9/3/2012 1:06:13 PM
Sea Ice Hits New Record Low
Most sea ice experts and enthusiasts check the Arctic sea ice cover daily at this time of year because late August or September is when records are set. The Arctic set a new record in late August for lowest extent of sea ice ever. And as of the first week of September, it hasn't stopped yet.
The sea ice normally has a very large seasonal cycle, with a maximum in February/March and a minimum in August/September. The figure below shows the season cycle of Arctic sea ice area for each year since 2007 and for the average prior to 2007. The largest change in recent years has been at the end of summer, when the area has been 20-30% below the average. The red line is 2012.
The areas with unusually low ice coverage in summers in the last decade are just north of Alaska and Siberia (see figure below).
The reason for the low sea ice area in recent years is primarily because the sea ice is thinner at the start of the melt season, so it melts out more easily. Sea ice thickness varies across the Arctic, but typical sea ice is a few meters thick at the end of winter. It used to be thicker by about a meter or so.
July temperatures on Northern Hemisphere land reached record highs this year, and the previous three months also ranked very high. It is no wonder that the Arctic sea ice was low this summer. Other factors such as a the big storm that occurred off the north coast of Alaska two weeks ago may have enhanced the area loss by causing ice to blow into warm water and melt or pile up on top of itself. In 2007, unusually clear skies contributed to the area loss. Such factors as these have occurred throughout time and cause natural variability in the sea ice. But now they have a greater effect because the ice is thinner. Natural variability combined with climate change have given rise to the record loss in recent years. The question among experts is whether the sea ice can recover, even a little, in the coming decade. Climate models say it can, but only temporarily.
What does the ice loss mean for polar bears? This summer, the area that is ice free is precisely the region where polar bear habitat is most dense and widespread in the Arctic Basin. Sea ice on Hudson Bay melted out early this year, too, which presents a problem for polar bears because are unable to feed on their main prey, seals, when they come inland in summer.
Polar bears prefer to inhabit sea ice in relatively shallow waters, such as Hudson Bay, and near the continental shelf because these areas have higher biological productivity than the much deeper central basin. Seals, the favorite prey of polar bears, are also found more often on sea ice at the perimeter of the Arctic Ocean and on Hudson Bay. Polar bears use the sea ice as a platform to hunt because they are not as successful at hunting in water. It is normal for some areas of the Arctic Ocean perimeter to be ice free in summer. However, in recent years, a greater portion of the shelf is ice free and the ice has been absent longer. Hudson Bay sea ice has been absent earlier in spring and late into fall.
These factors stress the polar bear. The projected continued decline in sea ice this century is the reason that polar bears have been listed as threatened. A decision on whether ringed seals will be listed too is expected within a few weeks. It is hard to imagine that they won't when Arctic sea ice has taken another nose dive this year.
Photos copyright Daniel J. Cox/Natural Exposures.