This story initially appeared in Hakai Magazine and is a part of the Climate Desk collaboration.

In the Fram Strait off Greenland’s west coast, Véronique Merten encountered the foot troopers of an invasion.

Merten was finding out the area’s biodiversity utilizing environmental DNA, a way that permits scientists to determine which species live close by by sampling the tiny items of genetic materials they shed, like scales, pores and skin, and poop. And right here, in a stretch of the Arctic Ocean 400 kilometers north of the place they’d ever been seen earlier than: capelin.

And they have been in all places.

The small baitfish discovered within the northern Atlantic and Pacific Oceans is an ardent colonizer. Whenever the ocean situations change, it’s very easy for capelin to develop their vary, says Merten, a marine ecologist on the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany.

It is tough to estimate an animal’s abundance based mostly solely on the quantity of its DNA within the water. Yet in Merten’s samples, capelin was probably the most often encountered species—excess of typical Arctic fish like Greenland halibut and Arctic skate. To Merten, the proof of so many capelin thus far north is a daring signal of a worrying Arctic phenomenon: Atlantification.

The Arctic Ocean is warming shortly—the Fram Strait is nearly 2 °C warmer than it was in 1900. But Atlantification is about greater than rising temperatures: it’s a course of that’s reshaping the bodily and chemical situations of the Arctic Ocean.

Because of the oceans’ international circulation patterns, water routinely flows from the Atlantic into the Arctic. This alternate largely happens in deeper water, with currents carrying heat and comparatively salty Atlantic water north. This heat Atlantic water, nonetheless, doesn’t combine properly with the Arctic’s floor water, which is comparatively cool and contemporary. Fresher water is much less dense than saltier water, so the Arctic water tends to drift on high, trapping the saltier Atlantic water deep beneath the ocean’s floor.

As sea ice disappears, nonetheless, the floor of the Arctic Ocean is heating up. The barrier between the layers is degrading and Atlantic water is mixing extra simply into the higher layer. This is kicking off a suggestions loop, the place hotter floor water melts extra sea ice, additional exposing the ocean’s floor to daylight, which heats the water, melts the ice, and permits Atlantic and Arctic water to mix much more. That’s Atlantification: the transformation of the Arctic Ocean from colder, more energizing, and ice-capped to hotter, saltier, and more and more ice-free.

Merten’s discovery of plentiful capelin within the Fram Strait—in addition to the DNA she discovered from different Atlantic species, like tuna and cock-eyed squid, far exterior their typical vary—is additional proof of simply how shortly Atlantification is taking part in out. And its penalties might be monumental.

In the Barents Sea off Russia, for instance, a long-term study presents a grim image of how Atlantification can disrupt Arctic ecosystems. As the Barents Sea has grown hotter and saltier, Atlantic species have been “moving in and taking over,” says Maria Fossheim, a fisheries ecologist with the Institute of Marine Research in Norway who led that examine.

Fish communities within the Barents Sea, Fossheim says, have shifted north 160 kilometers in simply 9 years—“three or four times the pace that [previous studies] had foreseen.” By the top of her examine, in 2012, Fossheim discovered that Atlantic species had expanded all through the Barents Sea, whereas Arctic species have been largely pushed out.

Merten’s findings counsel the Fram Strait could also be heading in the same route. Because this examine is the primary to look at the variety of fish within the Fram Strait, nonetheless, it’s unclear how latest these modifications actually are. “We need these baselines,” Merten says. “It could be that [capelin] already occurred there years ago, but no one ever checked.”

Either means, they’re there now. The query is: what’s going to present up subsequent?