Several people have mentioned (I think the first I heard of it was from the Rabett) that at the moment, Arctic sea ice is exceptionally fragmented. It has even been suggested that we should expect a precipitous decline over the next week or so, by some accounts because of a possible imminent storm. I don’t know whether or not such prognostications are true, but I am motivated to look at the relationship between the two most common measures of sea ice, its area and its extent.
Sea ice extent is defined as the area of ocean which has at least 15% ice cover, whereas sea ice area is simply the area of the sea ice itself. When the ice is completely connected rather than fragmented, the two measures will be the same. It turns out, however, that the ice is usually somewhat fragmented, and when it is the extent of the sea ice is larger than the sea ice area. The two measures taken together can give us at least a rough idea of how much the ice is fragmented.
We can easily compare them in two different ways: by taking the difference (extent minus area) and by taking the ratio (extent divided by area). The Arctic Sea Ice Blog does one version of this (actually the ratio of area to extent rather than extent to area), but uses daily extent data from JAXA which only begin in 2002. I’ll use monthly data from NSIDC (and add back in the size of the “hole” at the pole which is unobserved, accounting for the fact that the size of the unobserved area changed in mid-1997) in order to extend the analysis over a longer time span. Here’s the extent-area difference over time:
The most obvious feature is an annual cycle, with the difference being greatest during late summer/early fall when ice cover is approaching its annual minimum. Here’s the ratio over time:
Again we see an obvious annual cycle, but for the ratio we can also see an increase over time during the peak summer/fall months.
It behooves us to remove the annual cycle by computing anomaly values as a given month’s value minus the average for that same month. Here’s the difference anomaly:
There’s no obvious pattern, although there are many extreme values during more recent years. Here’s the ratio anomaly:
For ratio, the overall increase over time is again evident.
We can look at the annual cycle in detail by plotting each year’s difference or ratio on top of the other years. Here’s the difference, by month, up through 2006 (the y-axis is mislabelled “Ratio” but the title is correctly “Difference”):
This serves to define the “normal” annual pattern in extent-area difference. It increases in June, is greatest in July, sometimes dips below average in September, and often shows a secondary rise during October. But during the record-smashing year 2007 a unique pattern element emerged:
The value during June was much larger than in previous years. The September dip was on the low side and the October rise on the high side, but it’s the exceptional June value that really stands out.
The following years, 2008 through 2011, returned to the previous pattern. Here’s the data, I’ve left the dots on 2007 and added 2008 through 2011, which are kinda hard to pick out since they don’t really deviate from what happened before:
But this year again shows essentially the same oddity observed in 2007, namely, an exceptionally high value during June:
Could this June extremity in extent-area difference be a harbinger of things to come? Might it signify that this year will emulate 2007 in showing an exceptionally low sea ice minimum? After all, 2007 wasn’t just low, it was exceptionally so — it didn’t just continue the declining trend, it deviated from that trend, with truly dramatic ice loss. If the June extent-area difference is a sign of that, then we might witness an astounding loss of Arctic sea ice again this year, not just a continuation of the trend, but an exceptional low, even well below the expected value due to trend.
A similar story emerges from examination of the ratio rather than the difference. Here’s the pattern up to 2006:
Again, something unique happened in 2007:
The ratio was much higher than before during June, and also set new records for July, August, and October. The following years 2008 through 2011 saw new records for August but a return to the previous pattern for other months:
Yet again, 2012 has repeated the unusual pattern of 2007:
There are new record highs for June and July. Again I wonder, is this a harbinger of a 2012 repeat of the “fall-off-the-cliff” drop in Arctic sea ice we witnessed in 2007? It certainly suggests that for June and July, the ice was more fragmented than in previous years.
And there is indeed quantitative evidence of this greater degree of fragmentation — both from the extent-area difference and the extent/area ratio. It will be interesting to see what August brings, and even more interesting to note this year’s minimum values (for both extent and area) in September. Signs point to a new record low, and there is the possibility that it could be not just a record-breaker, but a record-obliterator just like in 2007.
Finally, for those who enjoy the animated graphs here are the differences and ratios over time (click the graphs!):