THE Arctic is called by some the canary in the global-warming coal mine. Like that fated bird, carried in cages by pitmen well into the 20th century, it is sensitive to changes which might otherwise not be obvious. Canaries expired in contact with gases such as carbon monoxide and methane, warning miners to leave the area. The Arctic—or, rather, its sea ice—is similarly expiring as the Earth warms up in response to more of another gas, carbon dioxide.

The area of the Arctic Ocean covered by ice at the height of summer has been shrinking by 11% a decade for the past 35 years. But the details are obscure—because gathering data in the Arctic Ocean is hard. This year, therefore, a systematic approach to that gathering has begun. The Marginal Ice Zone (MIZ) programme, paid for by the United States Navy, is seeding the Beaufort Sea—the part of the ocean north of Alaska—with many sensors.

In March the programme’s researchers laid dozens of devices along a transect running 400km north from the ice’s edge. Some of these instruments are weather stations sitting on the ice’s surface. Others are buried in holes punched through it. These measure the thickness of the icy layer, and also the salinity, temperature, oxygen concentration, organic-matter composition and movement of the seawater beneath. All these sensors beam their data to satellites. The plan is that they will drift with the ice until it melts. Even then, some are designed to float, so they can continue their work.

Floating sensors are not all that is planned, though. At the end of July, the crew of the MIZ’s research vessel, the good ship Ukpik (“snowy owl”, in a local language), began the operation’s next phase by deploying four instrument-laden robots known as Seagliders, which will roam the depths in search of readings.

Seagliders are torpedo-shaped, but do not have conventional motors. Instead, each is fitted with an external oil-filled swim bladder and wings. When the bladder is emptied (by sucking the oil into the glider’s main body) the glider’s buoyancy is reduced. This causes it to sink, and water to flow over its wings, generating forward motion. As the vehicle nears the bottom, the oil is pumped back into the bladder, the wings change their attitude, and it sweeps gently upward—again generating forward motion. In this way, a Seaglider can cover 20km (around a dozen nautical miles) or more a day in a way that uses so little electrical power that its battery should last almost a year.

The Seagliders that MIZ has launched will range back and forth between the open ocean and the water under the ice sheet, following that sheet’s edge as it retreats. This presents a problem, for Seagliders usually navigate by surfacing and taking a fix from the satellites of the Global Positioning System (GPS). At the same time, they upload their data via another set of satellites, the Iridium network. Under the ice, both of these tasks are impossible.

The solution is borrowed from natural denizens of the deep—whales—whose songs travel thousands of kilometres by being contained and refracted within distinct ocean layers. The layers the whales employ are a long way down, but scientists at the Woods Hole Oceanographic Institution, in Massachusetts, have found a shallower one that serves in a similar way (and also has the bonus of not interfering with cetacean communication). The MIZ’s researchers are using this.

The snowy owl’s wisdom

Among the sensors the scientists placed on the ice in March were a set of eight acoustic navigation beacons. These have base-stations at the surface, which fix their locations using GPS. They then rebroadcast that information from loudspeakers hanging 100 metres down below the ice, in the transmission layer. If a Seaglider can detect two or more beacons while it is travelling through this layer, it can swiftly compute its own position.

This may not always work, because the Seagliders might stray too far from the beacons. In that case, the researchers have a pair of robotic guide dogs to assist. These are called Wave Gliders (pictured at the top of the story). One part of each Wave Glider stays on the surface, generating electricity from solar panels during the Arctic’s 24-hour summer daylight. The other part is an array of hydrofoils suspended four metres underwater. The difference in motion between the waves above and the calm below causes water to move over the hydrofoils and propel the Wave Glider forward up to twice as fast as a Seaglider. Although Wave Gliders broadcast far above the sound layer, and thus have shorter ranges than fixed beacons, they can be programmed to shadow the Seagliders, and keep them within earshot.

For the next two months the MIZ’s network of gliders, floats, buoys, icebound instruments and weather stations, together with satellites and aerial surveys, will gather the largest quantity of data yet collected on the seasonal melting of the Arctic ice sheet. An icebreaker will recover any surviving devices before the ice re-forms, late in September. Two years of analysis will follow, to try to turn the observations into new climate models. With luck, the MIZ’s researchers will thus find out exactly what song the Arctic canary is singing.