According to the NSIDC, weather conditions this fall were slightly less conducive for Arctic sea ice melt than was the case in 2007, the year that witnessed the record low extent. As such, December 2011′s extent was the 3rd lowest on record. Additionally, Arctic sea ice extent on December 31st measured just 13.25 million sq. km. That was only 561,000 sq. km. more than the 2010 record low extent. Because the dipole anomaly didn't set up in the same way or with the same intensity as in 2007, Arctic sea ice extent has measured slightly higher than record minima in recent months. We can't count on these types of weather variations in the future, of course. Another reason ice extent was low but didn't set another record was the difference in ice motion: sea ice was likelier to remain in the Arctic in 2011 than in 2007 or 2010.
Since September's record low, Arctic sea ice has refrozen rapidly. December's sea ice extent increased by 2.37 million sq. km. (vs. 1.86 million sq. km. average). This situation mimics that of recent years after ice extent reaches low values in September and the sun sets for the winter. The ocean released massive amounts of heat to the atmosphere, especially this fall since the AO index has been extremely positive. This has caused cold air from the continents to be bottled up in a stronger vortex than normal and has drawn heat from the Arctic Ocean as it passes over the warmer fluid.
In terms of longer, climatological trends, Arctic sea ice extent in December has decreased by -3.5% per decade These rates are more negative this year than the previous year (a trend that has continued). These rates also use 1979-2000 as the climatological normal. There is no reason to expect these rates to change significantly any time soon. Additional low ice seasons will continue. The specific value for any given month during any given year is, of course, influenced by local and temporary weather conditions. But it has become clearer every year that the establishment of a new normal in the Arctic is occurring. This new normal will continue to have far-reaching implications on the weather in the mid-latitudes, where most people live.
Arctic Pictures and Graphs
The following graphic is a satellite representation of Arctic ice as of December 12, 2011:
Figure 1 - UIUC Polar Research Group's Northern Hemispheric ice concentration from 20111212.
Compare this with January 7th's satellite representation, also centered on the North Pole:
Figure 2 - UIUC Polar Research Group's Northern Hemispheric ice concentration from 20120107.
Hudson Bay finally froze over completely. The sea ice in the Bering Sea formed normally. What is missing is the sea ice north of Scandinavia. This is the result of anomalously warm waters from the Gulf Stream being drawn further north than is normal. This is due to the exceptionally positive AO index during the past couple of months. As a side note, this phenomenon combined with the moderate La Nina in the Pacific Ocean has led to January being a warm and dry month for most of the U.S. so far. The AO index is returning to more normal values now, so cold air outbreaks will become more likely in February.
Overall, the health of the remaining ice pack is not healthy, as the following graphsof Arctic ice volume from the end of December demonstrates:
Figure 3 - PIOMAS Arctic sea ice volume time series through December 2011.
As the graph shows, volume hit a record minimum earlier in 2011 before returning to the -2 standard deviation envelope. I know most folks don't have a very good handle on statistics, but conditions between -1 and -2 standard deviations are rare and conditions outside the -2 standard deviation threshold (see the line below the shaded area on the graph above) are incredibly rare: the chances of 2 of them occurring in 2 subsequent years under normal conditions are very, very remote. Hence the assessment that "normal" conditions in the Arctic are shifting from what they were in the past few centuries.
Switching back from volume to area, take a look at December's areal extent time series data:
Figure 4 - NSIDC Arctic sea ice extent time series through early January 2012.
As you can see, the sea ice extent has spent all of the fall and early winter well outside of the -2 standard deviation region, just as it has for 5 winters in a row. It cannot be stated otherwise: these conditions are not indicative of a healthy system.
Antarctic Pictures and Graphs
Here is a satellite representation of Antarctic sea ice conditions from December 12th:
Figure 5 - UIUC Polar Research Group's Southern Hemispheric ice concentration from 20111212.
Compare that graphic with the same view from January 7th:
Figure 6 - UIUC Polar Research Group's Southern Hemispheric ice concentration from 20120107.
Ice loss is easily visible around the continent. High ice concentrations remain well into the austral spring east of the Antarctic Peninsula (the land mass that "points" to South America). Conditions of Antarctic sea ice remain good this year.
Here is the Antarctic sea ice extent time series from December:
Figure 7 - NSIDC Antarctic sea ice extent time series through early January 2012.
After conditions caused a slowdown in melt in late November and early December, the remainder of December and January was marked by normal ice melt rates. At this point, no news is good news. The Arctic is providing more than enough excitement for the time being.
Here are my State of the Poles posts from December and August.
You can find NSIDC's January report here.