Understanding tomorrow’s Arctic

Hundreds of international researchers are currently analysing observations from the year-long MOSAiC expedition, during which hundreds of environmental parameters were recorded with unprecedented accuracy and frequency over a full annual cycle in the central Arctic Ocean. Three review articles on the MOSAiC programmes for atmosphere, snow and sea ice, and ocean have now been published in the journal Elementa, highlighting the importance of looking at 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 affect weather and climate worldwide.

Sea ice retreat has become a symbol of global warming: In the Arctic, its extent in summer has been cut nearly in half since the beginnings of satellite recording in the 1980s. Less well studied, but equally relevant, is the thickness and other properties of the ice. The questions of what this means for the future Arctic, and how these changes will affect global climate, were the impetus for the historic MOSAiC expedition aboard the German research icebreaker Polarstern from September 2019 to October 2020. With the results now available, researchers are compiling the most complete observation-based picture of climate processes in the Arctic, where surface air temperatures have been rising more than twice as fast as on the rest of the planet since the 1970s.

A special approach was needed to study these processes for an entire year, partly because the central Arctic Ocean is still ice-covered in winter and therefore difficult to access. During the expedition, the icebreaker froze on a large ice floe and drifted with the natural transpolar drift across the Arctic Ocean. This is where the surprises began. ’We found more dynamic and faster drifting pack ice than expected. This not only challenged the teams on the ground in their daily work, but above all led to changed sea ice properties and sea ice thickness distributions,’ said Dr Marcel Nicolaus, sea ice physicist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and co-leader of Team Ice in the MOSAiC project.

Why the Arctic is warming so much faster than the rest of the globe is not yet fully understood - one possible reason could be the role of water vapour. ’Unfortunately, we cannot measure water vapour in the central Arctic from the satellite with sufficient accuracy to assess its influence,’ Dr Kerstin Ebell of the University of Cologne remarked. The University of Cologne operated a novel millimetre-wave radiometer (MiRAC-P) on board Polarstern to obtain new reference data. ’Our measurements show the enormous variability of water vapour in the Arctic and in particular the very small amounts during the polar night. This will help improve satellite estimates,’ Ebell said.

Another important component of the Arctic atmospheric system is long-lived mixed-phase clouds. ’What keeps Arctic mixed-phase clouds alive for several days and how much precipitation is associated with them is still not fully understood,’ said Dr Mario Mech of the University of Cologne, who was involved in the MOSAiC-ACA aircraft campaign. As part of this campaign, the Polar 5 research aircraft was deployed in the outflow region of the MOSAiC Arctic Ocean northwest of Spitsbergen and equipped with various remote sensing and measurement instruments, including the MiRAC-A radar and the HATPRO radiometer from the UoC’s Institute of Geophysics and Meteorology.

The three review articles serve as references for a variety of future scientific work. ’The physical observations are the basis for interpreting biogeochemical cycles and ecosystem processes, as well as supporting coupled models that we are using to learn even more about climate feedbacks and the global effects of Arctic change. These changes can affect weather and climate worldwide,’ said Professor Dr Markus Rex, head of MOSAiC and atmospheric scientist at AWI. ’It is fascinating how accurately we can map individual processes and set them in relation to each other. I am pleased to see how several hundred MOSAiC participants have contributed to these publications. The international collaboration with expedition participants from so many countries continues in a productive and well-coordinated way even after more than a year. This way, we can continue to provide important insights into climate change, which can serve as a knowledge base for societal change towards treating planet Earth in a sustainable way.’