Ces deux dernières années, les observations satellites avaient suggéré l’existence d’une reconstitution des glaces vieilles de plusieurs années dans la banquise Arctique. Le professeur David Barber, qui étudie la région depuis 25 ans, a effectué en septembre dernier une mission sur un brise glace pour vérifier in situ l’état de la banquise. Lors de cette exploration, le chercheur a constaté que les zones identifiées comme recouvertes par une glace épaisse par les satellites étaient en fait occupées par un amas de débris sur lequel s’était déposé une mince couche de glace récente. Dans cette banquise fracturée, le brise glace pouvait naviguer à 24 km/heure, sans presque rencontrer de résistance, alors qu’en eaux libres sa vitesse de croisière est de 25 km/heure. « La navigation à travers le pôle est imminente, car le type de glace qui s’y trouve ne représente plus un obstacle pour les navires à la fin d’été et en automne, » estime M. Barber.

Université du Manitoba, 27 novembre 2009

Arctic sea ice has duped satellites into reporting thick multiyear sea ice where in fact none exists, a new study by University of Manitoba researcher David Barber has found.

In 2008 and 2009 satellite data showed a growth in Arctic sea ice extension leaving some to reckon global warming was reversing. But after sailing an ice breaker to the southern Beaufort Sea this past September Dr. Barber and his colleagues found something unexpected : thin, “rotten” ice can electromagnetically masquerade as thick, multiyear sea ice. And contrary to what satellites recently suggested, we are actually speeding up the loss of the remaining, healthy, multiyear sea ice.

The results of the study have now been accepted for publication in the peer reviewed journal Geophysical Research Letters, of the American Geophysical Union.

“These are very significant findings since the scientists and public all thought that sea ice was recovering since the minimum extent in 2007,” says Barber, a professor of Environment and Geography and Canada Research Chair in Arctic System Science.

In September 2009 Barber and others went to various points in the southern Beaufort Sea aboard the research vessel (NGCC) Amundsen. They discovered the multiyear sea icescape was not as ubiquitous as it appeared in satellite remote sensing data. And much of the multiyear ice, which is integral to maintaining the ecosystem and its inhabitants, was so heavily decayed the Amundsen easily broke through floes six to eight meters thick. Indeed, through most of the journey the Amundsen sailed at an average speed of 24km/h ; its open water cruising speed is about 25km/h.

“Ship navigation across the pole is imminent as the type of ice which resides there is no longer a barrier to ships in the late summer and fall,” Barber says.

So why have satellites been fooled ? When studying sea ice, satellites shoot microwaves at the icescape and, among other things, record how they scatter. Each variety of ice was thought to have its own unique scattering characteristics which researchers could read to determine where certain species of ice reside. But Barber and his colleagues discovered that multiyear ice and the “rotten” ice have similar near-surface temperatures, similar near-surface salinities, and both have similar open water and new sea ice fractions at the surface. So when satellites try to identify who’s who, the microwaves behave similar enough that cases of mistaken identity abound.

“Our results are consistent with ice age estimates that show the amount of multiyear sea ice in the northern hemisphere was the lowest on record in 2009 suggesting that multiyear sea ice continues to diminish rapidly in the Canada Basin even though 2009 aerial extent increased over that of 2007 and 2009,” the paper concludes.

“This has significant implications for assessment of the speed of global climate change impacts in the Arctic and for increased shipping and industrial development in the Arctic,” says Barber.