much of the evidence for this comes from records when the planet's climate was already cool, some scientists argue. So the evidence may not hold lessons for today, when the planet is in a warm hiatus between glacial periods. A more timely test of the North Atlantic's role in abrupt climate change, they say, may come through unraveling the mystery of a rapid cooling event that peaked 8,200 years ago.

By the standards of the Little Ice Age, the 8,200-year event was frosty and global. Although the event lasted only about 100 years, Greenland's temperatures dropped by about 3 degrees C. Indeed, the consultants who argue for upgrading the national security status of abrupt climate change used this event as their model.

Several researchers say the cooling 8,200 years ago may have been triggered by the collapse of ice dams holding back the waters of Lake Agassiz - a vast glacial reservoir covering the Great Lakes region of the US and Canada at the end of the last ice age. It would have flushed enough fresh water into the right places in the North Atlantic to shut down the conveyor.

But here, too, the picture grows murky. At a meeting last fall at the WHOI, researchers from the US and Canada looked at the problem and raised more questions than answers. For instance, no one has yet seen conclusive topographical evidence of such a huge outflow. If the waters of Lake Agassiz did surge into the Hudson Straits and the Labrador Sea, they surely would have carved a path in the land. This lack of evidence has sent some searching for an outflow path to the north, into the Arctic Ocean.

Core samples from the ocean floor also fail to confirm the Hudson Straits-Labrador Sea theory, says Mr. Keigwin of the WHOI. Instead, he has found evidence of freshening in the water much farther south and hugging the coast. This suggests that the water may have entered the Atlantic too far south to affect the conveyor - at least directly.

"This is an important problem," he says. If scientists can prove that freshening of the ocean did occur during this current period between ice ages, then the possibility of global warming triggering an abrupt climate change would have to be taken more seriously, he adds. "We need a coordinated effort" to answer the riddle.