An entire Arctic ecosystem suddenly started shrinking within the last 10 years and could be gone within another decade. The collapse could be the largest, fastest impact of climate change seen to date.
âIt could happen within the next few years, or within 10 or 20 years,â says lead author of the Institute of Marine Research in TromsĂž, Norway. âWe canât say precisely.â
The region in question is the Barents Sea, a stretch of ocean covering 1.6 million square kilometres between the Arctic Ocean and the north coasts of Norway and Russia.
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The Barents Sea has always had two distinct zones: a warmer zone to the south, and a colder zone to the north, fed seasonally with sea ice that drifts south from thicker ice cover in the Arctic Ocean. The northern zone has a unique Arctic-like ecosystem supporting polar bears, seals and ice plankton.
It is this region that seems to be collapsing.
Ice no more
Lind and her colleagues compiled data collected from the Barents Sea by Norwegian and Russian research vessels over the past 50 years, to see how the Seaâs makeup has changed.
The cold northern zone exists because the sea there is divided into three unusually distinct horizontal layers. The surface layer supports sea ice in the winter. Below that is a middle âArctic layerâ, where fresh water from melting ice settles. At the bottom is the âAtlanticâ layer, a tongue of warm water from the Atlantic.
This stratification depends on differences in density between the three layers. Because the meltwater is relatively salt-free and cold, it is less dense and so âfloatsâ above the denser, warmer, saltier Atlantic water beneath.
This layering is sustained by a plentiful supply of fresh water into the Arctic layer each year from melting sea ice. But this crucial fresh water input has suddenly fallen away.
From 1970 to 1999, the influx of fresh water to the Barents Sea was constant. But annual fresh water input began falling from 2010 onwards. In 2015 and 2016 it was 40 per cent below the 1970-1999 average.
Seasonal sea ice cover fell by a similar amount between 2010 and 2016, compared with 1970-1999. The surface layer warmed by 1.5°C in the 2000s, and saltiness increased in all three layers. This eroded the density differences that keep the layers apart.
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Warmer and saltier
The underlying cause is the rapid warming of the Arctic due to our greenhouse gas emissions. The warmer temperatures are melting sea ice across the Arctic. In 2017, global sea ice shrank to its smallest extent on record.
âItâs all dependent on how much sea ice flows into the Barents Sea from the Arctic,â says Lind. âBut with declining sea ice cover in general, large sea ice inflows become less and less probable.â
The speed of the change is startling. Until a decade ago, Lindâs data suggests, the Barents Sea was stable.
âThis striking study could be the first example of a tipping point in the climate system being passed on a regional scale,â says of the University of Exeter, UK. âIt provides persuasive evidence that weâre seeing an abrupt shift in the state of the Barents Sea. With ongoing global and Arctic warming, itâs hard to see how such a shift will be reversed any time soon.â
In effect, the cold âArcticâ parts of the Barents Sea are being replaced by warmer âAtlanticâ waters. Ultimately the top two layers will be replaced, a process Lind calls âAtlanticisationâ.
Similar de-stratification effects have been reported further east in the Laptev Sea, north of Siberia, says Lind. This suggests the phenomenon could be occurring more widely. âI think they could be linked, but thatâs the next part of our work,â she says.
A new Arctic
The loss of the sea ice in the Barents Sea is bad news for those species that rely on it, such as polar bears, some seals and ice algae, says of the Institute of Marine Research in Bergen, Norway.
But the warmer waters are already attracting species from the southern zone of the Barents Sea: particularly cod, which have reached record high levels in the past decade. âFor species like cod, haddock and redfish, the warming would likely be positive,â says Bogstad.
While this is good news for the fishing industry, the invading cod could outcompete native predators like harp seals, minke whales and Arctic cod. âThis could have unexpected knock-on effects in the ecosystem, and cause loss of biodiversity,â says Bogstad. In Bogstad reported that, while cod levels were exploding, populations of harp seals had declined, and minke whalesâs body condition had deteriorated.
Whatâs more, the loss of the sea ice will make the Barents Sea darker, causing it to absorb more of the sunâs heat, says Lind. This will accelerate global warming, âbut we canât say for certain if there will be a significant effect,â she says.
However, the melting sea ice will not cause sea levels to rise more than they already are, because the ice was floating to begin with.
Lind also doesnât think the Atlanticisation will halt or slow the Atlantic Meridional Overturning Circulation (AMOC), a vast current that spans the Atlantic and ensures western Europe has a relatively warm climate given its latitude. Climatologists expect the AMOC to weaken due to climate change, and there is a possibility it could collapse entirely. But the changes in the Barents Sea should not affect this, says Lind.
