Arctic Ice 2009

[Admiral_Bobski]

That's not the case. The ice extent measurement treats any pixel with >15% ice concentration as being 100% covered with ice. The ice area measurement weights it according to the measured concentration. Let's illustrate it with a simple example data set. Say we have eight pixels in the data set, each one representing 100km^2. The concentrations for the eight pixels (sorted into order for convenience) are 10%, 10%, 20%, 50%, 80%, 85%, 90%, 95%.

Extent calculation: Two pixels below 15% - treat as zero. Six pixels above 15% - treat as 100%. Stated ice extent = 600 km^2

Area calculation: Two pixels below 15% - treat as zero. Weight the other 6 pixels by percentage. (20%+50%+80%+85%+90%+95%) * 100km^2 = 420 km^2

This is why ice area figures are always lower than ice extent figures. The ratio of area to extent is equal to the average concentration across the pack as a whole - I've never actually seen the figure worked out, but it might be a handy measure of the degree of compaction of the pack. You could say that the area gives a better impression of the overall ice pack health, however as I said above it's susceptible to interference from melt ponding. Those pixels at 80/85/90/95% could all actually be 95% pixels with varying amounts of surface melt. Using extent instead will get round this issue at the cost of obscuring the effects of ice fragmentation. As long as you're consistent which you use, and the degree of fragmentation / ponding is similar from year to year, the overall trends will be accurate. Really, it's all an artefact of the problem that we don't have infinite resolution cameras. There's no such thing as "50% ice" in reality - a given point of the ocean surface is either iced over or not.

Thing is, that's already taken care of in the area calculation. If you take a mass of 100% ice and spread it out so you now have twice as much 50% ice, then the ice extent measure will double, but the ice area measure will stay the same. The correction's already been done. Yes, the pack is a bit more compact this year than others (or possibly the melt ponds have iced over quicker). No, it's not sufficient to conclude that there's plenty of ice and the ongoing trend is all down to compaction. When you do the correction (i.e. use area rather than extent), it's still the third worst on record by quite some margin. Eyeballing pictures really isn't a good guide, unfortunately.

Thanks for that songster. I have already corrected myself above, though not in such detail as you have.

I think you have misinterpreted what my original post on this subject was all about - I wasn't trying to claim that Arctic ice is not in some kind of downward spiral, and that it's no worse off than it was back in 1979. That's not what I'm saying at all. I was merely pointing out the interesting phenomenon of increased ice concentration.

The ice area figures do give a much better figure than the ice extent figures, I will concede, but they are still subject to the vagaries of approximation. A visual representation of the ice concentration gives a clearer picture of how the ice is concentrated. I was not trying to draw conclusions only from, as you put it, "Eyeballing pictures". Regardless of ice areas or ice extents or anything else, it is interesting that ice concentrations seem higher than ever before.

I am surprised at being jumped upon for noticing a phenomenon (without, I might add, trying to draw any conclusions). My first post offered a possible explanation for this phenomenon:

Is it possible that changing synoptics are conspiring to keep the ice pack confined to a smaller area, thereby apparently reducing the ice extent, but increasing the ice pack concentration?

Effectively the same explanation was subsequently offered by Dev:

2. Winds seem to have being blowing from the south or to be light across the seas North of Siberia (which also looks to be relatively warm for the time of year atm). Perhaps pushing the pack ice together or out down east Greenland?

And then repeated by yourself:

As to why the concentration appears higher this year than others, it could be winds compacting the pack and closing up leads, or it could be melt ponds icing over. My understanding is that it's been a colder and cloudier summer than usual up there, so the latter seems at least plausible.

I'll drop out of this area now, since I've got better things to do with my time than defend myself against accusations of "spinning" and "eyeballing" thank you very much.

CB

[Gray-Wolf]

Bob Grumbine is also a good egg and has recently started blogging, to cover some of the enquiries he gets about science and GW: http://moregrumbines...e.blogspot.com/

if you have technical questions about the Arctic, he may well address them for you...

P

I approached him a few years back regarding the Ross ice shelf stability and he answered back in a matter of days. As you say " a good egg".

http://www.unep.org/compendium2009/

part of the recent compendium from the U.N. outlining recent papers ,and their contents, regarding changes in the cryosphere.

Edited September 27, 2009 by Gray-Wolf

[BLAST FROM THE PAST]

I'll drop out of this area now, since I've got better things to do with my time than defend myself against accusations of "spinning" and "eyeballing" thank you very much.

CB

CB

Thats what happens when anything questions, even inadvertently, the APPARENT onward irreversible affects of AGW. You have pointed out a mere visual fact, an interesting one and it is absolutely reasonable to post.

BFTP

[Thundery wintry showers]

I think the problem is that some latch onto anything that supports their own views, and there's a risk of others (like CB above) being lumped together with said individuals. It's very easy to do- I've been guilty of it myself on occasion- but it is something that we have to avoid, because otherwise it will deter some people from posting in here despite them having a lot of very good inputs to add to these discussions.

I don't see anything wrong with someone pointing out that the ice concentrations are unusually high for recent years. Those who say it is probably due to the ice pack compacting as a result of frequent southerly winds and melt ponds icing over are probably right. I imagine, though, that a more "compacted" ice pack might be harder to melt over the upcoming years than a fragile, thin ice pack, which could help to slow the rate at which any further melt occurs over the coming decade. In the meantime, though, a more restricted ice extent means albedo-related positive feedbacks in the areas that used to be ice-covered but now aren't.

I am (slightly) encouraged that, as shown by JACKONE's statistics, the ice has recovered for the second consecutive year. Unfortunately we still have a long way to go before we get back to 1990s levels.

[J10]

Today's provisional figure is 5,452,031, and this years low point is all but confirmed at 5,249,844 on the 13th September. How does this compare to past years.

The figure has been revised to 5,480,781.

I think I should wait until the end of day revision before posting the figures.

[J10]

As we move into the recovery phase for 2009, I will close this thread and create a new thread for the Autumn Winter Period.