This is a guest blog I wrote for Climate Progress and Skeptical Science. You may use it as a new open thread to discuss the cracking event. I will try and do a more detailed winter analysis in April, if Allah and time permit.
The sea ice cap on top of the Arctic Ocean is often imagined to be a monolithic, continuous sheet of ice floating on water. A closer look quickly shows it is rather a collection of larger and smaller pieces of sea ice. Of course, we have all seen the images of ice floes separated by open water during summer, but even during winter the ice pack gets fractured, leading to leads that quickly freeze over again. This explains how from the 1950's onwards submarines were able to emerge at the North Pole (the image on top is showing the USS Connecticut as it surfaces in the Arctic Circle on March 19th 2011; copyright: Kevin S. O'Brien, U.S. Navy). The subs couldn't break through the thick ice and had to look for a lead where the ice was thinner.
Strangely enough those who deny the reality and potential consequences of AGW still like to abuse this event and claim it somehow proves that nothing unusual is happening up North. It doesn't prove or disprove anything, as cracks and leads have always been a normal feature of the Arctic sea ice pack. But 'normal' is a word that has become less and less applicable to the Arctic in recent times. The 2012 melting season was the latest climax in a series of record years, that showed conclusively that the ice is thinner than it has been for a very long time.
We don't even have to await the coming melting season to see this re-confirmed. We can see it right now, at the end of the freezing season. Like I just said, cracks are a regular feature of the Arctic, but this video below, made by NOAA's Visualization Lab, shows a cracking event that is very rare, if not unique:
Ice, however thin, doesn't fracture by itself. It needs wind to pull the ice pack apart. This wind was provided by a big, intense and stubborn high pressure area that started about a month ago and kicked the Beaufort Gyre into action, which is an ocean circulation pattern that transports the ice in a clockwise fashion from the North American coast towards Siberia.
This short animation of ASCAT radar images shows the movement in 10 day intervals from January 1st onwards, compared to the previous three winters. The black dot represents the North Pole, the white mass below it is the northern part of the Greenland Ice Sheet, the brighter colours represent thicker multi-year ice that survived last year's melting season: