Is everything different in the Arctic?
Usually, vast quantities of ice algae grow under the Arctic ice. They form in the shallower shelf sea off Siberia and drift with the floes towards the Arctic. But during their Arctic expedition in September 2023, the researchers find no trace of the algae, which is an important food source for many marine organisms. The expedition crew gets to the bottom of the causes.
It's still not quite over, but already 2023 is a year of dramatic climate records. The summer was the hottest on record worldwide. The year as a whole is also well on course for a host of other records. Record heat in the oceans, record melting of glaciers, record minimum sea ice cover in the southern hemisphere in Antarctica. In addition, there are unprecedented extreme events: Gigantic forest fires in Canada and Siberia. Burning landscapes in the holiday regions of the Mediterranean. And deadly storms and flood disasters such as in Libya.
And what about the Arctic?
Even in the far north, the ice-covered ocean around the North Pole, it looked set for a new negative record in spring. But when the research icebreaker Polarstern returned to Bremerhaven from the Arcwatch-1 expedition in the Arctic ice, the scientific crew had surprising results on board. The sea ice of the central Arctic Ocean did not melt as much as expected during the summer. The minimum coverage, which is always reached here in September, was 4.33 million square kilometers in 2023. Although this was smaller than the previous year (5.03 million square kilometers), it was significantly higher than the previous record low in 2012 (3.48 million square kilometers). In addition, the ice was also slightly thicker than in previous years and the melt ponds at the surface, which are otherwise so typical for summer, were missing. Instead, the sea ice was covered almost everywhere by an unusually thick layer of snow, which probably protected the ice from the heat of the sun and atmosphere and thus from increased melting. And there were more unexpected findings from the Arctic in 2023.
"We really rubbed our eyes in amazement and asked ourselves the whole time: What is going on here?" says expedition leader Antje Boetius, Director of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). For almost two months, she along with 53 researchers and 43 crew members from 15 countries were on board the Polarstern in the far north to determine the current state of the Arctic. The icebreaker covered 5,311 nautical miles - a distance from Hamburg to Cape Agulhas at the southern tip of Africa - and stopped at nine "ice stations" to precisely explore every level of the Arctic ecosystem on, in, and under the sea ice down to the bottom of the deep sea. One of these stations was also the geographical North Pole.
"We were particularly surprised by the underside of the ice," explains Antje Boetius. "Normally there are huge amounts of ice algae here, especially Melosira arctica, which forms meter-long chains that hang deep down into the ocean. This year, however, the ice was practically dead at all the stations we examined. The algae were completely absent. And with it, the export of food to the deep sea." Instead of the typical "marine snow" of dead algae, during their investigations with underwater robots the researchers encountered swarms of tunicates, small crustaceans and winged snails under the ice. "The ice algae are an important food source for the entire Arctic ecosystem. In 2012, during the largest ice melt to date, they lay in large clumps on the deep-sea floor. A feast for bacteria, bristle worms and sea cucumbers," says the marine biologist. "We still saw their traces everywhere. However, after two years of abnormal ice drift, the contributions from clumps of algae on the seabed are now missing. This is an enormously rapid change from sea ice to the deep sea."
So what was going on in the Arctic in 2023? With the help of satellite data, the researchers were able to reconstruct the origin of this year's ice. Usually, the ice forms in the shallower shelf sea off the Siberian coast and then drifts with the so-called transpolar drift into the central Arctic and towards the European North Sea. This year is different. A chain of unusually stable low-pressure systems held the ice off Siberia together during the summer. Instead, floes drifted from the Canadian basin into the central Arctic. This means they did not form in the shallow shelf sea, but on the open ocean. Antje Boetius suspects that the completely different region of origin of the ice is also the reason for the lack of algae forest under the ice. This is because Melosira arctica is a typical inhabitant of the shallow Arctic shelf seas. It is still unclear whether the transpolar drift anomaly can repeat itself and how the Arctic will react to the onset of El Niño and the enormous water warming in the Atlantic in the coming years.
In addition to these rather worrying findings, there were also many beautiful and inspiring experiences during the expedition. For example, a previously unmapped seamount turned out to be a real hot spot of biodiversity. With their underwater cameras, the crew filmed extensive gardens of sponges, huge orange sea anemones and, repeatedly, the pink "Dumbo" octopus, which, with its ear-like lateral fins, is reminiscent of the little elephant from the classic cartoon. The researchers were also able to clarify the identity of a typical inhabitant of the North Pole.
"The ice station and the deep-sea research at the North Pole were of course something special for everyone," says Antje Boetius. "Since the northernmost point of the Earth's axis is not so easy to reach, we know little about what actually inhabits it at a water depth of over 4,000 meters under the ice. Our first evaluations show that the hedgehog worm lives here, an almost mythical creature that makes even hardened marine scientists smile." The worm is about 20 centimeters long, hides in the seabed and regularly extends its tongue, which can be up to two meters long, with which it licks food particles from the deep-sea floor in a large circular motion.
"The animal looks like an alien," explains the AWI director. "But the deep sea is not an alien and distant planet. Our expedition has shown that changes on the surface have an effect on the seabed within a very short time. A weather anomaly led to a different birthplace of Arctic sea ice in 2023, which is now likely to result in a food shortage due to a lack of ice algae. The sea ice minimum in September was not a new negative record, but it is fully in line with the trend of about 13 per cent sea ice loss per decade. Together with the rapid loss of mass in the Greenland ice sheet and the thawing of the permafrost, we have to realize that even well below a global warming of 1.5 degrees Celsius, there are massive impacts for the Arctic ecosystem and its inhabitants."