7/16/2013 5:23:32 PM
Watching the Sea Ice
By Dr. Celia Bitz
Sea ice in July ought to be a topic to relieve us from the summer’s heat! But following a decade with nine of the lowest Arctic sea ice extent minima in the instrumental record, I watch the sea ice conditions that play out at this time of year with growing concern.
The normal seasonal march of sea ice extent yields an oscillation between maximum coverage in April and minimum coverage in September. Satellites have been consistently recording sea ice coverage since 1979. They show the extent at the September minima has retreated by about a half in the last three decades.
Last year in September 2012, Arctic sea ice reached an all time record low. It is still uncertain what the extent will be in September this year. There are some clues now that indicate that September 2013 will not likely set a new record. The main basis for this reasoning is that the sea ice extent in early summer this year was higher than it has been since 2008.
I watch the loss of sea ice in early summer closely because it sets up a pattern: increased surface heating by sunlight is absorbed in the comparably less reflective open ocean. Arctic sea ice coverage today is low compared to the sea ice extent at this time in summers a few decades ago, but typical of the extent at this time in summers in the last decade. While it is some relief if the Arctic sea ice extent does not set a new record in 2013, it is by no means a return to normal. In the upcoming weeks, the Arctic Ocean is still likely to quickly become open over coastal regions, where polar bears rely most on sea ice to hunt.
As a volunteer sea ice expert for Polar Bears International, I pay close attention to the sea ice cover of Hudson Bay, where long-term observations of polar bears indicate populations are shrinking. Hudson Bay is normally sea-ice-free for up to three months each summer and fall. However, in the last decade, the Bay has been sea-ice free on average for three-four weeks longer. The Bay is open earlier by about 10 days and stays open later by about 10-20 days.
In early November 2012, cold weather descended on western Hudson Bay and the sea ice grew earlier than it had in the last five years or so. With a week or two of extra time to grow, presumably the sea ice grew thicker, though unfortunately there are no data of sea ice thickness across the Hudson Bay to confirm my speculation.
According to the Canadian Ice Charting Service, Hudson Bay should be 1/3 covered by sea ice and 2/3 by open water at this time of year. However, at present, the sea ice coverage is only about ¼. While below normal for this time of the year, the sea ice cover is unlikely to set a record for early sea-ice free conditions this year. This is hardly good news for polar bears, but the conditions are not as dire as they have been in the majority of years since about 1998.