Even before the annual summer minimum, typically seen in mid-to-late September, the Arctic sea ice covers 4,35 million square kilometres. The Northeast and Northwest Passages are mostly ice-free already. Scientists from University of Hamburg and the Alfred-Wegener-Institute (AWI) estimate that the ice extent will not hit a record low in 2015 but confirm the negative trend. During the International Polar Meeting in Munich, Germany, leading sea ice specialists will be available for interviews and background discussions.

Aerial view of the German research icebreaker Polarstern during its voyage through the Arctic ice. The vessel currently concludes its research trip in the central Arctic. Bild: Stefan Hendricks, AWI
Aerial view of the German research icebreaker Polarstern during its voyage through the Arctic ice. The vessel currently concludes its research trip in the central Arctic. Bild: Stefan Hendricks, AWI

"Despite contrary predictions, we have established that the arctic sea ice is not rebounding," says Professor Lars Kaleschke from University of Hamburg's Cluster of Excellence CliSAP, who has been tracing the ice development based on satellite data. "Only 4,35 million square kilometres of the Arctic Ocean are currently ice-covered sooner and lesser than minimums reached in 2013 and 2014. The climax of the summer melt is not expected before next week."

Arctic sea ice is considered a tipping element in the climate system and an early warning indicator of global warming. In the 1970s and 80s, summer minimums still averaged 7 million square kilometres. "Similar to developments in 2007, this year's summer minimum may decline to 4.2 million square kilometres," reckons Dr Marcel Nicolaus from the AWI.

Though not undercutting the record low of 3.4 million square kilometres in 2012, it would be the second-lowest extent since satellite observations began in the 1970s. The exact annual level also depends on short-term weather conditions.

The negative record of 2012 with 3,4 million km2 will not be touched this season. Nonetheless, this year’s minimum extent will be the second lowest since observations with satellites began.
The negative record of 2012 with 3,4 million km2 will not be touched this season. Nonetheless, this year’s minimum extent will be the second lowest since observations with satellites began.

"This July, for instance, a high-pressure area above the North Pole made the wind rotate clockwise," explains Lars Kaleschke. The resultant ice movement severed a relatively thick ice arm from the compact sea ice cover in the Beaufort Sea. The CliSAP researcher anticipates that the ice extent will further diminish substantially if this arm melts completely due to its isolated and more southern position. As the Northeast and Northwest Passages are mainly free of ice this summer, once again, both routes allow transit of ships.

The development of minimum Arctic sea ice extent in September since 1972. Grafik: Marcel Nicolaus, AWI
The development of minimum Arctic sea ice extent in September since 1972. Grafik: Marcel Nicolaus, AWI

In order to determine the total amount of arctic sea ice, its thickness must be taken into account, too. This is the only way to assess whether the total ice mass has been reduced or merely squeezed together. "The best assessments of ice-thickness in the Arctic expanse are provided by the satellites CryoSat-2, primarily thick ice, and SMOS, primarily new and thin ice. As yet, these methods are unsuitable in the summer when melt ponds are dominating the arctic landscape," says researcher Marcel Nicolaus. In situ ice thickness measurements complement satellite data.

The map shows the Arctic Ocean with an ice arm in the Beaufort Sea. Median ice concentration on September 1, measured over 15 years (21992 – 2007) as a September minimum sea ice extent, is shown in red, and possible sea routes (Northwest and Northeast Passage) in blue.
The map shows the Arctic Ocean with an ice arm in the Beaufort Sea. Median ice concentration on September 1, measured over 15 years (21992 – 2007) as a September minimum sea ice extent, is shown in red, and possible sea routes (Northwest and Northeast Passage) in blue.

Therefore, scientists aboard the research vessel Polarstern are collaborating to retrieve comparative ice thickness data during the arctic summer months. On-board, Stefan Hendricks from the AWI reports: "The current ice thickness compares to that measured in previous years. Over the past days, it snowed so much that a 20-centimetre layer of snow already covers the ice floes. In contrast to 2012, the snow came earlier and built up a thicker cover." What could become a problem: snow works like insulation - unusually heavy snowfalls slacken ice growth in winter.

In the next few days, Hendricks and his colleagues are to measure ice thicknesses with a towed pinger locator flown across the ice by helicopter. They will distribute autonomous measuring devices that continue to send snow and ice data directly to the research institutes via satellite after the expedition. Such field measurements help verify new computational models.

Source: AWI, Bremerhaven