Climate in the Balance

And yet, only a geologic blip ago, this was a frigid and forbidding place, encased in a mile-thick sheet of ice - ice piled as high as five Liberty Place towers, spires included.

Massive ice sheets have advanced and retreated repeatedly over aeons - at a glacial pace. But what researchers have discovered recently is that climate can change in a hurry.

Their findings have led to an ultimate irony: In the debate over global warming, one of the hottest issues is ice.

The planet's temperature has warmed robustly in the last 20 years - with 2005 being the second-warmest year on record - and the Arctic polar cap is disappearing. The same melting that has raised concerns about rising sea levels has prompted counterintuitive scenarios that it could produce a fresh and disastrous big chill.

Few foresee an imminent glacial outbreak, and some serious scientists insist that one is all but impossible, but ice-core records show clear evidence that rapid coolings and warmings have happened. And that was long before humans started burning the fossil fuels blamed for at least some of the modern warming.

Today, while the debate rages over how much humans are to blame for the planet's indisputable warming, scientists are still trying to figure out what conspired to bring on the flash-frozen ice ages.

But a long and tortuous trail of evidence leads to a surprising suspect at the heart of the conspiracy: the Gulf Stream.

Logically, it would be an unlikely culprit. It is hundreds of miles from the southern extent of the last ice sheet, and it covers only about 0.2 percent of the world's ocean surface.

Yet the mighty stream is a critical piece of something much larger: the North Atlantic current system that moves warmth out of the tropics toward the North Pole and sends cold water back toward the equator - the so-called conveyor belt.

It is estimated that the Gulf Stream transports about 20 percent of the heat moved by the oceans.

If the Gulf Stream were to slow down or take a more southerly route, the change would disrupt the whole system - the North Atlantic would cool off. Europe and eastern North America might turn colder as the rest of the world heated up. Scientists think that's what happened the last time ice invaded Europe and the United States.

The question is: Could it happen again?

Of ice and dwarves

When Benjamin Franklin crossed the Gulf Stream for the last time, in 1785, he was sailing into the sunset of a stunning climate epoch, one that had spanned more than four centuries and changed the world. It had certainly changed Greenland.

Greenland wasn't always a misnomer. When Eric the Red arrived there in roughly 1000, he was able to set up a thriving colony. By the mid-14th century, however, something horrible had happened. Growing conditions deteriorated. The eating habits of the colonists changed dramatically. At first, the Viking diet had consisted of 80 percent meat, fruit and grains - and 20 percent seafood, according to carbon-isotope analyses of Viking bones. In the last days of the colony, the Viking diet was 80 percent seafood. This had nothing to do with a new wave of excellent seafood restaurants. The once-productive land was locked under ice.

Over the centuries, the growing scarcity of food took its toll. The survivors were forced to move. In their final days, they were said to be diseased and dwarflike.

What had happened was that a cooling trend had taken hold in Europe and eastern North America. It lasted through the Age of Exploration, the Renaissance, the Enlightenment, through to the beginning of the Industrial Revolution. Coming after a long worldwide warming period known as the Medieval Optimum, it radically altered landscapes and growing patterns.

The North Atlantic did not warm the New World the way it tempered Europe, and the Little Ice Age added a dose of harshness to the colonial climate. It is frozen in images of white Christmases and of Washington's crossing an ice-choked Delaware on Dec. 26, 1776.

The Little Ice Age proved emphatically that climate is not static.

Researchers now believe the cold spell was tied to subtle changes in the sun's energy, and that the Gulf Stream had a hand in the cooling.

Core issues

In 1992, Richard Alley was in central Greenland, examining ice cores, when he saw something he could not believe. He and his colleagues were looking at the layers that told them about Greenland's temperature year by year, going back millennia.

But instead of a gradual change, they saw radical shifts in the layers representing the climate 12,000 years ago. The temperatures had plunged and risen suddenly. He saw a swing of 15 degrees in a matter of 10, or no more than 30, years.

"This was a flipped switch, not a slowly turned dial," he recalls. "Something really dramatic had happened."

This made the Little Ice Age look like a snow flurry.

Alley had come across a phenomenon described in 1985 by Wallace Broecker, a chemical oceanographer and paleontologist with Columbia University's Lamont-Doherty Observatory. Broecker called it the Younger Dryas period, for an Arctic shrub that mysteriously appeared throughout Europe.

But whereas Broecker drew upon a variety of research sources, Alley was looking at direct physical evidence.

The science of climate change was itself changing.

Until the 1950s, climate was viewed as essentially a stable system, says Spencer Weart, head of the history center for the American Institute of Physics. That view was stood on its head when researchers saw evidence that big swings could occur in just a couple of millennia. By 1980, scientists came across further clues that such changes could happen in a few centuries.

Broecker tightened the possible time frame in 1985 by publishing a paper on the Younger Dryas era. In the process, he indicted the North Atlantic and gave global warming an icon.

The article, which appeared in the journal Natural History, posited that tundra conditions overspread Europe as the Gulf Stream and the North Atlantic heat-transport system broke down. Europe turned arctic.

Naturally, the editors asked for graphics.

"They wanted a diagram," he says, "so they hired an artist in Hoboken. I never met the man, so I made sketches that went through them to him. I didn't really pay that much attention."

What Broecker gave him was a sketch of his famous "conveyor belt" to describe how warm surface water moves north, is chilled by the cooler air, sinks to the sea bottom, and returns southward.

It is a simplified model of what oceanographers prefer to call the Meridional Overturning Circulation, or MOC, and it was an instant hit.

"People started to pick it up right away," Broecker says.

That the North Atlantic would be so important underscores the complexity of oceanic circulation. The Pacific is triple its size, yet the Atlantic, Broecker explains, does a better job of moving heat northward than the wind-driven currents of the Pacific.

And the mighty stream is the engine driving it.

A secret agent

A critical ingredient in the recipe for climate change is one of the most prosaic and plentiful substances on the planet: salt.

The key to keeping the conveyor belt in motion is the sinking action of the water. Salt adds weight to water, so the more saline it is, the better it sinks; the better it sinks, the faster the conveyor moves.

Why is the Atlantic saltier than the Pacific? In part, says Broecker, it's because more fresh water from rain and snow drains into the Pacific than into the Atlantic.

The differences are subtle but important. Every quart of ocean water has between 1.1 and 1.2 ounces of salt. Add a mere 0.03 ounces of salt to the water, and there is the same sinking effect as cooling the water by several degrees, by Broecker's calculation.

This is why any buildup of freshwater is so troubling: It could dilute the ocean subtly but critically. In the case of the Younger Dryas era, Broecker theorized that a mighty pulse of freshwater from melting glaciers stopped the sinking action. Freshwater accumulated in the far North Atlantic, and it froze. The conveyor suddenly slowed, interrupting the northward flow of warm water - and warm air.

The Gulf Stream couldn't do its job.

What Alley found in his Greenland ice cores was that such a cosmic change could happen suddenly. In Weart's view, it marked a sea change in scientific opinion.

"The whole notion of rapid climate change was very hard for science to accept," he says. "The guys who said there could be rapid climate change had to drag the rest of the climate community kicking and screaming."

In the movie The Day After Tomorrow, the conveyor shuts down and the United States turns arctic in about 24 hours. While the film had some scientific ancestry, thankfully it bore little resemblance to cold reality.

Broecker says that recent findings have postponed The Day After Tomorrow indefinitely. Only a major icing-over of the North Atlantic would bring about a radical cooling of the Northern Hemisphere. Ice would repel the sun's energy and create more icing. That would happen only if all the Greenland ice melted and injected enough freshwater to shut down the conveyor belt. No computer models see that happening in the foreseeable future.

"The red flag went way up because of these abrupt changes, and it was scary," Broecker says. "It took us 15 years to get it in context. Now I think we can lower the red flag to half-mast."

But not lower it all the way. For scientists are now finding disturbing changes in the North Atlantic.

Fresh concerns

Oceanographer Ruth Curry is looking at the world on a computer screen, and she doesn't like what she sees.

She has been poking around the North Atlantic since she came to the Woods Hole Oceanographic Institution 25 years ago looking for "adventure."

She ultimately found it in an esoteric field: studying the salt content of the North Atlantic. She has become an expert on the subject, and it is suddenly a high-profile pursuit.

She displays a color-enhanced global map that shows a band of bright yellow in the tropics, indicating salty water.

"The tropics in the lower latitudes are getting more saline," she says, "and the high latitudes in both hemispheres are getting fresher."

That could be because as the planet warms, more water evaporates, causing more rain and snow in the higher latitudes. The freshwater precipitation is evaporating from the tropics, leaving the ocean saltier, and wringing out as rain and snow farther north, making the ocean there fresher.

Some of the freshwater pouring into the Atlantic is trickling off Greenland, she believes, and that is disconcerting. The Greenland ice sheet has shrunk by about 20 percent since the late 1970s, coinciding with dropping salt levels in the North Atlantic.

"It's been known for some time that it's been freshening," Curry says, "and we've just recently figured out how much. We've never experienced it in our time of taking measurements."

Curry says the freshening is almost assuredly the result of worldwide warming. The planet's rising temperature could be causing that extra rain and snow.

Greenland ice, however, is a far scarier source of freshwater. Curry says this in the modulated way she says everything, like a classical-music disc jockey informing the audience that it's just heard a Mozart piano concerto.

Greenland is an immense glacial repository. It is estimated that if all the Greenland ice were to melt, worldwide sea levels would rise more than 20 feet - roughly the equivalent of the worst of Hurricane Katrina's storm surge.

Realistically, for anything that catastrophic to happen, it would have to become a whole lot warmer, and given the current rate of increase in carbon-dioxide levels, that could take one or two centuries.

Unfortunately, no one knows exactly how sensitive the climate system is.

Does it have a "tipping point," beyond which it would change with a rapidity unprecedented in the period of recorded history?

Curry holds that we are in a particularly dangerous period because, as ice melts, temperature changes accelerate. Ice repels the sun's warmth, sending it back into space, which helps explain why the Arctic is a perennial ice box.

The power of frozen ground cover was evident in the Philadelphia area last week. On Wednesday morning, the temperature in the snow-covered suburbs fell to 3; while the low at the snow-less airport was 15.

Once ice melts, the repellent effect is lost. The bare ground absorbs the sun's energy, and the overlying atmosphere warms dramatically, compared with the ice-covered landscape. More ice melts, and that becomes contagious. It's one reason the Arctic warming has far outpaced that of the rest of the planet, raising worries about melting.

"From the geologic record, we see that there have been events that last centuries," Curry says, "but this is a global-warming world, not an emerging-from-an-ice-age world, so all the circumstances are different."

The chances of a shutdown of the conveyor are remote, if not out of the question. But any significant changes in the oceanic circulation would likely have major - and wholly unpredictable - impacts on climate. At the peak of the Little Ice Age, the Gulf Stream did not shut itself down. Researchers believe, however, that it slowed down, or maybe wandered from its usual trek. If that happened again, they don't want to be caught by surprise.

Today, concerns about the state of the ocean run so deep that an unprecedented international effort is under way from the Straits of Florida to Greenland to track changes in the flow of the North Atlantic.

Currents of change

Each and every week, a bulky freighter that looks like an aircraft carrier attached to a four-story apartment tower leaves Newark, N.J., bound for Bermuda bearing supplies.

Along the way, the M.V. Oleander, owned by Bermuda Container Lines, relays vital measurements across the Gulf Stream that help scientists at the University of Rhode Island keep track of the current. Once a month, the cargo includes a government volunteer who takes deepwater readings by ejecting temperature probes from a device that resembles a caulking gun.

Is the mighty stream slowing down? Is it speeding up? Is the water getting dangerously fresher?

The Oleander project is part of an immense and costly reconnaissance of the Gulf Stream and its tributaries. At least five countries are involved in the effort.

Next month, Rhode Island's Thomas Rossby is starting an Oleander-style program on the other side of the ocean. Rossby is the son of the legendary Carl Gustav Rossby, one of the first to propose that oceans drive climate. He and a colleague will install a measuring device in a ferryboat to take weekly readings from Scotland to Iceland.

A good 40 degrees in latitude and temperature away, one of Rossby's former students, William Johns, is keeping watch on a cluster of moorings off the Florida coast.

This is a cooperative venture between the University of Miami's Rosenstiel School, where Johns works, and the United Kingdom. They are maintaining a 3,000-mile-long measuring network all the way from Florida to the Canary Islands. The British have committed about $40 million to the project.

They all aim to answer the same question: Is the ocean changing?

So far, at least two new studies suggest that Curry's concerns about the freshwater buildup in the North Atlantic are warranted.

Satellite data have detected a slowing of the circulation from Ireland to Labrador, according to a research team led by NASA's Sirpa Hakkinen. The team said that if the slowing continues, it might lead to large-scale ocean and - eventually - climate changes.

In a second study, about 1,500 miles to the south, a group of British scientists this month reported a 30 percent slowdown in the movement of Atlantic deepwater.

Hakkinen and Henry J. Bryden, the head of the British team, cautioned that their results weren't conclusive.

Bryden looked at measurements taken at five intervals from 1957 to 2004.

Hakkinen said it was impossible to predict whether the slowing in the so-called North Atlantic gyre would continue or was part of a natural cycle.

Herein is a fundamental problem of oceanic research: the period of record is minuscule.

Carl Wunsch, a respected oceanographer at the Massachusetts Institute of Technology, says researchers are just beginning to build a baseline to track the movements of the North Atlantic conveyor. Right now, they have little basis for comparison.

"Only in the last 10 years have the observations begun to be available to allow you to know what's going on out there," Wunsch says. "Is the system changing? Yes. Is the system slowing down? Possibly. Are we undergoing a major climate change? We don't know."

Climate evidently obeys the first rule of weather, only on a grander scale: What might happen is almost always more interesting than what is happening.

If anything, however, the uncertainty makes it even more important to find out what the conveyor belt is up to. For the volatility of climate is inarguable.

"These scenarios are conceiveable," Wunsch says, "and we sure as hell want to know what's going on out there."

"It appears there can be changes within decades when you reach this tipping point," says Weart, the physicist-historian. "We may have reached it."

How will they know?

Scientists think the mighty stream might tell them. Endlessly moving along its 3,000-mile-long path, with 60 million years of stored knowledge, it is a force of nature that for millennia has stabilized the Earth's climate.

They continue to mine it, to probe its secrets, and to build on the legacy of Ben Franklin, the man who, 280 years ago, was so taken by the peculiarly warm waters coursing through the North Atlantic.