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You might be familiar with the narwhal – otherwise known as the “unicorn of the sea”. The whale species, with their iconic spiral tusks that can extend up to three metres long, live year-round in Arctic waters across Canada, Greenland and Russia. They attract committed tourists determined to see and photograph them in the wild, where, if lucky, they can be observed gathering and feeding in large groups.
Between 2017 and 2019, a group of scientists travelled to the eastern coast of Greenland also hoping to see some narwhals. Unlike tourists, however, they had a slightly more unusual task in mind: together with a group of local hunters from the town of Ittoqqortoormiit, they aimed to attach electronic tags that allowed the narwhals to measure water temperature and salinity as they travelled around the region. Their results are now published in the journal Science Advances.
To understand why these narwhals make such useful scientific instruments, it helps to understand the waters they travel through. The continental shelf of East Greenland has a complex network of ocean currents. At depths of hundreds of metres, a relatively warm (up to 3˚C), salty current of water is given the name “Atlantic intermediate water” after its origins in the North Atlantic. Above this – at depths shallower than 150m – is “polar water”, originating from the melting of sea ice and the Greenland ice sheet. Despite reaching temperatures down to a frosty -2˚C, the low salt content of polar water (due to its origins as ice) means it is less dense and can float on top of the warm water beneath.
For oceanographers, attempting to gather data about this complex system is extremely challenging. Heavy multi-year sea ice meant that being able to access the region with large vessels was uncommon until the motor-driven ships of the 20th century. Inside the long, deep coastal fjords, icebergs are an ever-present danger. And even if access were easy, polar research vessels are an expensive and scarce resource: there is no way that we could gather detailed observations of every nook and cranny of the East Greenland coast.
A solution, then, is narwhals. Scientists have been recruiting sealife into scientific studies for decades, attaching various instruments to animals ranging from whales to turtles to seals. Narwhals make a particularly good candidate for the problem at hand: they are known to perform uniquely deep dives (1,500 – 2,000m depth) in predictable areas, often under thick sea ice.
These almost perfectly match the movements that scientists need to extract vertical “CTD” (conductivity-temperature-depth) records of the ocean. By tagging six narwhals with temperature and salinity sensors, the study, led by the Greenland Institute of Natural Resources, could gather more than 2,000 records of the structure of the ocean, even in places such as fjords that ships struggle to reach.
Mads Peter Heide-Jørgensen
The ocean and the ice
As a glaciologist, I have a keen interest in understanding what is happening to the warm, salty Atlantic water at the margins of Greenland. Often, this water is prevented from entering fjords by a “sill” – an underwater ridge at the mouth of the fjord. However, when sills are deep and Atlantic water is abundant, warm water can spill over the sill and approach the front of glaciers.
At the ice front, warm ocean waters can make glaciers melt and retreat faster. This has contributed to the rapid acceleration of the loss of the Greenland ice sheet since the 1990s, as the waters around Greenland have experienced what the authors of the new study call “Atlantification”.
Laura Grier / Alamy
These complex ice-ocean interactions are difficult to implement in our models of future ice sheet behaviour. We do not have detailed records of where and how water is transported through fjords, making it hard to predict exactly how glaciers will respond to the changes in the wider ocean.
The narwhal records help to fill some of this gap: where dives are concentrated, the data can be used to reconstruct a remarkably high-resolution map of water temperatures at different depths within the fjord. Additionally, the depths that the narwhals dive to provide some record of the minimum possible depth of the fjord, giving us indications of where and how big the sill might be, even without maps of the ocean floor from ship records. The records indicate that shallow fjords of less than 400m depth can limit the penetration of warmer Atlantic water, making the glaciers that flow into them more resilient to a warming ocean.
Narwhals are not the whole answer: as tagging only began in 2017, historic records from instruments on ships are needed to assess longer-term trends. There is also some uncertainty as to whether any preferences in where and when narwhals dive may bias the results obtained.
However, they have given us an extraordinary new way see what is happening beneath the surface. As tagging continues, these unlikely oceanographers could help us spot new intrusions of warm Atlantic water – and understand what they mean for Greenland’s glaciers.
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