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The Arctic Ocean has been getting warmer far longer than previously believed - and it’s not directly due to climate change.
The Ocean began to warm at the end of last century due to a process called Atlantification - where warmer and saltier waters flowed in from the Atlantic.
But scientists fear that climate change could lead to more Atlantification, warming the ocean even further.
An international group of researchers reconstructed the recent history of ocean warming at the gateway to the Arctic Ocean in a region called the Fram Strait, between Greenland and Svalbard.
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Using the chemical signatures found in marine microorganisms, the researchers found that the Arctic Ocean began warming rapidly at the beginning of the 20th century.
Since 1900, the ocean temperature has risen by approximately 2 degrees Celsius, while sea ice has retreated and salinity has increased.
The results, reported in the journal Science Advances, provide the first historical perspective on Atlantification of the Arctic Ocean and reveal a connection with the North Atlantic that is much stronger than previously thought.
The findings could have important implications for sea-ice retreat and global sea level rise as the polar ice sheets continue to melt.
All of the world's oceans are warming due to climate change, but the Arctic Ocean, the smallest and shallowest of the world's oceans, is warming fastest of all.
"The rate of warming in the Arctic is more than double the global average, due to feedback mechanisms," said co-lead author Dr Francesco Muschitiello from Cambridge's Department of Geography.
"Based on satellite measurements, we know that the Arctic Ocean has been steadily warming, in particular over the past 20 years, but we wanted to place the recent warming into a longer context."
Atlantification is one of the causes of warming in the Arctic.
As the Arctic Ocean gets warmer, it causes the ice in the polar region to melt, which in turn affects global sea levels.
As the ice melts, it exposes more of the ocean's surface to the sun, releasing heat and raising air temperatures.
As the Arctic continues to warm, it will melt the permafrost, which stores huge amounts of methane, a far more damaging greenhouse gas than carbon dioxide.
The researchers used geochemical and ecological data from ocean sediments to reconstruct the change in water column properties over the past 800 years.
They precisely dated sediments using a combination of methods and looked for diagnostic signs of Atlantification, like change in temperature and salinity.
"The reason for this rapid Atlantification of at the gate of the Arctic Ocean is intriguing," said Muschitiello.
"We compared our results with the ocean circulation at lower latitudes and found there is a strong correlation with the slowdown of dense water formation in the Labrador Sea. In a future warming scenario, the deep circulation in this subpolar region is expected to further decrease because of the thawing of the Greenland ice sheet.
“Our results imply that we might expect further Arctic Atlantification in the future because of climate change."
The researchers say that their results also expose a possible flaw in climate models, because they do not reproduce this early Atlantification at the beginning of the last century.
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