The sea ice data available from NSIDC includes estimates of both sea ice extent and sea ice area. Extent is the area over which sea ice concentration is at least 15%, while area is … well, the area covered by ice. Necessarily, sea ice extent will be greater than sea ice area.
It’s mentioned in each data file for the northern hemisphere that there’s a “hole” in the satellite coverage right over the pole, i.e., an area which is not observed by the satellite sensor. Furthermore, this hole was larger in olden days (through June 1987) than in more modern times (since July 1987). In the before time, the hole covered 1.19 million km^2, but now it’s only 0.31 million km^2.
For estimating extent, the hole is assumed to be entirely covered with ice at a minimum of 15% concentration, i.e., the entire hole is counted as part of the ice extent. For estimating area, the hole is simply not counted, hence the NSIDC area data have a discontinuity at the June/July 1987 boundary, when the size of the hole changed. Perhaps that’s one of the reasons area estimates are not as popular as extent estimates.
In any case, one can study the area as well as extent data, and see how they compare. For that purpose, I assumed that the unobserved polar hole was entirely covered with ice at 100% concentration, i.e., that it should all be included in the area calculation. That may be unrealistic — but it does give me numbers to work with. So, for months up to and including June 1987 I added 1.19 million km^2 to the area estimates, since July 1987 I added 0.31 million km^2, to get estimates of northern hemisphere sea ice area for comparison with sea ice extent data.
The interesting comparison (or at least, one interesting comparison) is the difference between extent and area. This showed a characteristic annual pattern through 2006, with the difference being reasonably constant for much of the year at between 1.5 and 2 million km^2. However, extent minus area tended to rise in June, peak in July, decline in August, decline further to about “normal” in September, rise again in October, then decline to normal again in November. The pattern is evident here:
In 2007, the year of the great melt, an interesting thing happened. The extent minus area difference reached its peak not in July, but in June instead (2007 is plotted in blue):
Over the next 2 years the pattern returned to a more normal one. There was still a premature June increase in 2008 but not as pronounced as that of 2007, while the pattern for 2009 was easily in the “normal” range overall. Here’s the updated plot with 2008 in green and 2009 in brown:
Now we come to this year. Most of us are aware that June saw an unusually large decline in northern hemisphere sea ice extent. The interesting thing to note is that this year also shows premature increase in the extent-minus-area difference, but this time it’s less pronounced but more premature — the difference shows unusual increase in May (2010 is plotted in red):
Whether or not this is a sign, a harbinger of dramatic things to come for the 2010 summer minimum, I don’t know. Honestly: I doubt it. But it is a curiosity, and does seem to indicate that not only is the summer minimum of northern hemisphere sea ice in decline, the annual pattern is also changing. Certainly, curious.
And for those who are curious about the southern hemisphere, here’s the pattern of extent minus area throughout the year: