Instrumented narwhals put a number on the warm water reaching East Greenland's inner glaciers

The readings showed that warmer, saltier water of Atlantic origin had travelled as far as 300 kilometres (186 miles) inland, through channels into basins hundreds of miles from the open coast, and that research vessels often must turn back where narwhals can continue swimming.

Heide-Jorgensen said scientists already knew from other studies that atlantification is occurring in East Greenland, but it was surprising to see how much Atlantic water is flowing into areas 300 kilometres or farther from Greenland's outer coast.

The researchers combined the narwhal records with more than 2,200 earlier sets of temperature and salinity measurements and modelled how conditions changed with depth, location and season.

The narwhals carried conductivity, temperature and depth (CTD) sensors that recorded water temperature and salinity at different depths while tracking the animals' locations; the devices were satellite transmitters.

From August 2017 to July 2020 the six narwhals recorded data from a total of 2,060 dives.

Some of the narwhal readings extended beyond 1,500 metres (4,900 feet) depth.

Each whale was held for less than 30 minutes while its sensor was attached, and the tags stayed in place for 85 to 332 days.

Rather than recording every dive, the tags saved measurements from the deepest qualifying dive during each 12-hour period, producing up to two sets of readings per day.

The researchers also used ships to gather data about the area, and used both sources together.

Data collection occurred each time a whale dove at least 50 metres below the surface, with at least 15 minutes between dives.

The models show that the layer of Atlantic water has become thicker while the upper layer of Polar Water has become thinner, according to co-author Susanne Ditlevsen.

The researchers report that the warm water is likely contributing to melting of glaciers in Nordvestfjord in Scoresby Sound and in Nansen Fjord and Kangerlussuaq Fjord farther south, while glaciers in shallow coastal areas are less affected by atlantification.

East Greenland's fjords are layered, with colder, relatively fresh Polar Water near the surface and warmer, saltier water of Atlantic origin moving beneath it.

Most of the influx of Atlantic waters occurred at depths exceeding 150 metres.

Compared with glaciers terminating in shallow basins, glaciers terminating in deep basins tended to have significantly higher water temperatures at depths of 200 to 500 metres.

Scoresby Sound and Kangerlussuaq showed higher temperatures and salinity than the shallower Blosseville Coast.

Scoresby Sound spans about 10,000 square kilometres and its waters can reach depths exceeding 1,000 metres.

Heide-Jorgensen, a biologist at the Greenland Institute of Natural Resources and lead author, said there are very few oceanographic measurements from Scoresby Sound despite it being the world's largest fjord system.

Heide-Jorgensen said narwhals visit areas that are logistically difficult and expensive to reach with traditional methods and sample localities in winter that are usually only sampled in the ice-free summer months.

The work was published in Science Advances as 'Narwhals document atlantification of East Greenland', DOI 10.1126/sciadv.adr1424.

Ditlevsen said: 'Now we suddenly have measurements from places we can use in our models, giving us a much more complete picture of the ocean.'

When Greenland's glaciers lose ice, freshwater enters currents flowing toward the North Atlantic, where it can affect ocean circulation and climate patterns far beyond the Arctic.

Between 1960 and 2021, ocean temperatures in the region increased by about 0.047 degrees Celsius per year.

Narwhals favour cold water, so they may have sampled the warmer Atlantic layer less often than the Polar Water above it, and because they followed their own routes rather than survey lines the readings did not cover every fjord or water layer evenly.

Although narwhals have carried data-collection devices before, this is the first time devices attached to narwhals were used to measure salinity.

The project was reviewed and approved by the Institutional Animal Care and Use Committee of the University of Copenhagen.

The 2,060 dives average about 343 profiles per instrumented narwhal.

At the stated maximum of two profiles per day, the longest-lasting tag could have returned at most about 664 profiles.

Kristin Laidre, a polar scientist at the University of Washington's Polar Science Center who was not involved in the study, said narwhals have been used as ocean sampling platforms several times before because they sample remote ice-covered areas and deep depths, making repeated dives in one area over many months.

The 2,060 narwhal profiles are about 94 percent as many as the more than 2,200 previously archived measurement sets for the region, so the animal-borne data roughly matches the entire prior archive in size.

A trend of 0.047 C per year across the 1960-2021 record implies total warming of roughly 2.9 C.