University of Bremen: Deep insights into the Arctic of tomorrow

During the MOSAiC expedition, hundreds of environmental parameters were recorded with unprecedented accuracy and frequency over a full annual cycle in the central Arctic Ocean. Researchers have now published the physical properties of the atmosphere, snow and sea ice, and ocean in three review articles in the journal Elementa and have highlighted the importance of considering all components of the climate system together. These results provide the first complete picture of climate processes in the central Arctic, which is warming more than twice as fast as the rest of the planet – processes that influence weather and climate worldwide.

The disappearing sea ice is a symbol of progressive global warming: in the Arctic, its extent has almost halved since satellite recordings began in the summer of the 1980s. Less well studied, but just as relevant, are the thickness and other properties of the ice. The question of what this means for the Arctic of the future and how these changes will affect the world was the driving force behind the historic MOSAiC expedition with the German research vessel Polarstern from September 2019 to October 2020.

Most complete picture of climate processes to date
Researchers from institutions in 20 countries had been preparing the expedition for around ten years, the total cost of which was around 150 million euros, most of which was financed by the Federal Ministry of Education and Research. With the results now presented, the researchers are creating the most complete picture of climate processes in the Arctic based on observations.

Being able to study the processes for a full year required a special concept because the central Arctic Ocean is still covered with ice in winter and is therefore difficult to access. As a result, the icebreaker froze to a large floe and drifted through the Arctic Ocean with the natural transpolar drift.

Four researchers from the University of Bremen are on board
The University of Bremen was involved in the expedition with four people on board in the areas of sea ice, satellite remote sensing and oceanography. The scientists from Bremen have now contributed to all three review articles.

Satellite data form the backbone of our understanding of sea ice change in the Arctic. Due to the harsh climatic conditions and low population density in the Arctic, reliable satellite observations are particularly important in winter. In addition to the sea ice surface, statements about the ice thickness, ice types, melt pools or the snow on the sea ice are now also possible with the help of satellites. “Many of these processes are being developed at the Institute for Environmental Physics at the University of Bremen,” says Dr. Gunnar Spreen, scientist and head of the working group for remote sensing of the polar regions at the University of Bremen.

Remote sensing instruments built on MOSAiC floe
In order to develop new methods for future satellite missions by ESA, NASA or the EU Copernicus program (e.g. CIMR microwave radiometer or CRISTAL altimeter), remote sensing instruments were set up on the MOSAiC floe. The University of Bremen coordinated and managed the remote sensing activities at MOSAiC. “We placed the largest array to date of 14 remote sensing instruments on the ice floe to all observe the same snow and ice at the same time. In space there are corresponding counterparts on satellites. Together with the detailed physical snow and ice observations, these measurements will pave the way for future satellite missions.

First results now show that a combination of radar altimeters at different frequencies can be used to determine snow depth – but also that some of the current methods can provide erroneous data if warm air intrusions or rain change the microwave properties of the ice surface. Gunnar Spreen: “Such weaknesses can now be improved thanks to the MOSAiC measurements.”

New insights into the exchange processes possible
The oceanography working group, also in the Institute of Environmental Physics, coordinated the sampling of anthropogenic trace substances (CFCs) and noble gas isotopes (helium and neon) during the MOSAiC expedition and the measurements were carried out in the in-house laboratories after the expedition. This data set promises new insights into the exchange between the atmosphere, the upper ocean, and the underlying water layers under changing ice conditions.

“Taking water samples in the arctic winter, especially in the ice camp outside the Polarstern, posed considerable logistical and personal challenges for the scientists on board. Thanks to their use, we are now able to better understand vertical and horizontal exchange processes under the ice using trace substances,” says Dr. Maren Walter from the Oceanography working group.

The three review articles serve as references for a variety of future scholarly work. “The physical observations are the basis for the interpretation of biogeochemical cycles and ecosystem processes, as well as for the coupled models that we use to learn even more about climate feedbacks and the global impact of Arctic change. These changes can influence weather and climate worldwide,” says Professor Markus Rex, head of MOSAiC and atmospheric researcher at AWI.

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