The year’s increased Antarctic sea ice levels cannot be seen as undercutting the long-term record decline in Arctic sea ice and the global sea ice decline generally.
Arctic sea ice extent this past September reached the lowest point recorded since satellites first started measuring sea ice in 1978. Arctic sea ice blew past 2007′s record low, ending at 3.37 million square kilometers, roughly half the size of the summer minimum ice cap during the period from 1978 to 2000.
At the same time, sea ice around Antarctica has been approaching a record high. The Antarctic situation has led some to dismiss the dramatic events in the Arctic as a simple fluke, pointing to the growth of sea ice around Antarctica as a counterpoint. What’s missing from that comparison is that the modest growth of sea ice around Antarctica in no way compares to the dramatic 2012 declines seen in the Arctic nor, even more importantly, over the past decade.
The National Snow and Ice Data Center (NSIDC) at the University of Colorado, Boulder, provides an authoritative dataset on both Arctic and Antarctic sea ice cover, using data from NASA satellites. The figure below shows 2012 Arctic and Antarctic sea ice cover (as of October 8th, 2012) compared to the typical values from 1978-2000 for the same days of the year. The typical values are shown as a grey band representing the 2.5 and 97.5 percentile confidence intervals with a dotted line showing the average value.
The data make clear that the changes in Arctic and Antarctic sea ice cover are not remotely comparable. While Antarctic sea ice is high, it is barely outside of what would be considered normal based on the 1978-2000 period. Arctic sea ice, on the other hand, is barely half of what it was three decades ago.
To better represent the change in sea ice cover over time, it helps to convert the daily sea ice values for the Arctic and Antarctic into anomalies relative to a 1978 through 2000 baseline period. These anomalies allow scientists to remove the large annual cycle in sea ice to reveal the underlying trends, as shown in the figure below. The tiny red and blue dots represent all daily sea ice anomalies for the Arctic and Antarctic respectively, and the solid lines represent a smoothed fit to the data.
Seen side-by-side, the vast differences in changes between Arctic and Antarctic sea ice become readily apparent. The smoothed data actually somewhat under-estimate the magnitude of the change in minimum ice cover, as Arctic sea ice tends to recover to nearly normal levels during the winter somewhat independent of the magnitude of the summer melt.
It is instructive also to compare the smoothed data to more easily see differences in longer-term changes in sea ice cover between the poles:
While Antarctic sea ice lately has indeed been on the high side of normal, the situation is in no way comparable to the dramatic declines seen in the Arctic. Furthermore, what is important from a scientific standpoint isn’t so much the record lows seen this particular year, but rather the longer-term trends over the decades. The long-term trends point to a clear and significant downward trend in Arctic sea ice extent, which accelerated starting in the late 1990s. While there is a modest upward trend in Antarctic sea ice, that increase makes up for only a fraction of the decline in the Arctic, and global sea ice as a whole has been decreasing.
The actual data makes it hard to conclude that those wanting to point to the Antarctic as a counterpoint to what is happening in the Arctic may simply be trying to change the subject from the recent unprecedented global sea ice declines.