The Arctic Ocean is one of the most distinct and isolated ecosystems on earth, rich in unique wildlife and sparkling with ice. But when the climate warms up, it looks less like itself – and more like the warmer, salty Atlantic to the south.
“Atlantification,” as scientists call it, is a major threat to the sensitive, highly specialized marine life of the far north.
The Arctic Ocean has a unique structure from the start – it contains superimposed layers of warmer and colder water covered by a sheet of sea ice that shrinks in summer and expands in winter. Arctic organisms, from tiny algae to majestic polar bears, are closely adapted to this particular system.
But Atlantification is accelerating the melting of the Arctic sea ice, which is already rapidly disappearing. And it can disrupt marine food webs, driving off certain arctic species, and attracting new ones from warmer climates.
Scientists already knew, thanks to the data collected by satellites, that Atlantification had been going on for decades. And they know that as the Arctic warms, things are likely to get worse. But new research finds it has been a problem for much longer than scientists previously suspected.
A to learn, appeared in the Journal last week Scientific advances, notes that the Arctic Ocean began to change more than a century ago.
The sea ice cover began to shrink. More sunlight could get through to the water. Algae began to bloom earlier in the season. The water warmed up steadily in summer. In general, the sea became warmer and salty.
The study focuses on a region of the Arctic Ocean near the Fram Strait, a narrow water passage between Greenland and the Norwegian islands of Svalbard. It is an important gateway between the Arctic and the Atlantic.
The Barents Sea, which connects to the east side of the Fram Strait, is now one of the regions most affected by the Atlantic Ocean. It has experienced one of the strongest warming in the entire Arctic Ocean. And with the decrease in sea ice cover, the water has become warmer, saltier and more mixed.
Studies have shown that the process is a kind of self-reinforcing cycle.
Because the Arctic Ocean is so structured and layered, the Atlantic water flowing north into the region is usually trapped in a warm, salty layer that underlies the colder, fresher surface water just below the sea ice.
But the warming in the region is causing the sea ice to melt. And as that protective cap disappears from the surface of the water, the ocean begins to swirl and mix. The warmer layer of Atlantic water bubbles to the surface and causes even more melting.
A Study 2018 found that the Barents Sea has become significantly warmer and salty in recent decades. The top 200 feet of water, typically the coldest and freshest layer, have warmed about one and a half degrees Celsius in the past few decades, and their freshwater content has decreased by nearly a third.
The new study used a special technique to look back in time to see how the region has changed. It analyzed ancient marine sediments drilled from the ocean floor, where they had been trapped for decades or even centuries. The chemical makeup of these sediments and the tiny marine animals they contain can tell scientists how the ocean has changed over the years.
These analyzes revealed that the ocean suddenly atlantic at the beginning of the 20th century. The study cannot definitively explain why this happened. But the authors have some ideas.
Their main theory involves a large ocean current that carries warm water north from the equator and brings cold water back from the Arctic. Scientists often describe it as a giant ocean conveyor belt.
Studies suggest that this flow – known as the Atlantic Meridional Overturning Circulation, or AMOC – slowed down more than a hundred years ago. The researchers suspect that as the current weakened, the structure of the ocean began to change, allowing masses of warm, salty water to seep into areas along the Fram Strait.
There is some debate as to why the flow began to weaken first. The slowdown coincides with the end of a natural cold period in Earth’s history known as the Little Ice Age. In the northern hemisphere, temperatures rose and glaciers melted. Some of these changes may have interrupted the flow of electricity.
At the same time, the industrial revolution was in full swing. Human societies pumped more carbon dioxide into the atmosphere – ushering in the age of anthropogenic climate change. Those early greenhouse gas emissions and the onset of modern global warming may also have played a role.
It’s not exactly clear how much of the early Arctic warming is due to natural processes compared to the early days of man-made climate change. But even if natural processes played a dominant role a century ago, scientists agree that ongoing climate change is making these processes worse.
An influx of cold fresh water from the melting Greenland ice sheet is expected to further weaken the AMOC over the coming decades. In fact, recent research has shown that the current has already reached its slowest point in the past thousand years (Climate wire, February 26).
At the same time, the rising temperatures in the Arctic will continue to melt the sea ice. Some studies warn that the Arctic Ocean could experience ice-free summers within a few decades.
The result: a warmer, salty, heavily modified Arctic Ocean.
A look into the past is often a useful preparation for the future, say scientists. It is a window into the types of changes the world could experience if the climate warms.
In this case, the study suggests that the Arctic Ocean is likely to continue to change as the climate warms.
How exactly these changes might look is still uncertain. The researchers find that climate models – which scientists use to simulate past and future climate change – do not do a good job of capturing the changes they documented in their new study.
Making sure these processes are well represented in the latest models could be a way for scientists to improve their predictions about future Arctic warming.
“Climate simulations generally do not reproduce this type of warming in the Arctic Ocean, which means that understanding of the mechanisms driving Atlantification is incomplete,” said lead study author Tesi Tommaso, a scientist at the Institute of Polar Sciences of the Italian National Research Council in a statement.
“We rely on these simulations to predict future climate change, but the lack of any signs of early warming in the Arctic Ocean is a missing piece of the puzzle.”