Six tagged narwhals returned more than 2,000 readings from fjords ships rarely reach

The team sorted the water by potential density. Each tag logged temperature, salinity, depth and position, and the team converted those into a measure of potential density; the density then assigned the water around the whale to an Atlantic or a polar origin ^[7][8].

Seals, belugas and bowheads have carried instruments before ^[16]. Narwhals were preferred because they come back to the same places, and repeated measurements at one place are what a time series requires ^[16]. "We have used a completely novel technique," Mads Peter Heide-Jorgensen of the Greenland Institute of Natural Resources told AFP ^[4][5], adding that "we can sit at our warm offices and drink coffee while the data comes in" ^[6].

Ships are the alternative. Heide-Jorgensen said icebreakers are relatively few and prohibitively expensive, that their crews are at risk if a vessel is damaged, and that a ship near a calving glacier could capsize ^[15]. Fixed electronic sensors are possible, but deployment has normally been confined to summer, when the water is open ^[14].

Six animals over four years produced more than 2,000 profiles, an average above 330 each, with no breakdown by animal ^[23]. The whales set the sampling pattern. Where a narwhal forages is where the column gets measured, so this technique will not produce a chosen transect or a fixed depth the way a mooring line does ^[7][16].

Getting from these readings to European weather takes several steps. Meltwater from East Greenland's fjords and coastal shelf feeds two major Arctic currents ^[10]; one of them, the East Greenland Current, is a key regulator of the Atlantic Meridional Overturning Circulation ^[11]; and scientists believe an eventual AMOC collapse could trigger profound cooling in Europe's winters ^[12]. The whale data do not measure the overturning itself. They document warm Atlantic water penetrating deep into the fjords, which the study links to intensifying glacial melt ^[3].

Catching a narwhal in a net and fitting the instrument takes about 20 minutes, described in the account as a brief, stressful episode after which the animals return to normal behaviour; the tags eventually fall off on their own ^[17]. "I wouldn't say they enjoy interacting with humans," Heide-Jorgensen said, noting that "they do it under protest" ^[18][19]. He also called narwhals "losers in the climate game", and said a warming climate is reducing their habitat and that they are predicted to move farther north to stay near cold water ^[21].