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  EurekAlert 7 Dec 05
Global warming could halt ocean circulation, with harmful results

National Geographic 30 Nov 05
"Mini Ice Age" May Be Coming Soon, Sea Study Warns
James Owen

PlanetArk 1 Dec 05
Key Warming Ocean Current Slowing Down - Scientists
Story by Jeremy Lovell

LONDON - The Atlantic Conveyor, a life-giving ocean current that keeps northern Europe warm, is slowing down, scientists said on Wednesday.

If the 30 percent slowdown seen over the past 12 years is not just a blip, temperatures in northern Europe could drop significantly, despite global warming, they added.

Scientists have long forecast that the Atlantic Conveyor that carries warm surface water north and cold deep water back to the equator could break down because of global warming.

According to the theory, rising air temperatures cause ice caps to melt, making the water less salty and therefore less dense so it can't sink and flow back south.

The scientists on Wednesday said this was the first time that observations had put flesh on the bones of the theory.

"This is the first time we have observed a change in the current on a human timescale," oceanographer Harry Bryden said, noting that it had completely shut down during the ice ages.

But he said the latest figures were far from proving a trend and that constant and long-term monitoring was needed. "It is like a radiator heating the atmosphere and is too important to leave to periodic observations," Bryden told a news conference to flesh out a paper he co-authored in Nature science journal.

FROM MOROCCO TO MIAMI

The Hadley Centre for Climate Prediction and Research has calculated that if the current stopped, temperatures in northern Europe could drop by up to six degrees centigrade in 20 years.

The latest figures, collated last year, are from a string of monitoring devices at various depths in the Atlantic from Morocco to Miami. It was the fifth snapshot since 1957 taken in the same area of the temperatures and currents in shallow, mid and deep ocean.

While measurements in 1981 and 1992 had shown little change, those in 1998 and 2004 had shown a major shift, with less of the warming Gulf Stream getting up to Greenland and less of the cold, deep returning current coming back. The so-called Atlantic Meridional Overturning Current is known as the Atlantic Conveyor, of which the Gulf Stream is the surface component.

"This is tantalising evidence that there may be a large change in ocean circulation under way that paradoxically could cause regional cooling," said Phil Newton of Britain's independent Natural Environment Research Council (NERC).

However, the scientists stressed that they could not be completely sure what was driving the change or how it might alter or be compensated for by winds that pick up the radiated heat and circulate it.

National Geographic 30 Nov 05
"Mini Ice Age" May Be Coming Soon, Sea Study Warns
James Owen for National Geographic News

Chilling new evidence from the Atlantic Ocean is raising fears that western Europe could soon be gripped by a mini ice age. Global warming is slowing down the ocean current that carries warm waters from the tropics to the North Atlantic, scientists say.

In the 2004 eco-disaster movie The Day After Tomorrow, a similar scenario spurred sudden, catastrophic climate change, with much of Europe and the United Stated transformed into frozen wastelands within days. That scenario remains far-fetched.

But British scientists say their new findings indicate that the threat looks all too real for northern Europe and marine animals.

Researchers at the National Oceanography Centre in Southampton, England, found that the flow of warm ocean currents toward northwest Europe has declined by 30 percent since the 1950s. The research, to be published tomorrow in the journal Nature, is based on data collected in a great swath of the Atlantic between West Africa and Florida.

Led by oceanographer Harry Bryden, the team detected other key changes in the overall Atlantic circulation system. For one thing, there appears to be a 50 percent reduction in the amount of cold, deep water flowing from the North Atlantic to the tropics, the team says.

Also, the researchers found a 50 percent increase in currents circulating within subtropical seas without reaching higher latitudes. More warm waters, that is, are staying put in the tropics.

The study supports computer model predictions suggesting that global warming will switch off the North Atlantic current in the next 50 to 100 years. (See "Global Warming May Alter Atlantic Currents, Study Says.")

"This provides the first evidence that such a slowdown is actually occurring," said Detlef Quadfasel, oceanographer at the University of Hamburg in Germany.

Important Observation

Quadfasel, who was not involved in the study, says the British team's findings aren't conclusive. They are, he said, based on limited samples of water salinity and temperature collected over five decades.

Still, Quadfasel said, "This observation is really important, even though it's at the limit of accuracy. The main message, I think, is right."

Many scientists have predicted this effect. Global warming already appears to be injecting more fresh water into polar seas due to increased precipitation and the melting of the Greenland ice cap. This freshening of the North Atlantic current makes its waters less dense—so they don't sink down to depths at which they would then be transported back south. As result, the circulation stalls, with warmer water no longer being drawn north.

Measurements of salinity levels in the North Atlantic over the last 50 years show "a significant trend toward lower salinity all over the place," Quadfasel said.

Evidence from prehistoric times shows that it is possible for northern air temperatures to drop by 10ºC (18ºF) within decades. These abrupt changes are intimately linked to switches in ocean circulation, experts say.

Full Stop?

Some computer models predict an abrupt stop to the North Atlantic current sometime in the next hundred years. The current is responsible for Europe's relatively mild climate.

"Personally I don't think it's going to happen in the next few years, but it's like a nuclear power plant—you've got to look at the risks, even if you don't expect the thing to blow up next week," Quadfasel said.

If the current does stop, he says, it would have devastating effects on northwest Europe. The freezing conditions would affect everything from agriculture to energy demand.

Marine life could also be seriously affected, according to Andreas Schmittner, assistant professor at Oregon State University's College of Oceanic and Atmospheric Sciences. Stalling of the North Atlantic circulation would halt the flow of nutrients from the deep ocean.

"Plankton abundance will strongly decline," Schmittner said. "Since plankton builds the base of the ocean food chain, animals higher up the chain—such as fish, squid and whales—can also be expected to suffer," he said.

Such impacts would not be confined to the Atlantic Ocean, he adds. Water that sinks in the North Atlantic resurfaces in the Indian and Pacific Oceans and the seas surrounding Antarctica. "Reduced sinking leads to reduced upwelling of nutrient-rich waters and hence slower growth of algae and other plankton," he said.

How Real? How Soon?

But Schmittner says a Day After Tomorrow scenario isn't likely to be appearing anywhere near you soon. "There is no reason for immediate concern or panic," he said.

It's still too early to say whether sea changes identified in the new study are part of the ocean's natural variability or the first signs a decline caused by human-induced global warming, he said.

"I received a phone call a few weeks ago from a concerned citizen who plans to sell his property and move abroad because of a possible collapse of the Atlantic overturning circulation," Schmittner added. "Such reactions are clearly unwarranted."

And what does Quadfasel, the German oceanographer, think of Hollywood's cautionary eco-tale?

"Basically I thought the movie was okay. What was wrong was the timescale. Such things don't happen within a couple of days," Quadfasel said.

EurekAlert 7 Dec 05
Global warming could halt ocean circulation, with harmful results

Absent any climate policy, scientists have found a 70 percent chance of shutting down the thermohaline circulation in the North Atlantic Ocean over the next 200 years, with a 45 percent probability of this occurring in this century.

The likelihood decreases with mitigation, but even the most rigorous immediate climate policy would still leave a 25 percent chance of a thermohaline collapse.

"This is a dangerous, human-induced climate change," said Michael Schlesinger, a professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. "The shutdown of the thermohaline circulation has been characterized as a high-consequence, low-probability event. Our analysis, including the uncertainties in the problem, indicates it is a high-consequence, high-probability event."

Schlesinger will present a talk "Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation" on Dec. 8 at the United Nations Climate Control Conference in Montreal.

He will discuss recent work he and his colleagues performed on simulating and understanding the thermohaline circulation in the North Atlantic Ocean. The thermohaline circulation is driven by differences in seawater density, caused by temperature and salinity. Like a great conveyor belt, the circulation pattern moves warm surface water from the southern hemisphere toward the North Pole. Between Greenland and Norway, the water cools, sinks into the deep ocean, and begins flowing back to the south.

"This movement carries a tremendous amount of heat northward, and plays a vital role in maintaining the current climate," Schlesinger said. "If the thermohaline circulation shut down, the southern hemisphere would become warmer and the northern hemisphere would become colder. The heavily populated regions of eastern North America and western Europe would experience a significant shift in climate."

Higher temperatures caused by global warming could add fresh water to the northern North Atlantic by increasing the precipitation and by melting nearby sea ice, mountain glaciers and the Greenland ice sheet. This influx of fresh water could reduce the surface salinity and density, leading to a shutdown of the thermohaline circulation.

"We already have evidence dating back to 1965 that shows a drop in salinity around the North Atlantic," Schlesinger said. "The change is small, compared to what our model needs to shut down the thermohaline, but we could be standing at the brink of an abrupt and irreversible climate change."

To analyze the problem, Schlesinger and his colleagues first used an uncoupled ocean general circulation model and a coupled atmosphere-ocean general circulation model to simulate the present-day thermohaline circulation and explore how it would behave in response to the addition of fresh water.

They then used an extended, but simplified, model to represent the wide range of behavior of the thermohaline circulation. By combining the simple model with an economic model, they could estimate the likelihood of a shutdown between now and 2205, both with and without the policy intervention of a carbon tax on fossil fuels.

The carbon tax started out at $10 per ton of carbon (about five cents per gallon of gasoline) and gradually increased.

"We found that there is a 70 percent likelihood of a thermohaline collapse, absent any climate policy," Schlesinger said. "Although this likelihood can be reduced by the policy intervention, it still exceeds 25 percent even with maximal policy intervention."

Because the risk of a thermohaline collapse is unacceptably large, Schlesinger said, "measures over and above the policy intervention of a carbon tax -- such as carbon capture and sequestration -- should be given serious consideration."
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