"Rain," sighed Diana Henderson, a National Weather Service meteorologist, when asked for the latest forecast. "We're looking at rain or a chance of rain through next Tuesday."

Rain pounded the North Bay and the Santa Cruz Mountains on Tuesday, and delivered powerful but intermittent bursts of heavy precipitation almost everywhere else.

The weather service issued flash flood warnings for southern Marin County and Capitola in Santa Cruz County, when radar images showed heavy rains approaching, but the creeks stayed in their banks.

The warning, however, drew wary residents in Marin County out of their dry homes and businesses to watch the San Anselmo creek rise. On New Year's weekend, the creek jumped its banks and flooded businesses and some residences in San Anselmo.

"At this point, I've got more raincoats than pajamas in my closet," said Katie Hogan of San Anselmo, a real estate broker who was watching the creek in the town of Ross.

The rains continued to slow Bay Area commutes, temporarily flood low-lying roads and chew potholes in pavement. Highway 1 in San Mateo County remained closed at Devil's Slide. Caltrans officials continued monitoring the slide, which has caused the road to sink and crack.

"It's slowly inching downward, which is the modus operandi of Devil's Slide," said Caltrans spokesman Jeff Weiss. "It doesn't slide quickly, but slowly and surely, until all of the water is out of the slide plain."

In the Central Valley, water and public safety officials paid close attention to levees, rivers and reservoirs. The border of Sacramento and San Joaquin counties forms a kind of flood-threat line in the Central Valley. North of the border, conditions are fair: There appears to be sufficient capacity left in the reservoirs bordering the Sacramento Valley, and levees generally are in good repair. South of the line, in the San Joaquin Valley, the situation is ominous. Reservoirs east of the valley are nearing capacity, and many levees are in marginal condition.

San Joaquin Valley flooding was considered probable by today, but officials say reservoir operators recently were able to reduce downstream releases, keeping the valley's rivers within their banks -- barely.

By Tuesday afternoon, the San Joaquin River near Vernalis in the south delta region neared flood stage, but was not expected to go higher.

"We are at 28.5 feet at Vernalis," said Michael Miller, a spokesman for the California Department of Water Resources, on Tuesday. "We technically are at flood stage at 29 feet, but we don't declare a danger flood stage until 29.5 feet, when we can expect levee overtopping."

Also on Tuesday, the state Department of Boating and Waterways closed delta waters in San Joaquin and Contra Costa counties to all boating to prevent levee overtopping by watercraft wakes. The closure is expected to remain in effect until Friday.

While the possibility of inundation has diminished for the next day or two, the flood threat will be high for weeks or months as the San Joaquin River and its tributaries remain swollen.

"The Sacramento (Valley) system can accommodate roughly 600,000 cubic feet of water a second," said Don Strickland, an information officer with the Department of Water Resources. "By contrast, the San Joaquin Valley system can accommodate 52,000 cfs. The San Joaquin simply isn't designed to handle as much water."

Officials say the current rainy season is shaping up as the fifth wettest on record for the state, and could result in unprecedented water releases.

"I've been at this job for 20 years, and last year, a wet year in its own right, was the first time I ever saw flood control releases on the Fourth of July," said Jeff McCracken, a spokesman for the U.S. Bureau of Reclamation.

But this year, said McCracken, flood control releases on California rivers could extend until Labor Day.

"With all the snow in the high country, there is a tremendous volume of water in the system," he said, "and it takes a long time to move it through. The water we release from Friant Dam (on the upper San Joaquin River) today will end up in the delta in about a week."

The San Joaquin River is running at well over 20,000 cubic feet per second by the time it hits the delta, and water levels are unlikely to diminish significantly for weeks. Such an extended period of high water will strain the river's levees to the utmost, officials say.

In other words, major flooding could occur anywhere along the San Joaquin system due to a levee breach.

"Along the Sacramento River, the levees were built and are maintained either by the state or the U.S. Army Corps of Engineers, and the standards are very high," McCracken said.

It's different in the San Joaquin Valley, McCracken said, because many of the levees there are maintained by local reclamation districts; construction and maintenance standards are lower, he said.

"The state is doing what it can to strengthen some of them, but it's difficult," he said.

Strickland said Department of Water Resources crews are bolstering levees near Firebaugh, Vernalis, Stockton and at the confluence of the Stanislaus and San Joaquin rivers.

Perhaps most worrisome is the simple fact that most of the water has yet to enter the San Joaquin's system: It's locked up as snow in the high elevations of the Sierra.

"We're talking about a snow pack that's 150 percent of average, and it's mostly still up there," said Strickland. "Usually by this time, a lot of the snow is gone -- but we're still adding to it."

The worst-case scenario, said Strickland, would be a spate of hot spring weather immediately after the passage of the current cycle of storms.

"That would present some real problems," he said. "Ideally, we'd like to see some dry, moderate weather to give things a chance to dry out."

Although rapid warming at the surface of Antarctica has been well-documented, this is the first report of climate change much higher above the continent. What's more, it is the largest warming of its kind found anywhere on Earth (Science, vol 311, p 1914).

John Turner and colleagues of the British Antarctic Survey in Cambridge, UK, monitored the temperature of the middle troposphere, at altitudes of up to 10 kilometres. They used data from 11 weather balloons stationed around the Antarctic coast between 1971 and 2003. The balloons measured air temperature, humidity and pressure.

They found that winter air temperatures over Antarctica have increased at a rate of 0.5 to 0.7 °C per decade over the past 30 years. "The warming could have implications for snowfall across the Antarctic and sea-level rise," Turner says.

While the exact causes of the warming are hard to pin down, "changes in cloud amount and increases in the greenhouse gas concentration may well be playing a part", he says.

However, if you live in Europe, don't order that snowcat just yet. The conclusions reported last year have been dismissed by many climate scientists, who say their models show the current will keep going for at least another hundred years or so. So what is really going on? Are changes in ocean circulation about to turn our lives upside down, or is this something only our grandchildren will have to cope with?

This vital question is in doubt because the behaviour of ocean currents is still remarkably obscure. On a crude level, the oceans of the world are linked together by a network of currents sometimes called the global conveyor, with warm surface flows connecting to cold deep currents. The conveyor is driven by winds and by a more complicated process called thermohaline circulation - and this is the process that has climatologists worried.

As its name implies, thermohaline circulation depends on heat and salt. An offshoot of the Gulf Stream called the North Atlantic Drift flows all the way to the seas off Greenland and Norway. Evaporation makes the water saltier, so as it is chilled by Arctic winds it becomes denser than the waters underneath it and sinks. It then spills back southward over the undersea ledges between Greenland and Scotland to form a slow, cold, undersea river called the North Atlantic Deep Water. This flows all the way to the Southern Ocean, with some water going as far the Indian Ocean, where it gradually wells up again, perhaps a millennium after it sank.

The weak link is the sinking process. Climate change is injecting ever more fresh water into the Arctic by increasing river flows and accelerating the calving of icebergs from Greenland. This fresh water dilutes the North Atlantic Drift, reducing its density and making it more buoyant. If the fresh water input reaches a critical rate, around 100,000 tonnes per second, sinking could stop entirely. The northern branch of the conveyor would stop, and warm tropical waters would no longer flow past the west coast of Europe.

With that million-gigawatt heat supply switched off, climate models suggest that air temperatures in the region could fall by between 5 and 10 °C, and parts of the US and Canada would suffer too. A switch-off like this is blamed for a cold snap 12,000 years ago called the Younger Dryas, which turned the forests of Scandinavia into tundra.

Could it be happening again? That spectre was raised in December by Harry Bryden of the National Oceanography Centre in Southampton, UK. His team took a ship from Florida to the coast of North Africa, stopping at 120 points en route to lower a bundle of instruments all the way to the sea floor. The researchers compared their results with similar measurements made at irregular intervals since 1957. According to their analysis, the deep, cold return leg of the circulation has weakened by 30 per cent (New Scientist, 3 December 2005, p 6). If that has slowed, they reasoned, then the northward branch of warm water must have slowed too.

In fact, the slowdown seems to have started nearly a decade ago. When the US National Oceanic & Atmospheric Administration made a similar survey of the Atlantic in 1998 it was interested in carbon dioxide levels and did not calculate the flow rate. When Bryden's team did the sums, they found the flow had been relatively steady between 1957 and 1992, dropped off by 1998 and remained low.

Bryden's paper prompted some nervous press coverage. "There were alarming stories saying that the sky is falling," says Carl Wunsch, a physical oceanographer at Massachusetts Institute of Technology. "It's a complicated story reduced to a fairytale". In fact, Bryden's measurements are not proof of imminent cataclysm.

One question mark is whether his team has simply seen short-term fluctuations in the ocean. "The ocean is a very turbulent beast. We tend to assume that at great depth it is quiet, but that's not necessarily so," says Stefan Rahmstorf of the University of Potsdam in Germany. Wunsch likens it to the vagaries of the weather: "It might get colder for a few days in England, but you don't necessarily say we're entering a new ice age." Bryden, however, thinks his team has found more than a stutter. "If we just had the 1998 data we'd be nervous, but 2004 is similar to 1998," he says.
Are the models wrong?

There are other reasons to be cautious. Climate models do not predict any substantial slowdown in Atlantic currents until near the end of this century. "It would mean all our models are wrong," says Rahmstorf. Bryden thinks they might well be: "I think if we measure a slowdown, the models will follow."

The trouble for Bryden is that not all observations fit in with his conclusions. If less warm water is flowing north, the seas off western Europe ought to be cooler than normal. They are not. In fact, these waters are slightly warmer than a decade ago. And direct measurements of the cold, deep currents that spill southwards over the ledges joining Scotland, Iceland and Greenland do not show a downward trend. Although these currents did slow between 1995 and 2000, they have picked up again. "We are faced with conflicting evidence," says Rahmstorf.

How to resolve this conflict? It is possible that Bryden's group has got its physics wrong. Like other groups, the team did not measure flow rates directly, but instead calculated them from measurements of temperature and salinity. Rahmstorf and Wunsch both point out that these calculations rely on assumptions that are far from proven.

Or it could be that the currents are changing in ways that no one has anticipated. There is a vast stretch of ocean between Bryden's measurements at 25° north and the overspill at around 65°. In between, the warm surface current becomes meandering and unstable, and difficult to measure, says oceanographer Tore Furevik of the University of Bergen in Norway. "There are certainly large changes going on beneath the surface of the North Atlantic, but we are still missing too many pieces in the puzzle to get the picture clear," he says.

For the moment, this rather unsatisfactory answer is the best we have. Oceanographers and climate scientists agree that thermohaline circulation will slow as the world warms, but most think it will happen later rather than sooner. In its report due next year, the Intergovernmental Panel on Climate Change is likely to predict a slowdown of at most 50 per cent by the end of this century.

This prediction, however, relies on estimates of the freshwater input. While the discharge from Siberian rivers is being monitored, that from Canadian rivers is not. The input from the Greenland ice cap could change too. The glaciers that drain the ice cap are accelerating, and in the past decade the amount of ice they spit into the ocean has doubled. Nobody can predict with confidence what they will do in the coming decades. "At the moment models don't represent the dynamics of Greenland glaciers, which may or may not start moving faster," says Richard Wood of the Hadley Centre for Climate Prediction and Research in Exeter, UK. What all this means is that even if Bryden is wrong and Atlantic circulation is not yet slowing, a shutdown could still happen sooner than most models predict.
Farming hit hard

For Europe, the timing of any slowdown or shutdown is critical. If it does happen soon, the weather will certainly get chilly. Average temperatures would be about 5 °C lower, and winters could be as harsh as those in Newfoundland. In bad years the Thames might freeze over, and even in good years farming will be hit hard.

On the other hand, if currents hold fairly steady until the late 21st century, the cooling effect of a shutdown would help to mitigate warming. There might be drastic changes in other aspects of the climate - not to mention a relatively rapid rise in sea level around the northern Atlantic (see "Rising waters") - but Europeans might escape much of the warming that occurs elsewhere.

Their distant descendants might need those snowcats, though. If greenhouse gases do eventually fall to pre-industrial levels and the world cools down again, there could be a lag of a thousand years before ocean circulation restarts. So Europe still faces the big freeze - just not for a few hundred years.

Coming back to this century, other parts of the world face even more serious consequences than Europe. A slowdown in the thermohaline circulation would reduce the transfer of heat from the southern to the northern hemisphere, shifting the Earth's "thermal equator" to the south. "One of the things that really struck us is that rainfall patterns over the whole world change dramatically," says Wood.

When he tried artificially switching off thermohaline circulation in one climate model, he found that monsoon rains weaken over India, and parts of central and south America lose half their rainfall. "It would have a huge impact on the climate of those regions," says Wood. He estimates that agricultural productivity in parts of India could fall by 30 per cent. And in the Americas? "If you lose the rain then the rainforest tends to die out." Although all of this is based on an imminent shutdown, which climatologists think very unlikely, even a delayed slowdown could seriously disturb rainfall patterns.

Wood's model also predicts that a shutdown would warm the southern hemisphere by 0.2 °C on average - not much, but against a background of rising global temperatures any extra warming will hardly be welcome. In 2004, Brazil was hit by the first ever hurricane recorded in the South Atlantic, perhaps a consequence of rising sea-surface temperatures. Could a slowdown in circulation have contributed? "Theoretically, this is possible," says Michael Mann of Pennsylvania State University in Philadelphia, "but I think it would be a leap to tie any observed change to thermohaline circulation. It could just be fluke."

While the precise effects on the climate remain uncertain, there is little doubt that a shutdown will wreak serious damage beneath the waves, since upwelling waters supply vital nutrients to the phytoplankton that are the basis of ocean food chains. A study last year predicted that the productivity of the world's oceans would fall by a fifth if the Atlantic thermohaline circulation shuts down.

There's more. At the moment, the oceans are soaking up a lot of the excess carbon dioxide pouring into the atmosphere. Without the thermohaline circulation, however, surface waters will soon become saturated and greenhouse gases will build up faster still. Any reduction in carbon fixing as forests and ocean ecosystems fail would only compound the effect.

A lot therefore depends on what is really happening in the Atlantic. To find out, the UK has launched a project called RAPID, an unprecedented effort to monitor North Atlantic currents. In 2004, on the same voyage that found the controversial signs of a slowdown, Bryden's team planted a series of 22 moorings along a line from Africa to America (see Diagram). Cables fixed to the seafloor tether instrument packages that are constantly measuring ocean properties such temperature and salinity.

With continuous measurements now coming in from the Atlantic, it should be possible to distinguish between short-term fluctuations and a longer-term trend. "Soon we'll find what seasonal variability there is and know whether what we said was a 30 per cent slowdown was above the noise level. I'm hoping we'll eventually be able to get 10 years' worth of measurements."

Then again, monitoring a single cross section of one ocean might not be enough. "People are obsessed with the North Atlantic and it's only 10 per cent of the ocean," Wunsch says. "There is a danger we're neglecting the rest." For example, there are areas of sinking ocean water around Antarctica that also help to drive the global ocean circulation. "Recent papers have started to suggest there are changes happening in the Southern Ocean," says oceanographer Steve Rintoul, based in Hobart, Australia.

Some studies show that the water in some Antarctic seas is getting less salty, for instance, but the picture in the Southern Ocean is even less clear than it is in the Atlantic. "We have to observe this system globally and indefinitely," says Wunsch. "But how the devil do you get governments to do that?"
From issue 2547 of New Scientist magazine, 15 April 2006, page 42


Rising waters

In the Hollywood disaster movie The Day After Tomorrow, the shutdown of the Atlantic circulation somehow results in a giant wave crashing over New York. That is nonsense, of course, but a shutdown of the Atlantic thermohaline current would raise sea levels in the North Atlantic.

At present, sinking waters in the Arctic produce a kind of plughole effect, lowering the sea level of this region and slightly raising it elsewhere. If sinking stops, sea levels will fall in the Southern Ocean and rise by up to a metre on some North Atlantic coasts.

A group led by Anders Levermann of the Potsdam Institute for Climate Impact Research in Germany has run a simulation that shows northern Norway getting a metre rise by the time the circulation stops altogether, while the UK, north-eastern Canada and the US see rises of up to 80 centimetres (see Map). With global sea levels also rising as the oceans warm up and glaciers melt, these regions face total rises of well over a metre by the end of the century.

One NASA study shows that if the sea level was just a metre higher on the New York coast, storm surges that now occur just once in a century would happen almost every year, flooding some low-lying suburbs. In the Thames estuary, planners believe they can handle total rises of up to two metres, but if there is also a large increase in the frequency and ferocity of storms, as some models predict, then an extra half-metre rise from a thermohaline shutdown could mean abandoning some areas to the sea.

To determine whether the San Andreas Fault is moving with the tremors, scientists with the San Andreas Fault Observatory at Depth (SAFOD) are installing instruments to measure the tremors' activity. Located near Parkfield, Calif., SAFOD is part of the EarthScope Project, an effort to study the North American continent's geology.

"Unlike the sharp jolt of an earthquake, tremors within Earth's crust emerge slowly, rumbling for longer periods of time," said Kaye Shedlock, program director for EarthScope at the National Science Foundation (NSF), which funds the project. "Although not in this case, tremors are usually produced by magma moving in cracks or other conduits beneath a volcano."

The rumblings are the first recordings of non-volcanic tremors in a deep borehole, providing scientists with data to better understand such mysterious underground movements.

The results will help geologists understand whether the deeply buried rocks of the San Andreas Fault - which are derived from an ancient subduction zone - behave in a similar way to the rocks of the Cascadia Subduction Zone, still active today.

"In the Cascadia Subduction Zone off the Pacific Northwest, for example, tremors are associated with the slow slip of the undersea Juan de Fuca tectonic plate as it submerges beneath the North American tectonic plate," said Greg van der Vink, EarthScope facility project director.

Researchers used submersible vehicles and other technology to explore the Vailulu'u Seamount, an active volcano lying off the coast of the Samoan archipelago. They found that the volcano had sprouted a new 1,000-foot cone at its summit since it was last explored 5 years ago.

Toothpaste and pillows

Dubbed Nafanua, the cone is made from large pillow lava, a type of rock that forms when hot magma spewed by underwater volcanoes or cracks in the Earth comes into contact with water and congeals into large blobs, or "pillows."

"Imagine a replica volcano on your tabletop and somebody squirting toothpaste out from underneath," said study team member Hubert Staudigel of the Scripps Institution of Oceanography in La Jolla, Calif.

"If you keep squirting toothpaste from this one place, the squirts will overlie one another. Depending on their viscosity, the squirts will have a very flat slope or just stay in place and pile on top of each other," Staudigel told LiveScience.

Moat of death

Nafanua, which is still about 2,300 feet below sea level, is creating new niches for undersea creatures as it creeps slowly towards the ocean surface.

Near its summit, eels swarm around warm hydrothermal vents and feed on shrimp carried their way by volcanic currents.

The inside of Nafanua, however, is deadly. The same currents that provide the eels with food also deposit sea creatures into a cavity of turbid and toxic waters, creating what the researchers call a "moat of death."

The bodies of fish, squid and crustaceans rot within this moat, and bright red bristleworms feed on the bacteria that slough off their carcasses.

The finding was detailed in this week's issue of the journal Proceedings of the National Academy of Sciences.

There were no immediate reports of injuries or damage from the 1:27 p.m. (0427 GMT) undersea earthquake, the agency said.

It said the quake was felt strongest in the town of Urakawa, a Pacific coastal town on Japan's northern island of Hokkaido, about 780 kilometers (480 miles) northeast of Tokyo.

The tremor was centered about 40 kilometers (25 miles) under the ocean floor off Urakawa.

The agency said there was no threat of a tsunami, potentially dangerous waves triggered by seismic activity.

Japan, which rests atop several tectonic plates, is among the world's most earthquake-prone countries.

A magnitude 5 quake can damage houses and buildings in densely populated areas.

The mumps epidemic is the nation's first in 20 years. Health officials say 515 suspected cases have been reported in Iowa, and the disease also has been seen in six neighboring states, according to the U.S. Centers for Disease Control and Prevention.

As of Monday, Nebraska has 43 reported cases; Kansas, 33; Illinois, four; Missouri, four; Wisconsin, four; and Minnesota, one.

The Iowa health department identified two people who were potentially infectious when they were traveling in late March and early April.

Officials in other states have not yet linked any cases to the air travelers. But because the illness's incubation is two to three weeks, cases may not begin appearing until about now, Teale said.

This week the CDC put out an advisory about the passengers to state health departments. "Infectious diseases can travel easily on planes and other modes of transportation," said Dr. Jane Seward, acting deputy director of the CDC's viral diseases division.

The first traveler is executive director of a Waterloo, Iowa, downtown development organization who in late March was in a delegation that traveled to Washington, D.C.

The woman, Terry Poe Buschkamp, had earlier visited the Dominican Republic where she thinks she may have caught the bug. Health officials did not release her name, but she has acknowledged her infection to the media.

Buschkamp, 51, left Waterloo, Iowa, on March 26 on a Mesaba Airlines flight to Minneapolis and then flew Northwest Airlines to Detroit. On March 26, she flew to Washington, D.C.'s Reagan National Airport. During her visit, she shook hands with Iowa's two U.S. senators, Tom Harkin and Charles Grassley, she said.

She returned to Waterloo on March 29 on Northwest and Mesaba flights, with a stop in Minneapolis.

She said she developed a scratchy throat upon her return, and after hearing reports of a mumps outbreak, went to a doctor for testing. She got confirmatory test results six days later.

During those six days, she had been to church and numerous work events, including an April 1 pub crawl that involved about 370 people. Mumps has been a mild disease for most people, but Buschkamp found the length of time she was able to spread the virus before learning her test results alarming.

"That's the real story," she said.

She said two of her fellow travelers have told her they have mumps-like symptoms, but have declined to see a doctor about it.

The second person was a young man returning from vacation in Arizona on April 1, Teale said.

He flew American Airlines, from Tucson to Dallas, then to Fayetteville, Ark., to St. Louis and finally to Cedar Rapids, Iowa.

Two people - and nine flights? "It's hard to get anywhere (from Iowa) without connecting," Teale explained.

Mumps is a virus-caused illness spread by coughing and sneezing. The most common symptoms are fever, headache and swollen salivary glands under the jaw. But it can lead to more severe problems, such as hearing loss, meningitis and fertility-diminishing swollen testicles.

No deaths have been reported from the current epidemic.

A two-dose mumps vaccine is recommended for all children, and is considered highly - but not completely - effective against the illness. About a quarter of the Iowans who have suspected cases got the vaccine, Teale said.

© 2006 The Associated Press.