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North Pole. Atlantic. Long ago, the two oceans existed in harmony with the warm, salty Atlantic waters flowing gently into the Arctic. The layered nature of the Arctic – sea ice at the top, cool fresh water in the middle, and warm, salty water at the bottom – helped maintain the boundary between the arctic ocean and the warmer Atlantic.
But everything changed when the larger ocean began to flow faster than the polar ocean could accommodate, weakening the separation between the layers and transforming Arctic waters into something closer to the Atlantic. This process, called atlantification, is one reason the Arctic is warming faster than other oceans.
“This is not a new invasion of the Arctic,” said Yueng-Djern Lenn, a physical oceanographer at Bangor University in Wales. “What’s new is that the properties of the Arctic are changing.”
The satellites provide some of the clearest measurements of changes in the Arctic Ocean and sea ice. But their record dates back just 40 years, obscuring how oceanic climate has changed in previous years.
“We need some kind of time machine to go back,” said Tommaso Tesi, a researcher at the Polar Sciences Institute-CNR, Italy.
In an article published Wednesday in the journal Science Advances, Dr. Tesi and colleagues were able to turn back time with one-yard-long sediment cores from the seafloor that archive 800 years of historical changes in Arctic waters. Their analysis found that Atlantification began in the early 20th century, decades before the process was documented by satellite imagery. The Arctic has warmed by about 2 degrees Celsius since 1900. However, this early Atlantification did not occur in current historical climate models; an inconsistency that the authors say may reveal gaps in these estimates.
Dr. “This is a little troubling because we rely on these models for future climate predictions,” Tesi said.
Mohamed Ezat, a researcher at the Norwegian Arctic University’s Tromso campus and not involved in the research, described the findings as “outstanding”.
Dr. “Information on long-term past changes in the Arctic Ocean hydrography is needed and it is too late,” Ezat wrote in an email.
In 2017, researchers extracted a core of sediment from the seafloor of Kongsfjorden, a glacial fjord at the eastern end of the Fram Strait, a gateway between the Norwegian archipelago of Svalbard and Greenland, where Arctic and Atlantic waters mix.
Researchers regularly sliced the core and dried these layers. Then came the painstaking process of sifting through and identifying the samples’ foraminifera, which are single-celled organisms that use minerals from the ocean to form complex shells around them.
When foraminifers die, their shells drift to the seafloor and accumulate in layers of sediment. Living things in sediment samples are very important clues; By identifying which foraminifera are present in a sample and analyzing the chemistry of their shells, scientists can gather features of past oceans.
The team’s original idea was to reconstruct the oceanographic conditions of a region that includes both Arctic and Atlantic waters and dates back 1,000 to 2,000 years. But in slices of the core dating back to the early 20th century, researchers noticed a sudden, large increase in the concentration of foraminifera that favor saline environments – a sign of Atlantification much earlier than anyone has documented.
“One study came as quite a surprise,” said Francesco Muschitiello, an oceanographer at the University of Cambridge and author of the paper.
The amount of sediment was so high that the researchers were able to piece together a chronology of past climate in increments of five or 10 years. Additionally, a molecular biomarker can identify the specific year 1916, when coal mining began in Kongsfjorden. Because the foraminiferal shift occurred just before this marker, the researchers estimate that Atlantification began around 1907, give or take a decade.
When the researchers compared data from paleoclimate models with others to see if they overlap, they found that current climate models did not have this early sign of Atlantification. Researchers suggest a number of possible reasons behind this absence, such as underestimating the role of freshwater mixing in the Arctic or the region’s sensitivity to warming.
Not involved in the research, Dr. Lenn sees a difference between this early Atlantification and the current rapid Atlantification driven in large part by the melting of Arctic sea ice. Dr. “After the start of the industrial revolution, it’s too early for us to have accumulated too much heat in the planetary system and it’s anthropogenic at this point,” Lenn said.
The authors are unsure of the exact reasons behind the early Atlantification. Dr. If this is due to human effects, “the whole system is much more sensitive to greenhouse gases than we previously thought,” Muschitiello said.
In another possibility, earlier natural warming may have made the Arctic Ocean much more susceptible to the accelerated Atlantification of recent decades. “Could we have destabilized a system that was already changing?” Dr. Tesi said.
This is the maddening mystery of any paleoclimate model. “There weren’t any of us there,” said Dr. Laughing.
While this applies to humans, it does not apply to corals in the Fram Strait. Long-lived animals record changes in climate and other parameters, making them excellent watchdogs of climate history. Dr. Tesi hopes to study the strait’s cold-dwelling corals to see what information they can offer about the Atlantic’s invasion of the Arctic.
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