Drifting blocks of ice detached from the Antarctic glaciers, frozen cubes of a couple of metres – or even tens of kilometres.
They are a polar symbol, an environmental marker – and a huge hazard for the Volvo Ocean Race teams, sailing across the Southern Ocean from Auckland to Itajaí.“So far, we’ve detected about 20 of them for the race,” says Franck Mercier of CLS.
The French company has been contracted by the organisers to deliver ice analysis and data that Alicante Race Control then uses to place the ice limits* for Leg 5.
So how exactly do you find an iceberg?
Well, it’s a tiny bit technical, and CLS has to use a lot of different data and up to four satellites for that.
First, the sea temperature. The colder the water, the more likely they are to find drifting ice. Currents have an effect, too, pushing it all around the Southern Ocean.
Secondly, the historical data CLS has gathered these past few years – they’ve been assisting several sailing races including the Vendée Globe and the Barcelona World Race.
And finally, the radar techniques.
Now we’re talking.
Picture: The brown and red squares are potential icy areas; the green and blue stars are detected icebergs.
CLS is working with space agencies, using their satellites to scan the sea surface in search of ice. They use altimetry techniques to draft a first exclusion zone, and SAR imagery to target the areas identified as potentially dangerous and scan them thoroughly.
Altimetry only spots the biggest icebergs, that is, ones bigger than 300m. SAR imagery is much more precise, every standard image covering a 500km x 500km area. SAR satellite sensors can detect smaller bits of ice – up to 50m in the case of a high-resolution picture. That’s as accurate as it gets, and it’s pretty expensive.
“Remember it’s not an exact science,” adds Franck, a French researcher who knows the icebergs by name. “We’re contributing to decrease the risks, but we don’t suppress it.
“The idea is to detect the biggest icebergs to anticipate the position of the smaller ones, which are still very dangerous for the boats, all of this part of the ocean dynamic.”
Their biggest catch this time around? A 1km long iceberg, first spotted because of a cold water plume, then “photographed” three times.
Franck and his colleagues have done the calculations – it’s 150m wide, 300m high, underwater part included, and it weighs 25 millions tons, the equivalent of 50 super tankers.
And the current was pushing it north at 1.1 knot, straight towards the fleet’s predicted position.
So they’ve warned Race Management, who moved the ice limits north. They can change one of the points of this virtual line no later than 30 degrees of longitude before the first boat reaches it.“There’s definitely plenty of ice around,” comments Simon Fisher.
Abu Dhabi Ocean Racing’s navigator is sat at the chart table and looks at a satellite picture of the area, wearing a nervous grin. His boat is currently 2,000 nautical miles away from land – they simply cannot afford to run into drifting ice. In 2001-02, News Corp sailed through ice and it's not something the sailors recall fondly.
“All the blue dots are icebergs that have been picked up by the various radars," adds Simon. "So far they’re doing a good job at keeping us out of it.”
It’s a matter of safety – but it goes way beyond that.
“There is very little scientific literature about Antarctic drifting icebergs. The existing data only goes back 10 years or so, when sailing races took an interest.
“Because there is no record yet, we cannot really link the ice activity in the area to the global warming theme. We see more of these icebergs, that’s true, but that’s also due to our improved techniques.
“The icebergs of the northern hemisphere come from the Artic polar icecap and are studied more. They’re directly linked to the climate change - and the quantity of sea ice decreases indeed.
“But in Antarctica, there is little change. In fact, the southern ice field tends to increase in winter… it could be one of the consequences of the global warming that causes an increase in the precipitations. These are only hypotheses.”
* Ice limits:
A virtual line the fleet must leave to starboard, it can be modified by Race Management depending on the movement of the ice in the southern part of the globe. An imaginary point has been placed every five degrees, drawing a precise contour that can be adapted.
Ice limit changes on March 25:
Leg 5 Sailing Instructions Amendment 8 has been posted and communicated to the boats – waypoints 11 and 12 have been moved further north after the detection of a new iceberg close to the ice limit line had been confirmed, between 95 W and 100 W.