Global polar sea ice area in September 2013 was slightly below climatological normal conditions (1979-2008). This represents a change from early 2013 conditions when sea ice area was at or above the average daily value. Antarctic sea ice continues to exist abundantly while Arctic sea ice fell below normal again during the month.
The NSIDC made a very important change to its dataset in June. With more than 30 years’ worth of satellite-era data, they recalculated climatological normals to agree with World Meteorological Organization standards. The new climatological era runs from 1981-2010 (see Figure 6 below). What impacts did this have on their data? The means and standard deviations now encompass the time period of fastest Arctic melt. As a consequence, the 1981-2010 values are much lower than the 1979-2000 values. This is often one of the most challenging conditions to explain to the public. “Normal”, scientifically defined, is often different than “normal” as most people refer to it. U.S. temperature anomalies reported in the past couple of years refer to a similar 1981-2010 “normal period”. Those anomalies are smaller in value than if we compared them to the previous 1971-2000 “normal period”. Thus, temperature anomalies don’t seem to increase as much as they would if scientists referred to the same reference period.
Arctic Sea Ice
According to the NSIDC, September 2013′s average extent was only 5.35 million sq. km., a 1.17 million sq. km. difference from normal conditions. This value is the minimum for 2013 as less sunlight and cooler autumn temperatures now allow for ice to refreeze. September 2013 sea ice extent was 1.72 million square kilometers higher than the previous record low for the month that occurred in 2012. The shift from a record low value one year to a non-record low the next is completely normal. Indeed, had Arctic sea ice extent fallen to a new record low, conditions this year would have been much more inhospitable to sea ice than they were. To be clear, I do not cheer new record lows. They are worthy of discussion not simply because of the record they set, but because they are part of a larger ongoing trend. This year’s minimum extent value did not break that trend, it continued it.
Overall, conditions across the Arctic Ocean this summer prevented record-setting ice loss. There were more clouds in 2013 than 2012. Clouds reflect most incoming solar radiation, which means less sea ice melts. At the end of the melt season, many small seas had normal sea ice extent, which is to say none. Anomalous areas include the East Siberian Sea and the Arctic Basin, which recorded less sea ice extent than normal.
September average sea ice extent for 2013 was the sixth lowest in the satellite record. The 2012 September extent was 32% lower than this year’s extent. The September linear rate of decline is 13.7% per decade relative to the 1981 to 2010 average, as Figure 1 shows. Figure 1 also shows that September 2013′s mean extent ranked sixth lowest on record. You can see from the graph that although a new record minimum was not set in 2013, the negative multi-year trend continued.
Figure 1 – Mean Sea Ice Extent for Septembers: 1979-2013 [NSIDC].