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Climate scientists go with the floe
04 June 2008
From New Scientist Print Edition.
Ben Crystall

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When Norwegian explorer Fridtjof Nansen and his crew moored their ship Fram to an ice floe 700 kilometres north of Siberia, they knew they faced a long imprisonment in the Arctic ice. They had calculated that the slow movement of the ice would carry them northwards, eventually placing them within striking distance of the then-unconquered North Pole. Although they never reached this goal, their three-year journey became an epic adventure. By the time Fram broke free of the ice in August 1896, Nansen and his companions had survived dangerous sled journeys, scurvy and had nearly starved to death.

Now, more than a century later, another expedition has followed Fram's path. In September 2006, Tara, a 36-metre schooner crewed by eight scientists and engineers, moored up on the Arctic sea ice and spent the next 15 months moving slowly with it across the top of the world. This time the expedition wasn't aiming for the pole: it was an ambitious attempt to record what is happening to the polar climate in unprecedented detail.

Sixteen months later, in January 2008, Tara finally emerged from the ice some 5000 kilometres from her starting point. She had survived two harsh winters, experienced the crushing power of ice quakes, sudden ice-shattering storms and the threat of hungry polar bears. Most of the expedition's measurements have yet to be analysed, but the initial findings tell of an Arctic that's changing even more rapidly than we thought.

Tara's expedition forms part of a four-year project called Damocles, which began in 2005 to observe climate changes in the Arctic. It involves researchers from 48 institutions across Europe and Russia who have set up a unique monitoring and forecasting system that is helping to unpick the complex interactions between the region's atmosphere, ice and ocean, and to predict how climate change will take its toll.

Perhaps the most important question that Damocles hopes to answer is whether the Arctic ice pack will disappear for good and, if so, when. If the ice vanishes it will take an entire ecosystem with it. Worse, without ice to reflect sunlight, the temperature of the ocean is expected to rise enough to trigger melting of the ice sheets that cover Greenland. Sea levels could rise by up to a metre over the next century, and once the Greenland ice sheets disappear, this could reach as much as 6.5 metres in total.

These kinds of predictions come from global climate models; complex computer simulations which calculate how changes in greenhouse gases influence temperatures, and how this might affect things like weather patterns, ice coverage and sea level. The most sophisticated models tie the atmosphere, the ocean and the ice caps together, but even these failed to predict the rapid climate changes that the Arctic has experienced in the past two years.

To really know what lies ahead, climatologists need real-world measurements. That's a problem in the Arctic because, unlike Antarctica, it has no solid land close to the pole on which to build permanent monitoring stations. There is, of course, a huge expanse of sea ice, but while much of it is still present all year round, it doesn't stay put. Instead it drifts on two major currents. One, the Beaufort Gyre, in the Beaufort Sea off the north coast of Alaska, is driven by winds associated with high-pressure weather systems, and tends to rotate the ice clockwise (see Map). The other, the Transpolar Drift Stream, is driven by strong westerly winds and forces vast quantities of ice along a more or less straight line from the north coast of eastern Siberia, past the pole and south towards the Atlantic.

While this makes it difficult to obtain long-term data from any one geographical point, Arctic researchers make use of this motion to sample weather conditions across the polar region. Every year, aircraft operated by the International Arctic Buoy Program drop dozens of buoys across the Arctic ice. As they drift, they transmit meteorological and oceanographic information back to base, allowing researchers to take measurements from a variety of locations.

Although useful, this information gives little more than a series of snapshots of the region to go on. Using it to model the polar climate would be like trying to identify a symphony from just a few dozen scattered notes. And while researchers can learn a lot from satellite data, nothing beats actually going to the Arctic and taking continuous measurements day and night. That's where Tara comes in.

The expedition has been a long time coming. Originally conceived in the 1980s by French explorer Jean-Louis Etienne - who in 1986 became the first man to walk solo to the North Pole - and scheduled for 1996, the first planned journey in the Transpolar Drift Stream (TDS) was cancelled at the last minute when its main sponsor pulled out. The team of French scientists who helped plan the expedition had to wait a decade until Damocles revived their dream of following in Fram's wake.

Tara finally set off from the port of Lorient in France in July 2006. Two months later, she was sailing past huge chunks of drifting ice hundreds of kilometres north of Siberia. With the help of a Russian ice-breaker which carved out a route, she made her way north and eventually tied up to an ice floe about 5 kilometres long by 3 kilometres across.

By the time the ice-breaker left, a week or so later, the surrounding sea was completely frozen, and Tara began to drift with the TDS. With the long journey across the Arctic now under way, the crew unloaded their supplies and equipment for the Damocles study onto the ice, ready to start their observations. Then, a week later, things turned sour. A heavy swell, whipped up by a storm to the south, hit the ice floe. It disintegrated "like a mirror being shattered", recalls Grant Redvers, the expedition leader. "We had already deployed all of our scientific instruments onto the ice, so we had to try to recover them." It was 10 days before they could reach the last of them, but luckily no key equipment was lost.

As the sun disappeared in October, the team settled down to a daily routine of scientific measurement and maintenance chores - in freezing temperatures and 24-hour darkness. As well as monitoring the ice thickness and atmospheric chemistry, they drilled through the ice and lowered probes several thousand metres down to profile the temperature and salinity of the water.

It was during the expedition's first winter that Matthieu Weber, the boat's engineer, stumbled upon the team's first surprise. While making routine measurements of salinity and sea temperature, he noticed that his sonar equipment appeared to have developed a fault. Rather than picking up the expected reflection from the seabed several thousand metres below, the sonar signal seemed to suggest the sea was less than 50 metres deep.

Checks confirmed that the instrument was working properly, so Weber radioed Jean-Claude Gascard, project leader of Damocles, at the Pierre and Marie Curie University in Paris, France, for advice. Initially Gascard was puzzled. Then he began to suspect that the sonar might be recording the formation of ice crystals - a precursor to sea ice known as frazil ice. To test this idea, Gascard asked Weber to take a close look at the seawater through a hole in the ice. "He came back and said he saw a lot of ice crystals coming up like ping-pong balls," Gascard recalls.

Rising snow

Explorers as far back as the 1800s had documented this strange phenomenon but they didn't have the technology at the time to find out where the crystals were coming from. Tara's sonar measurements revealed that frazil ice forms between 20 and 30 metres below the surface. The pressure and temperature at these depths holds water in a supercooled state, but any turbulence is enough to destabilise it, triggering the formation of frazil ice, which rises towards the surface like a snowfall in reverse.

"This phenomenon isn't included in the models," notes Gascard. Yet it could be an important factor in influencing the fate of the ice. A layer of cold water about 30 metres thick is known to insulate the Arctic pack ice from warm water deeper down, brought by currents from the south. Gascard suspects that rising frazil ice may disrupt this cold layer in unanticipated ways, mixing the cold and warm water layers and increasing the heat flow to the underside of the ice. Little is known about the long term implications of this, but Gascard plans to further investigate the effect later this year on Vagabond, a yacht used as a polar base in Norway's Svalbard archipelago.

After this initial insight, Tara's crew had to sit out the long, sunless winter before they could begin their full programme of experiments. When spring finally arrived, the team deployed their full range of instruments once more. These included sensors attached to a balloon that travelled up 1.5 kilometres into the atmosphere to record air temperature, humidity, atmospheric pressure, wind speed and wind direction. Radiometers on, above and below the ice were set up to measure its albedo, or reflectivity, in sunlight.

The team also monitored atmospheric ozone levels close to the ground - and there were surprises here, too. When the sun returned in March, they found that ozone levels dropped rapidly, and for three weeks from late April, there was no measurable ozone in the air.

Arctic ozone levels close to the surface were already known to fall each spring as sunlight triggers a photochemical reaction between ozone and bromine present in sea salt. These sustained losses, though, are unprecedented, says Christian de Marliave, the expedition's scientific co-ordinator. "We have seen levels drop before, but only for a few days at a time." The researchers speculate that this low-altitude ozone "hole" will become worse as more of the Arctic's older ice disappears, since fresh ice formed each winter seems to release more bromine than older ice. The long-term implications of the missing ozone have yet to be determined, but since ozone helps to break down airborne pollutants, it may affect the Arctic's ability to deal with increasingly dirty air.

Spring and its accompanying daylight made it easier for the team to work, but it also brought new dangers. Female polar bears and their cubs began to emerge from their winter quarters and were on the lookout for food. Tara's first line of defence was its two guard dogs, Zagrey and Tiksi, whose job was to warn the crew of any approaching bears. "During the summer, we had bears and their pups passing close by every couple of days," recalls Redvers. One day, a bear got a little too close for comfort. Zagrey needed a few stitches after that encounter, Redvers says. Luckily for both dogs, that was the closest shave of the trip.

The ice itself brought more danger. Much of the time it would have been easy for the team to forget that Tara was afloat, recalls Redvers, but occasionally the floe would suddenly move, compressing and fracturing the ice around the ship. "I don't think we will ever become completely accustomed to the distant growl of breaking ice," Redvers wrote in his diary, "the screeching sound inside Tara, the vibrations reverberating throughout the boat and the unstoppable indiscriminate force of bus-sized blocks of ice coming our way."

These "ice quakes" were certainly unsettling, but they were also illuminating. A network of five seismometers placed around Tara allowed the team to collect important information about the how the ice pack deforms and breaks in response to changing weather patterns. They aim to use these measurements to investigate the long-term durability of the ice.

The team also beamed sonar signals through the ice and used changes in their pitch, plus tiny movements in the ice generated by ocean swell - to work out the thickness of the ice pack. Twenty years ago it was 3 metres thick, and while scientists estimate that warming in the Arctic has thinned the ice by more than 1 metre on average, direct measurements had been hard to come by. The team from Tara has established that, on average, Arctic ice in the TDS is now just 1.5 metres thick.

Changes in the thickness of the pack ice might explain one of the biggest surprises to come out of the expedition so far. At around 10 kilometres per day, Tara's transpolar drift was two to three times as fast as had been predicted. Originally, Tara was expected to emerge around July this year from the ice in the Fram Strait - named after her predecessor - yet she actually broke free six months earlier, in January (see "Ship shape"). "Something is wrong with the models," says Gascard. "But there may also be something in the ice itself. As the ice thins it might become more mobile."

Gascard and his team hope that the first fruits of Tara's journey will be published later this year. That won't be a moment too soon, he says, as warming in the region seems to be accelerating. Satellite measurements have shown that the Arctic lost around 1.5 million square kilometres of pack ice between 2006 and 2007 - an area three times that of France. This caught everyone by surprise, says Gascard. And if ice continues to melt at this rate, the Arctic ocean is likely to be ice-free in the summer by 2030.

In fact some observers suspect that this milestone will occur much sooner. Mark Serreze of the US National Snow and Ice Data Center in Boulder, Colorado, suggests that summer ice will be gone in the next couple of years. If he's right, Tara's journey may turn out to have been our last chance for an intimate look at a permanently frozen Arctic.

From issue 2659 of New Scientist magazine, 04 June 2008, page 42-45

Ship shape Sailing a schooner into the Arctic might seem like madness, but French explorer Jean-Louis Etienne commissioned Tara with this very mission in mind. Her designer, Michel Franco, borrowed many of the tricks that Norwegian explorer Fridtjof Nansen incorporated a century earlier into his vessel, Fram. While Fram's oak hull was up to 70 centimetres thick, Tara relies for its strength on an aluminium alloy which in many places is just 2.5 centimetres thick. Unlike steel, which becomes brittle at low temperatures, this alloy can flex without breaking, even while surrounded by ice. And while Fram relied on reindeer fur, felt and linoleum for insulation, Tara's hull is lined with a thick layer of foam and plywood, which insulates the cabin and living quarters (see image, in red). Nansen's greatest contribution to Tara's design was the shape of the hull - which is critical to surviving an Arctic winter. Following Nansen's lead, Franco designed Tara's hull to be broad, smooth and round so that, rather than being crushed like an egg, the boat would pop up like an olive stone squeezed between finger and thumb, and sit on top of the pack ice. It also featured a lifting centreboard instead of a fixed keel, and removable propellers and rudders. These precautions worked: Tara suffered just a small dent at the stern, and another stretching a metre or so along the hull.Tara's next mission is yet to be decided, but her owners hope she will return to the Arctic within five years.

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