The state of global polar sea ice area in early June 2012 has once again fallen below climatologically normal conditions (1979-2009). Arctic sea ice loss is primarily responsible for this change in condition since just last month. Arctic sea ice melted quickly in May because it was thinner than usual; Antarctic sea ice has refrozen at a near normal rate during the late austral autumn. Polar sea ice recovered from an extensive deficit of -2 million sq. km. area three months ago to a +750,000 sq. km. anomaly one to two months ago before falling back to a -1 million sq. km. deficit. After starting the year at a deficit last year, sea ice area spent an unprecedented length of time near the -2 million sq. km. deficit in the modern era in 2011. Generally poor environmental conditions established and maintained this condition, predominantly across the Arctic last year. The last time global sea ice area remained near 19 million sq. km. through May was in 2007, when the Arctic extent hit its modern day record minimum.
According to the National Snow and Ice Data Center (NSIDC), weather conditions during the latter part of the previous winter and spring were less conducive for Arctic sea ice freezing on the Atlantic side of the Arctic while conditions were more conducive than usual for freezing on the Pacific side. Sea ice melt during May was more than normal: 1.62 million sq. km. instead of 1.38 million sq. km. As such, May′s extent was below average for the month in the satellite record. Arctic sea ice extent on in May averaged 13.13 million sq. km. Barents and Kara Sea ice remained very much below normal, more so than in recent years. The Bering Sea, which saw ice extent growth due to anomalous northerly winds in previous months, instead witnessed above normal conditions. Overall, near surface temperatures were warmer than average across the Arctic Ocean.
In terms of longer, climatological trends, Arctic sea ice extent in April has decreased by -2.3% per decade. This rate is lowest in the spring months than the late summer months. Note that this rate also uses 1979-2000 as the climatological normal. There is no reason to expect this rate to change significantly (more or less negative) any time soon. Additional low ice seasons will continue. Some years will see less decline than other years (like this past year) - but the multi-decadal trend is clear: negative. 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 humans are establishing a new normal in the Arctic with respect to sea ice. 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 April 28, 2012:
Figure 1 - UIUC Polar Research Group's Northern Hemispheric ice concentration from 20120428.
Compare this with June 7th's satellite representation, also centered on the North Pole:
Figure 2 - UIUC Polar Research Group's Northern Hemispheric ice concentration from 20120607.
The sea ice in the Bering Sea, as mentioned above, formed more quickly and to a further southern extent than is normally seen, but has largely melted already due to the ice's young age and thin depth. What remained missing this winter and early spring was 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 positive AO index & NAO index during the last boreal winter and spring. As a side note, this phenomenon combined with the most recent, moderate La Niña in the Pacific Ocean has led to Dec-May being anomalously warm and dry for most of the U.S. Indeed, this year has been the warmest Jan-May period on record in the US, as I will detail in a separate post.
Overall, the health of the remaining ice pack is not healthy, as the following graph of Arctic ice volume from the end of May demonstrates:
Figure 3 - PIOMAS Arctic sea ice volume time series through May 2012.
As the graph shows, volume hit a record minimum earlier in 2011 before returning to the -2 standard deviation envelope. I understand that most readers don't have an excellent 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 indeed. Hence my assessment that "normal" conditions in the Arctic are shifting from what they were in the past few centuries. Note further that after conditions returned to near the -1 standard deviation envelope in late 2011/early 2012, volume has once again fallen rapidly outside of the -2 standard deviation area. The chances that this would happen three separate times in three consecutive years is exceptionally remote. That means that natural conditions are not the likely cause; rather, another cause is much more likely to be responsible for this behavior.
Switching back from volume to area, take a look at May's areal extent time series data:
Figure 4 - NSIDC Arctic sea ice extent time series through early June 2012.
This winter allowed the extent to do something it had not done for the most recent handful of winters: a return of ice extent to within the -2 standard deviation envelope. Indeed, the extent in April briefly matched average conditions before a relatively warm spell melted ice quickly in mid-April. The reason for this is a shift in wind conditions: speed and direction both changed from late winter through this last month. Those winds piled sea ice up instead of pushing it apart. The disadvantage: ice extent decreased, as seen in Figure 4. The advantage: ice volume grew, as seen in Figure 3. The effect on this September's minimum extent will indicate how helpful the early season winds were in building sea ice that doesn't melt every year back up. During May, as I wrote above, melting occurred at a slightly faster rate than normal, resulting in a return to near-record low extent conditions.
Occasionally, I also like to include a supplemental time series graph that the NSIDC report contains. Here is this month's supplemental graph:
Figure 5 - NSIDC Arctic sea ice extent time series of ice extent conditions comparing previous years' data and 2012 data through May.
This graph contains all of the same data as the previous graph and adds the time series lines from the previous 5 winters. As you can see, extent varies during the same month from year to year. The recent decline in extent, caused by a change in wind direction and speed, has reduced Arctic ice extent back to ~13 million sq. km., which is well below normal for May. The past three winters also saw similar magnitude reductions through May, although the starting and ending values were obviously different. Despite these differences in subsequent years, the minimum ice extent values were quite similar: at or near the record low set in 2007. Will fall 2012 be any different or will the surge in ice growth on the Pacific side of the Arctic help to stave off the worst effects seen in the past five years?
Antarctic Pictures and Graphs
Here is a satellite representation of Antarctic sea ice conditions from April 28th:
Figure 6 - UIUC Polar Research Group's Southern Hemispheric ice concentration from 20120607.
Compare that graphic with the same view from June 7th:
Figure 7 - UIUC Polar Research Group's Southern Hemispheric ice concentration from 20120607.
Ice gain is easily visible around the continent. As a reminder, this is largely and somewhat confusingly due to the ozone depletion that took place over the southern continent in the 20th century. This depletion has caused a colder southern polar stratosphere than it otherwise would be, reinforcing the polar vortex over the Antarctic Circle. That vortex has helped keep cold, stormy weather in place over Antarctica that might not otherwise would have occurred to the same extent and intensity. As the "ozone hole" continues to recover during this century, the effects of global warming will become more clear in this region, especially if ocean warming continues to melt sea-based Antarctic ice from below. For now, we should perhaps consider the lack of global warming signal due to lack of ozone as relatively fortunate.
Finally, here is the Antarctic sea ice extent time series from June 7th:
Figure 8 - NSIDC Antarctic sea ice extent time series through early June 2012.
Antarctic sea ice extent had remained above average to some extent through the austral summer and early spring, which is good news. The difference in conditions from the first part of 2011 to the similar time period in 2012 is obvious: NSIDC measured last year's extent near the bottom of the standard deviation envelope while this year's extent is much healthier. Despite the shift in preceding conditions, extent in May 2011 and 2012 returned to normal.
Here are my State of the Poles posts from May and March.
You can find NSIDC's June report here.
Cross-posted at WeatherDem - the blog.