The devil in the deep blue sea

The last time the planet endured a dramatic temperature shift, with ice invading Europe and North America, there were no smokestacks and SUVs to blame.

But there were changes in a mighty engine that balances global temperatures, a meandering, mysterious force that flows unseen only a few miles off the beaches of Miami and Fort Lauderdale and the Florida Keys.

It's the fabled Gulf Stream. Traveled by whalers and sailors for centuries but never accurately charted until it aroused the curiosity of none other than Benjamin Franklin, the current has emerged in the last decade as a focal point for scientists studying global climate change.

The concerns about the state of the ocean today run so deep that an unprecedented international effort is underway from the Straits of Florida to Greenland to track changes in the North Atlantic.

And the Gulf Stream is a narrow but critical piece of the larger system: It moves warm surface water from the tropics toward the North Pole and pumps cold water back toward the equator in a deep-sea current -- a mechanism scientists have come to call the North Atlantic ``conveyor belt.''

Scientists believe if the Gulf Stream were to slow or take a more southerly route, the change could disrupt the system and the world's weather. Over time -- how much time is a key uncertainty in the theory -- the North Atlantic could cool, turning Europe and eastern North America colder as the rest of the world heated up. That's what many researchers think happened the last time there was a global big chill 12,000 years ago and during a less severe ''Little Ice Age'' that chilled Greenland and North America just 700 years back. The question today is: Could it happen again?

Some studies suggest it already is happening, with the Stream showing signs it may be slowing.

''This is going to be one of the big issues facing humans in this century,'' said Ruth Curry, a researcher at the Woods Hole Oceanographic Institution in Massachusetts.

Debate may rage over how much humans are to blame, but there is little doubt the planet is warming, which has prompted dark and counterintuitive scenarios envisioning a fresh Ice Age.

The 10 warmest years on record all have occurred since 1990, with 2005 the second hottest. Last hurricane season exhausted the alphabet. Water temperatures in the North Atlantic and the Gulf of Mexico have been near record highs. Arctic ice is melting at alarming rates. Few foresee an imminent glacial outbreak, and some scientists insist one is all but impossible, but ice-core records show clear evidence that rapid coolings and warmings have happened -- long before humans started burning fossil fuels many scientists blame for at least some of the modern warming.

It transports more heat northward than the Pacific Ocean, although it is only about a third the Pacific's size. The Gulf Stream -- really the western flank of a vast circle or ''gyre'' of water -- is what moves the heat, a ceaseless cycle powered by one of the most prosaic substances on Earth: salt.

Salt makes water heavier, and the Atlantic is saltier than the Pacific. Heavier water sinks faster. So as salty tropical water moves into the far North Atlantic, it begins to sink, pulling yet more warm water northward and keeping the conveyor moving.

William Johns, a professor at the University of Miami's Rosenstiel School of Marine and Atmospheric Science, is helping monitor a 3,000-mile-long measuring network all the way from Florida to the Canary Islands in a cooperative venture with the United Kingdom. His research has tracked the source of the stream, which moves along at from 2 to 6 miles per hour like an immense serpent, all the way to the Brazilian coast and the South Atlantic.

The stream pours through the straits of the Caribbean islands, a main branch shooting north up Florida's East Coast, with offshoots squeezing into the Gulf of Mexico through the Yucatán Strait, making the Gulf waters warm and deep. The water swirls into ''loop currents,'' such as the particularly deep one that fueled Hurricane Katrina.

The 2004 disaster movie The Day After Tomorrow used the conveyor as a plot device, shutting it down and turning the United States arctic in about 24 hours. While the film had dipped a toe in scientific waters, thankfully it bears little resemblance to cold reality.

Satellite data have detected a slowing of 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 last month reported a 30 percent slowdown in the movement of Atlantic deepwater, based on measurements between 1957 and 2004.

But Hakkinen and Henry Bryden, the head of the British team, cautioned their results weren't conclusive. Hakkinen said it was impossible to predict whether the slowing in the North Atlantic gyre would continue or was part of a natural cycle.

The problem is a fundamental one of oceanic research: The period of record is short.

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.''

Still, Woods Hole oceanographer Ruth Curry doesn't like what she sees when she looks at a color-enhanced version of the world on her computer screen. A band of bright yellow covers tropical waters -- an indication of 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 believes 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. 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.

The very notion of such instability in the climate was unheard of until the 1950s.

''The whole notion of rapid climate change was very hard for science to accept,'' said Spencer Weart, a physicist-historian and head of the history center for the American Institute of Physics. ``The guys who said there could be rapid climate change had to drag the rest of the climate community kicking and screaming.''

It wasn't until the 1980s when Wallace Broecker, a chemical oceanographer and paleontologist with Columbia University's Lamont-Doherty Observatory, published a paper that helped launch global-warming research and gave the science its icon -- a conveyor belt. 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 during what he called the Younger Dryas period, after an Arctic shrub that mysteriously appeared throughout Europe. 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.

Naturally, the editors asked for graphics. ''They wanted a diagram,'' he said, ``so they hired an artist in Hoboken, N.J. 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.

The reality of rapid change was further re-enforced in 1992 when Richard Alley, in central Greenland examining ice cores, 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 recalled. ``Something really dramatic had happened.''

Now, two decades after his landmark paper, Broecker believes recent findings and more refined computer modeling 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.''

Unfortunately, no one knows exactly how sensitive the climate system is, or how rapidly changes might occur. What is the tipping point?

Many researchers stress the changes they see and the uncertainty of what may happen make it even more important to find out what the conveyor belt is up to. For the volatility of climate is inarguable.

"...but there is little doubt the planet is warming..."

After a statement like that you might as well not read any further.

There is PLENTY of doubt

"...but there is little doubt the planet is warming..." now give them a break, with record freezing in Europe, snow in Delhi and across Asian, it should be obvious.

"Or are people sometimes stuck between the devil AND the deep blue sea?"

When they're not stuck between the moon and New York City or caught between a rock....and a hard place.

"global warming is a hoax."

Sure...

All you need is tens of thousands of scientists and weathermen 105 years ago to engage in a massive century long conspiracy spanning 100+ countries and to have this conspiracy continue til modern days.

Then you need modern scientists to fake new data from satellites, tree rings and glaciers and have all the governments of the world, including NASA and the Bush administration to be in on it.

1. Global warming: Real
2. Human Causation: Almost impossible but advocated by the Left
3. Kyoto treaty: pointless treaty to redistribute wealth advocated by the Left for political reasons
4. Conspiracy theories that global warming is not real: Response by well meaning people unable to distinguish between #1 and #2 and scared of #3.

The Greenland ice sheet has shrunk by about 20 percent since the late 1970s, coinciding with dropping salt levels in the North Atlantic.

It is estimated that if all the Greenland ice were to melt, worldwide sea levels would rise more than 20 feet

Have worldwide sea levels really risen by 5 feet in recent decades?

Global warming has been occuring for 18,000 years.

http://www.clearlight.com/~mhieb/WVFossils/ice_ages.html

We are in an interglacial warm period. Typically they last 15-20,000 years so wer are in the twilight of a warming period.

Climate change is controlled primarily by cyclical eccentricities in Earth's rotation and orbit, as well as variations in the sun's energy output.

The idea that man-made pollution is responsible for global warming is not supported by historical fact. The period known as the Holocene Maximum is a good example-- so-named because it was the hottest period in human history. The interesting thing is this period occurred approximately 7500 to 4000 years B.P. (before present)-- long before human's invented industrial pollution.

Since the end of the Ice Age (18,000 years ago), Earth's temperature has risen approximately 16 degrees F and sea levels have risen a total of 300 feet ! Forests have returned where once there was only ice.

It ain't blue and it ain't deep; it's green and it runs close to the surface.

What I can't figure out is why nobody seems to pay any attention to the fact that our moon seems to be escaping the gravitational pull of earth.

"Have worldwide sea levels really risen by 5 feet in recent decades?"

No.

I'm guessing the "20%" refers to a reduction in land area covered, not ice volume. Anyone know for sure?

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