Sunday, November 28

Arctic Ocean warmed up earlier than expected –

Man-made climate change ensures that the world’s oceans are getting warmer and the poles are melting. In the Arctic Ocean, the problem is exacerbated by the influx of warm water from the Atlantic. A new study now shows, based on sediment analyzes of the last 800 years, that this so-called atlanticization of the Arctic Ocean probably already started at the beginning of the 20th century – decades earlier than previously assumed. The new results also indicate a gap in previous climate models.

Of all the world’s oceans, the Arctic Ocean is hardest hit by global warming. As the smallest and flattest of the world’s oceans, it warms up particularly quickly – with serious consequences not only for the sensitive polar ecosystems, but also for the global climate. Due to the higher water temperatures, the ice in the polar region is melting and the sea level is rising. In addition, the incident sunlight is reflected less strongly from the sea surface than from the ice, so that more heat is absorbed and temperatures continue to rise. In addition, the Arctic permafrost stores large amounts of the greenhouse gas methane, which are released when the permafrost melts.

look in the past

The influx of warm water with a higher salinity from the Atlantic also plays a role in the warming of the Arctic Ocean. Instrumental records that document this process, for example by satellite measurements, have only been around for about 40 years. A team led by Tommaso Tesi from the Institute for Polar Research of the National Research Council in Bologna has now taken a look back into the past. “We know from satellite measurements that the Arctic Ocean has been warming steadily, especially over the past 20 years, but we wanted to put the recent warming in a longer context,” says co-author Francesco Muschitiello of the University of Cambridge.

To do this, the researchers examined sediment cores from the area of ​​the Fram Strait between Greenland and Svalbard, which is a connection between the Atlantic and the Arctic Ocean. In the sediment samples, which cover a period of 800 years, Tesi and his colleagues analyzed the chemical signatures of microorganisms such as foraminifera. Depending on the temperature and salinity of the water in which they live, these unicellular marine organisms store different oxygen isotopes in their housings and thus provide researchers with clues about the environmental conditions at the time when the respective sediment layer was formed. The researchers combined this information with other data, including lipid analyzes of the sediments, local climate records and various dating methods.

Rapid warming at the beginning of the 20th century

In this way, the scientists were able to trace the temperature and salinity at the transition between the Atlantic and Arctic oceans over centuries. The result: “If we look at the entire 800-year time scale, the temperature and salinity are fairly constant for a long time,” says Tesi. “But at the beginning of the 20th century there was suddenly a significant change in temperature and salinity – that’s really noticeable. In connection with existing reconstructions, our results show a rapid and early atlanticization of the eastern Fram Strait at the beginning of the 20th century. “

The researchers’ analyzes show that at this time the sea ice was receding rapidly, causing the plankton bloom to shift seasonally, the water becoming warmer, especially in summer, and increased mixing of the water layers in winter. According to the researchers, a likely cause for the increasing atlanticization at the beginning of the industrial age is that the Little Ice Age ended in the middle of the 19th century, after which the ocean circulation in the North Atlantic changed and enabled more efficient heat transport to the North Pole. “Our results indicate that we will have to reckon with further atlanticization of the Arctic in the future due to climate change,” says Muschitiello.

Gap in the climate models

“It is still controversial what effects natural and anthropogenic influences had on the North Atlantic system,” the researchers write. They also suggest that their results suggest a possible flaw in climate models. “Climate simulations cannot reproduce the type of warming we have observed in the Arctic Ocean, which means that understanding of the mechanisms that drive Atlanticization is incomplete,” says Tesi. “We rely on these simulations to predict future climate change, but the lack of any evidence of early warming in the Arctic Ocean is a missing piece of the puzzle. Solving these modeling problems will be critical to improving the accuracy of the forecast atlantization in response to future Arctic warming. “

Those: Tommaso Tesi (Institute of Polar Sciences, National Research Council, Bologna) et al., Science Advances, doi: 10.1126/sciadv.abj2946

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