By way of introduction, I have written as WeatherDem at a couple of blogs: SquareState (Colorado’s progressive community blog) since 2005 and my own since 2007. My primary issue is global warming. While not a climatologist, I am an atmospheric scientist and understand the very real potential threat that global warming poses to global human societies and ecosystems. Simply put, if we choose delay over action any longer on this issue, this planet will be a very different place in a very short time frame. I first wrote about Arctic sea ice in March 2008. I started this monthly series in approximately the same form in March 2009. I added Antarctic sea ice to my posts about a year ago, in December 2009. This is the latest incarnation of that series.
The state of global polar sea ice at the beginning of November 2010 is once again poor compared to climatological conditions (1979-2008). The Arctic sea ice extent remains well below average for this time of year. The Antarctic sea ice extent is above average, but not nearly so much as the Arctic sea ice is below average, which is why the global sea ice extent continues to track below average also. Global sea ice area has made a nice recovery since the mid-year low of ~17.5 million sq.km., as the time series at the above link shows. Some portions of the Arctic Sea weren’t as warm as they were after the 2007 or 2008 melt seasons, which has allowed Arctic Sea ice to refreeze in those areas quite rapidly. One of the plots below will demonstrate nicely which areas were cooler and which were warmer. The prime melt season for Antarctic sea ice is about to begin. The global sea ice area will start to plummet again towards its yearly minimum as the southern sea ice melts before bottoming out in a few months’ time.
In October, Arctic sea ice continued rebounding from the yearly minimum recorded in the middle of September. Ice growth was slightly faster than the climatological normal rate over 1979-2000 (plus 35,800 square miles per day compared to 31,700 square miles per day). Because of the extreme difference between 2010′s yearly minimum extent and the median minimum extent, the extent today has not attained climatological normal values (i.e. it remains outside of the +/- 2 s.d. range), just as it has for 5 out of the past 6 fall seasons.
In terms of monthly averages, 2010′s October extent was 1.60 million square kilometers (618,000 square miles) below the 1979 to 2000 average for October, and 920,000 square kilometers (230,00 square miles) above the record low extent for October measured in 2007. A time series graph of ice extent anomalies maintained by the University of Illinois at Urbana-Champaign’s Polar Research Group shows that ice extent spent quite a bit of time this year near -1.5 million sq. km. It was only within the past month that the anomaly finally started to move back toward the 0-line. This low anomaly has only been recorded once before, in the record low 2007 year. While still negative, the anomalies in 2008 and 2009 were not as severe as this year’s. The change in October ice extent has been measured at -6.2% per decade by the NSIDC. Prior to 2010, the change per decade was -5.9%. As I’ll discuss in a little more below, as the Arctic continues to transition into a new regime, these changes will be more and more negative until a new stable point is reached in which there isn’t much, if any, summer sea ice anymore. You can also see at the linked plot how dramatically low the extent was in October 2007.
More important to polar climate processes than areal extent, however, is the volume of ice. Why? If two years had the exact same minimum areal extent, say 3 million square kilometers, but the second year had thinner ice than the first year, the second would have had less ice volume. What that means physically is that if too much ice in the vertical dimension melts, the ice in the horizontal dimensions become easier to melt summer after summer. To give you a better idea of how Arctic ice volume has changed over the satellite record, click the following link, which shows that ice volume has been decreasing for decades, but has worsened considerably in the past 5 years. The graph shows how a particular day’s ice volume compares to the climatological record, which in this case extends from 1979-2009. A record low volume was recorded this summer. Keeping my simplified explanation above in mind, look at the difference between the 2007 minimum and the 2010 minimum. The 2010 minimum was 3 thousand cubic kilometers lower than the 2007 minimum, even though the 2007 areal extent was 600,000 square kilometers lower than 2010′s.
What has happened is as the areal extent has shrunk significantly over the past 30+ years, to the point where the extent now falls below the -2 standard deviation on an annual basis, the ice volume has also been melted. In 2007 and 2009, the volume anomaly fell slightly below the -2 standard deviation. In 2010, however, the overall condition of the Arctic sea ice has worsened to the point that the volume this year fell dramatically below the -2 standard deviation. This year’s volume anomaly is very significant to the overall condition of the Arctic sea ice’s future. Without the kind of volume that the Arctic sea ice had decades ago, it would be easier in upcoming years to effectively completely melt the ice pack, even if overall conditions got no worse than they are today. Unfortunately for the ice pack, conditions are likely to continue to deteriorate because feedback loops have been activated.
Arctic Pictures and Graphs
Here is a satellite representation of Arctic sea ice conditions from November 2nd:
Compare that with September 19th’s satellite representation, when this year’s minimum occurred:
The areas of greatest ice growth correspond to the areas which have the coolest sea surface temperatures. This NSIDC temperature map and anomaly map show 2010′s conditions in context with the past three years, when temperatures were actually warmer for most of the seas. As the NSIDC noted last month, higher sea temperatures contribute to additional melting after surface melt stops. Areas with the warmest temperatures in 2010 are now the slowest to see new ice growth. Ice will likely continue to grow along the edge toward the Atlantic and Pacific Oceans until next spring. Before that happens, a great deal of absorbed heat has to be transferred from the ocean to the atmosphere. Interestingly, this has an effect on lower latitude atmospheric circulations. Warmer lower atmospheric Arctic air temperatures were part of the cause of the cold air and severe winter weather outbreaks that affected places like the eastern U.S. last winter. Since the most negative Arctic Oscillation on record and moderate El Nino conditions last winter were among the strongest factors that induced those winter weather outbreaks, it remains to be seen what effect warm autumn Arctic air temperatures mean this winter. We already have a strong La Nina in effect, so some changes are inevitable.
Here is the time series graph of Arctic sea ice extent with the +/- 2 standard deviations as a light-gray envelope around the climatological average through November 2nd:
The alternative Arctic sea ice extent time series that I found earlier this year shows conditions since 2002, with different years represented by different colored curves. It provides additional context for comparing current conditions against those from the past. I will point out again that April 2010 witnessed the latest maximum sea ice extent on record. May and June weather conditions caused some of the fastest melting on record before shifting to more a benign pattern, allowing the melt rate to slow down and keeping 2007 as the low record holder, followed by 2008. September conditions almost allowed 2010 to slide past 2008 as second worst.
Antarctic Pictures and Graphs
Here is a satellite representation of Antarctic sea ice conditions from November 2nd:
For comparison purposes, here is the similar picture from September 8th:
Here is the time series graph of Antarctic sea ice extent with the +/- 2 standard deviations in light gray and the climatological mean in dark gray through yesterday:
The two satellite representations and the time series graph all tell the same story: there hasn’t been much change in Antarctic sea ice extent since August. The maximum extent (~19 million sq. km.) was bumped up against a few times in the past 2.5 months. The melt season has taken hold within the past few weeks, however. The extent is following along the +2 st. dev. area on its way toward the Southern Hemispheric minimum, which will occur sometime in February 2011. The other significant story the Antarctic time series tells that is different from the similar Arctic time series is Antarctic sea ice conditions remain near climatological norms. While the continent of Antarctica has warmed over the past few decades, and while the Antarctic region as a whole has warmed, the effects on sea ice haven’t reached a point to be as obvious as the warming over the Arctic. Some of this has to do with the relative magnitude of warming over both poles, which isn’t the same. Another factor to consider is the presence of the ozone hole over Antarctica, which has allowed stratospheric temperatures over Antarctica to warm relative to climatological norms. This has had the effect of increasing the strength of the circumpolar vortex, which has kept cold air nearer to the South Pole and maintained the integrity of land and ice sheets. Recall that, in contrast, the near-surface air over the Arctic has warmed over time, which combined with other features has pushed Arctic air away from the pole and has weakened the ice sheets over time. A strong Antarctic Oscillation this year helped keep storms around the continent, keeping away enough sunlight to keep ice present far out to sea.
The Arctic sea ice extent has been very significantly below the average value for a number of years in a row now. The Antarctic sea ice extent has bounced around the average value during that same time period. It hasn’t spent anywhere near the same amount of time above or below the mean that the Arctic’s extent has spent below its mean. It hasn’t been anywhere near as far away from the mean, especially on a regular basis, as the Arctic’s extent has. The year-after-year Arctic sea ice loss that has been recorded during the past few years (regardless of month of year, which means the signal is very strong indeed) is a result of the warming trend seen in the Arctic over the past 30 years or so. That warmth has manifested over the Arctic more so than over the Antarctic to date. Without changes to global greenhouse pollution rates, the Antarctic is only biding its time until its land-based ice sheet undergoes its own melt scenario, which will have real-world consequences that its sea ice sheet melt doesn’t.
You can find the NSIDC’s November report here. The page is dynamic, so if you’re reading this after November 2010, that month’s report will show up first. If that’s the case, you can look for any report in their archive on the top pull-down tab on the right-hand side of the page.