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L. Hamilton

Neven, my apologies, there's a typo in the 7/30 version of my cycle plot. A corrected 8/1 version is here:


Thanks, Larry. I've switched to your new graph.

Andrew Xnn


So, 2010 is standing out with the lowest area/extent percentage (65.7%) compared to the previous low years; lower than 2007 and in stark contrast to 2009. In other words, it is primed for compaction/export more so than any previous year.

Also, from the "Trends in ice area and extent by month 11/1978 - 6/1010" graph, it looks like October, September, August and July pretty much in that order have had the largest anomalies over the past few years. It will likely be an interesting next 3 months.

L. Hamilton

Andrew, graph's title should of course read "11/1978-6/2010", not through "6/1010" as I mistyped it. If anyone else wants a typo-free version, it's here:


Once the July numbers are posted by NSIDC, and for that matter August & September, I'll update the graph at that same URL.

Artful Dodger

Neven, could you comment on how you are handling the 'data hole' at the North Pole for your computations? Thanks!


AD, I'm using a very useful technique called 'ignoring'. ;-)

Do you think I should add a certain number to the area total? The 0.31 million square km Tamino mentions, or do they use a different number over at CT?

Artful Dodger

Neven, no single technique is perfect; each has merits and drawbacks. It is most important simply to document your method, so your results are reproducible.

Lord Soth

With such a huge difference between area and extent, there is a possibility that the extent may decrease late into September or early October. It would take the right weather condtions, but a little bit of nilas or thin ice, between the pack in early autumn; will not stop the compaction with the fall storms blowing in the right direction.


AD, what I did was download IJIS extent data and CT area data and arrange all the numbers in this spreadsheet.

You guys tell me what I did wrong, and then I'll continue tomorrow. Because if I did something wrong I have made a fool of myself enough as it is.

L. Hamilton

Here is what the ratio of area (adjusted as described earlier) to extent looks like, by month, 11/1978 to 6/2010. The ratio tends to be highest, just below 90%, in March. There is no apparent trend in these high points.

Most years the ratio of area to extent reaches a low point in August. In the 1980s and early 90s such low points remained above 70%, but unlike the maximum ratio, the minimum has a downward trend over time -- so that within-year variation in area/extent has increased.


Andrew Xnn

Another interesting graph L. Hamilton...

So, as late as 2006, the August Minimum could have passed for "normal".
However since then, every year has been exceptional with the previous extreme year being 1998.

The last 4 years, including 2010 have been exceptional.


Larry, as usual I have stolen your excellent graph for this blog post. Thanks a lot.

Jon Torrance

Hard to see how we can do anything other than ignore any north pole data hole the CT area data may have until we know for sure how large such a hole might be and how, if at all, they adjust their area numbers to deal with it. For that matter, I have no idea how IJIS deals with any data hole at the north pole in their area data - they explain what they do with extent just fine but I don't recall reading a similarly detailed explanation of how they produce their area chart.


Great blogging, Neven.

But I noticed something wrong with the example that you gave on the definition of area versus extent. According to nsidc,


they apply a "filter" of 15% to their area computation, so the 10% cell in your example would count as 0, rather than 6.25 km^2.

And, finally, yes, following the fate of the arctic ice is more exciting that following cricket.


Thanks, Bfraser, I'll change the text. Jon Torrance told me the same thing in a previous thread, but I misunderstood, thinking the 15% threshold only applied to extent.

I read that NSIDC explanation only a short while ago, but overlooked this definition for area. I practically copied their example off the top of my head, haha. Copypaste could have saved me the trouble and the mistake. :-)


Very interesting graphs and meaningful analysis, just a few thoughts though.

I am not so sure about what meaningful value we can attach to the area/extent ratio inasmuch as the ice area represents a surface of of ice that can be distributed very differently within the ice extent. What I mean by that is that you can 4 million sqkm of ice made up of a big chunk of 95% concentrated ice and a large area of say 15-30% concentration. This ice pack would behave very differently from an ice pack of the same area but with an homogeneous concentration between 70 % and 90%. Ice at 25 % concentration is likely to melt away within a few weeks, while 70% + concentration ice is more likely to be a temporary melt which will quickly refreeze.
I am sorry if I haven't made myself clear enough. What I want to say in a nitshell is that an analysis with a breakdown of ice extent into different rates of concentration might be a much better predictive tool that just an area/ extent ratio. This being said, I undertsand that this may not easy to do.

Artful Dodger

Yes Phil, the Area/Extent statistic as formulated will not capture fine detail. Primarily because we do not have 'per grid' data, A/E masks a good deal of signal due to spatial averaging. Further, there is temporal averaging (2 days for IJIS, 5 for NSIDC) that masks changes that occur over a shorter period.

Still, as Larry Hamilton's graph above clearly demonstrates, there is a useful signal embedded in the A/E statistic, which is what I was hoping to see when I suggested it's use.


And it more or less confirms what we have been seeing on satellite images and ice concentration maps in the past 2-3 weeks, with the 'holes' and everything. Despite its flaws I still think it's an interesting metric to be comparing with previous years in this final phase of the melting season. It might give a sense of how much potential for compaction there is (the final outcome of which is depending on atmospheric conditions of course).

Artful Dodger

Lord Soth said | August 01, 2010 at 21:23

"With such a huge difference between area and extent, there is a possibility that the extent may decrease late into September or early October. It would take the right weather condtions, but a little bit of nilas or thin ice, between the pack in early autumn; will not stop the compaction with the fall storms blowing in the right direction."

Hi Lord Soth,

I made a similar prediction on June 27. If the DA returns this Fall, we will continue to loose Sea Ice due to advection through Fram Strait well after the melt is over. I stand by my earlier prediction of Sep 24 - Oct 4 as the date for Sea Ice Minimum 2010.


Area decline has slowed down the last two days:

Extent decline is picking up speed again.
The latest value : 6,820,469 km2 (August 1, 2010)
A 101,562 sq km drop (pre-correction):

This suggests some compaction at the sea ice coasts, or perhaps some melt ponds draining through ice that has melted through.

Beaufort Sea ice is poised for more heavy melts:

Both Arctic sea ice area and extent are currently 2nd lowest on record for August 1st.


In terms of average loss from here on in, we should see it getting to around 5.3 million by the end of August. If there is some ice loss in September, we could get to 4.7 or 4.8. Maybe my pessimism was misplaced. I would really like it if sea ice extent this year reflected the true state of the ice, if you understand what I mean by that.

Artful Dodger

To further handicap the August horserace, here is the average daily SIE change for 2002-09: (sorted from largest to smallest)

Year  Avg/Day
2008  -70,121
2004  -64,657
2007  -57,041
2002  -54,797
2009  -48,654
2003  -46,764
2005  -45,000
2006  -37,697

Hi Neven,

You are correct, I downloaded the area graph and read the values off that, just counting pixels. Its far from perfect:
1. Data from later years obscures data from earlier so there's a bit of guess work involved.
2. Resolution - at high res, the line is 2-6 pixels thick, so I had to pick a reasonable centre
3. Trying to smooth out the data - my counts yielded about 52 data points (pixels) per month which I then smoothed out into 30/31 days. To do that I simply rounded the fraction (for example, in July: x / 52 * 31) to the nearest day of the month. Where there was one data point for a day I used that, where there were two I took the mean. That gave me one value per day.

I then divided the result by the published daily extent data. and graphed the resulting percentages, using the same colours as IJIS.

So in the detail its quite crude, but I figured it was close enough for my intent of examining gross trends. Perhaps IJIS can be persuaded to release the raw area data.

The original cut out at 8th July because I was doing this while cooking soup for the hungry tribe and didn't pay enough attention to the detail. Problem now fixed, new area graph downloaded and comparison charted. I've uploaded a new version to 31 July here:

Still haven't had a chance to really look at it and work out what it means, but I note that the percentage troughs in mid-August. After that, I guess the ice gets a little more compacted (same area, smaller extent), or we start to see leads and polynyas freezing over (greater area, same extent). Finally, a month after bottoming out, area and extent both start to shoot up.



FrankD, I'm impressed with what you have come up with, especially after having struggled yesterday to get that Google Chart Wizard to do what I want. I'm putting a second area/extent post up today with just the graphs, including yours. Thanks again.

Steve Bloom

Fun Arctic trivia:

Today I happened across this map of the De Long Islands (just NE of the New Siberian Islands) showing a boundary for the permanent ice. I don't know when the map dates to, but not any more!

A couple of them may be technically U.S. territory (a result of yet another failed attempt to find a Northeast Passage), although if the Russians thought there was any threat the claim would be carried through on they probably would have changed the names from Henrietta and Jeannette. Anyway, there's material for a joke there somewhere.


Excellent chart, FrankD, it shows that 2010 is more like its "sisters" 2007,8 & 9 than the earlier years. An argument against a recovery!



Nice discussion on the extent vs. area issue and a nice figure showing the trend. We've been calling the ratio of the two "compactness". See quote from this paper:

"The compactness of the ice pack (the ratio of ice
area to ice extent) in August, the month of minimum
compactness, has been decreasing in the last 20 yr because
the area is declining faster than the extent. A less
compact ice pack is more easily moved to one side of
the basin and thus allows for greater variability in the
ice extent"




Thanks a lot, Axel! So compactness it is.

That's a great paper. Am reading it as we speak.

I hope you saw that there is a follow-up to this post.

Artful Dodger

Nice to know that the experts are on the ball, too! Thanks, Iceman. I hope you'll stay with us here at Neven's blog and continue to share your knowledge. Welcome!

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