September is the time for annual stocktaking. As the Arctic melt season nears its end, the remaining ice cover called September minimum is an important climate change indicator. "In the winter 2015/16, air temperatures over large parts of the Arctic Ocean ranged more than 6 °Celsius above the long-term average," explains CEN sea ice physicist Kaleschke. "Higher temperatures stunt winter sea ice growth."

The ice thickness was examined as well. High resolution measurements via aircraft in various Arctic regions reveal: "Especially the newly formed first-year ice was very thin this year, hardly more than one meter. Normally, it is almost twice as thick," explains Christian Haas. "The multiyear ice, by contrast, was about as thick as in previous years, about 3 to 4 meters. This delayed the June and July ice losses considerably before strong winds kicked off the melt in August after all." Aiming for continuous ice thickness measurements, Universität Hamburg and the AWI jointly developed a new data product. A first in combining measurements of the two ESA satellites CryoSat and SMOS it can indicate trends. "Thus, by the end of the Arctic winter, we could already see that the ice was 10 centimeters thinner than in previous years-a significant decline" according to Lars Kaleschke.

The current sea ice extent is determined based on satellite data. A procedure enhanced by Kaleschke's team allows for exact imagery resolutions up to 3 kilometers. Hitherto, exact resolutions ran at not less than 12 kilometres. The innovative visualization yields particularly clear images of details, such as vortices, channels, and ice edges, providing valuable clues about ice dynamics and thus its stability. Hence, we can see the unusually early breakup of sea ice in April north of Alaska, the so-called Beaufort Sea vortex. Compared with past years, the May and June ice extents were indeed much lower than ever before. Also extraordinary: even the central sea ice in the immediate vicinity of the North Pole is showing numerous open water areas this year.

The Northeast and Northwest Passages have been ice-free again since the end of August 2016. For the past few weeks, Yachts and a cruise ship have been using the southern route of the Northwest Passage. In 2008, both passages had been passable simultaneously for the first time.

Arctic sea ice is considered a tipping element in the climate system and an early warning system for global warming. In the 1970s and 80s, summer minimums still averaged around 7 million sq km. "The retreating Arctic sea ice clearly indicates unmitigated global warming," says Lars Kaleschke.