Comments 1 to 26:

1. BDS2020 at 11:00 AM on 25 August, 2010
   Is the data available to take the volume chart back to 1950? Would be interesting to see data for the same period for both extent and volume.
2. Robert Way at 11:25 AM on 25 August, 2010
   No the data is not available back to then for volume from the polar science center unfortunately.
3. pikaia at 18:52 PM on 25 August, 2010
   I really hate the idea of trying to fit a linear trend line to a curve which is certainly not linear! Early in the 20th century the minimum ice extent was fairly constant, but it has accelerated downwards in recent decades. The way things are going, the ice will completely disappear much sooner than the straight line would suggest.
4. skywatcher at 19:46 PM on 25 August, 2010
   In that context, fitting a quadratic trend to either the sea ice extent or sea ice volume since 1979 produces a much better fit - ie the losses are accelerating through time, rather than simply declining linearly. Those trends reach zero (in September) for extent around 2030, and volume before 2020.
5. Mikemcc at 21:19 PM on 25 August, 2010
   I thought albedeo was primarily a measure of reflection of visible light, since that is where most of the energy is from sunlight. Dark water absorbs more of this energy than white ice. The energy absorbed by the water leads to heating, and some of the absorbed energy is then re-radiated as heat radiation (IR).
6. Ned at 21:37 PM on 25 August, 2010
   Mikemcc, yes, you're right -- "albedo" is the average reflectance across either the visible spectrum or the visible & near-infrared range (say, 0.4 to 0.7 or 1.0 or 1.3 micrometers) depending how one wants do define it. The albedo of a surface determines what fraction of the incident light from the sun is reflected. The portion that isn't reflected is, of course, absorbed, raising the temperature of the surface.
7. ecotox at 22:42 PM on 25 August, 2010
   pikaia is right. The curve would not be linear, as the albedo effect of more dark ocean each year would act as a catalyst. Hence, curving the relationship between ice volume and time. (However, adding another difficult variable to an otherwise simple concept could prove troublesome for politician simpletons and John Q public.)
8. HumanityRules at 00:35 AM on 26 August, 2010
   Again the PIOMAS isn't measured data but modelled data, an important distinction. That's why it can be, and sometimes is, wrong.
9. dorlomin at 00:44 AM on 26 August, 2010
   HumanityRules at 00:35 AM on 26 August, 2010
   Again the PIOMAS isn't measured data but modelled data, an important distinction. That's why it can be, and sometimes is, wrong.
   
   = = = = = = = = = = = = = = = = = =
   It is a model derived from measurements like almost every other metric used on a similar scale (100-1000 square kms).
   
   Even the isobars on an MSLP chart are effectively models.
10. Gordon at 07:02 AM on 26 August, 2010
    HumanityRules, you may be right, however, the graph shows a drastic acceleration in the decrease in ice volume, are you saying it is drastically wrong? If it is even 10% or 20% wrong, the decline is alarming. Furthermore, are you saying the declining trend it shows is wrong? Even if the apparent acceleration in decline in ice volume is exaggerated, that is small consolation, as the straight line itself is quite steep. Even if, as you say, the model is sometimes wrong, it is still very useful to see the problem we are facing. Unless it is always wrong? Is that what you are saying, that it is useless?
11. ProfMandia at 10:35 AM on 26 August, 2010
    See the article about John Cook at The Guardian.
    
    BTW, do not ask people there to donate to John's site because your comment will be removed. I wonder how I know this....? :)
12. HumanityRules at 10:40 AM on 26 August, 2010
    10.Gordon
    
    What I'm saying is that for the past two seasons the PIOMAS model has under-estimated the extent. In 2008 it estimated extent would be lower than 2007. This year it has a fairly low estimate of extent which looks like again being an under-estimate.
    
    There are some actual measurements of ice thickness. Here's one that's fairly easy to follow (http://epic.awi.de/Publications/Haa2010b.pdf).
    It's conclusion is that in 2009 there was very little difference in ice thickness compared to 2007. This is the first line from their conclusion
    
    "We conclude that older sea ice in much of the Arctic Ocean was of similar or even slightly larger thickness in April 2009 relative to conditions in 2007, but within the expected range of interannual variability."
    
    You might argue that this covers only a small fraction of the arctic but unfortunately this is probably as good as it gets.
    
    If ice thickness is roughly the same, maybe slightly higher and if the extent and concentration are both greater than 2007 then where does the recent dramatic fall seen by POIMAS come from? The PIOMAS authors in a 2008 paper highlight the fact that the 2007 conditions are weighing heavily on their 2008 predictions, my guess is it is still doing so. I think that this is an over-estimation of the influence that 2007 ice conditions is having on model outputs moving forward.
    
    I do think in general the past decade or two have shown a decline in arctic sea ice. It's worth concidering though how long this obsession with trend has been going on. The arctic ice and weather conditions have long been known to have high levels of natural variability. Comment on the arctic seems to blind itself from this variability to focus wholy on the recent downward trend, which is extrapolated forward indefinately as you seem to do. I think it's worth considering the recent conditions in relation to those long term natural variations.
13. Gordon at 13:30 PM on 26 August, 2010
    12. HumanityRules
    
    Thanks for your response. We'll see very clearly within a few years who's right about arctic ice volume. It will be interesting.
    
    I don't think people are 'obsessed' with this trend. Using the term slights people who disagree with you, implying they are not critical thinkers. The stakes are high, and therefore intense interest is justified. No one is unaware of past variability in arctic ice and weather conditions.
    
    People are not blindly extrapolating this trend indefinitely. Offhand, I can't think of any trend I've seen that continues indefinitely. Certainly, if the PIOMAS graph above is accurate, it will go horizontal in the not too distant future. Flatlining, so to speak. Let's hope the trend reverses before that. I'm all for that, I just see no sign of it now.
14. HumanityRules at 15:07 PM on 26 August, 2010
    13.Gordon
    
    "obsessed" is the wrong word to use but the intention was to show that the recent trend seems to have overtaken the long term variability of the arctic in many peoples minds. And certainly in the discussion of arctic and AGW.
    
    I was looking further into this idea and came across the website of Igor Polyakov , an arctic researcher. He seems to have published plenty on the subject and his website has some interesting short, but detailed write-ups. There are some extraordinary lines on the website and in some of his publications such as this.
    
    "If long-term trends are accepted as a valid measure of climate change, then the SAT and ice data do not support the proposed polar amplification of global warming. Intrinsic arctic variability obscures long-term changes, limiting our ability to identify complex feedbacks in the arctic climate system."
    
    From GRL, VOL. 29, NO. 18, 1878, doi:10.1029/2001GL011111, 2002
    
    
    I know nothing about this guy except that he doesn't seem to be a perpherial figure in the science.
15. HumanityRules at 15:59 PM on 26 August, 2010
    http://www.gi.alaska.edu/~bhatt/publications/polyakovetal_2010.pdf
    
    A more upto date paper about trends versus natural ossilations in the north atlantic. The indroduction is worth reading just to get the sense of how the author seems to think there is still much to argue for on the subject.
16. Dappledwater at 19:25 PM on 26 August, 2010
    HR, Polyakov has an interesting take on the Arctic, including this from his 2002 paper:
    
    " Extending our SAT time series by 25 years back to 1875 (years associated with an extended and cold negative LFO (phase) leads to a two-fold increase of the arctic trend compared with the Northern Hemispheric trend (Table 1,Figure 3). While this appears consistent with polar amplification, we believe it is more appropriately described as a statistical artefact resulting from biased sampling of the LFO."
    
    "In an analysis of long-term air temperature changes Vinnikov et al. [1980] used gridded northern-hemispheric SAT for 1891–1978, the first half of which was dominated by the negative, cold LFO phase prior to the 1920s, and the second
    by the positive, warm LFO phase of the 1930–40s. Averaging these data within zonal bands they also found a two-fold polar amplification of SAT trends (Table 1)."
    
    Of course much has happened in the Arctic since 2002.
17. CBDunkerson at 21:07 PM on 26 August, 2010
    HR, there is obviously a vast difference between PIOMAS predictions of minimum ice extent made months in advance and PIOMAS estimates of ice volume made after the fact.
    
    There is no evidence of significant discrepancy in those volume calculations. Indeed, they were very close to the measurements yielded by IceSat until it went offline... if anything they slightly UNDER-stated the decline in ice volume.
    
    Cryosat II data should be coming out some time in the next couple of months. From the available evidence it seems likely to me that data will be in close agreement with the PIOMAS results.
    
    Your citation of the Haas paper is interesting... as you cut it off right before; "However, the volume of older ice may have been less overall due to a lower areal coverage, and because our surveys were still spatially limited."
18. HumanityRules at 09:43 AM on 27 August, 2010
    16.Dappledwater
    
    My understanding is that both your quotes relate to the idea that due to the natural variability wherever you start and end your data set will determine the trend. It's a different way of saying we should look beyond the recent arctic ice trend to the bigger picture.
    
    17.CBDunkerson
    "There is no evidence of significant discrepancy in those volume calculations."
    
    I've shown you evidence! While PIOMAS is in it's own death spiral in 2009 scientists have been out in planes with radars showing that the ice is slightly thicker than it was in 2009.
    
    Planes with radars
    
    I mentioned in the post there is no good spatial coverage of ice thickness in the arctic. To turn things around where is the evidence that the post-2007 drops in PIOMAS do match reality? I've genuinely looked for them and can't find them. The Haas paper is probably the best actual measure of arctic ice thickness post-2007.
    
    I agree Cryosat II will be a big step forward. It should be noted Haas and his radar are part of the consortium generating thickness data in order to calibrate the satellite.
19. Peter Hogarth at 21:21 PM on 27 August, 2010
    HumanityRules at 09:43 AM on 27 August, 2010
    
    There are some more post 2007 articles in Arctic ice, a reality check. I have a small amount of even more recent information, but will try to collate this. There are however multiple smaller scale studies for ice thickness, using all sorts of methods, but difficult to draw arctic wide conclusions as thickness and melt rate are not uniform over the sea ice area.
20. Peter Hogarth at 21:38 PM on 27 August, 2010
    HumanityRules at 09:43 AM on 27 August, 2010
    
    There is also an update (to April 2010) of the Maslanik and Fowler 2007 "age of ice" animation which these guys kindly did for me in the second article on Arctic sea ice. As older ice is generally thicker, this is pertinent.
    
    
21. Dappledwater at 22:11 PM on 27 August, 2010
    HR @18 - that's not what those passages suggest, however who knows?, it's not discussed in the paper.
    
    Pete Hogarth - the multi year ice ain't what it's cracked up to be either
22. CBDunkerson at 22:21 PM on 27 August, 2010
    HR, ummm... yes, and I quoted the part of that 'scientists in planes' paper where they said the volume of older ice may have decreased.
    
    You're trying to use a small area study to advance a conclusion that the study itself contradicts based on insufficient data.
23. CBW at 01:36 AM on 28 August, 2010
    I realize that this is a basic post, but it makes no mention of ice area. Ice extent is a fundamentally flawed measure of ice because loose ice is subject to being moved around by wind and currents, and shows suppressed variability because thicker ice spreads out as the ice that restrains it disappears. The only reason it is used is because it has value for navigation, and hence a longer historical record.
    
    Ice area, on the other hand, tells you how much of the ocean surface is actually covered with ice, and so is a much better measure of the albedo and the potential feedback effects it implies.
    
    Extent tells you where there are chunks of ice floating around. Volume and area tell you how much ice there is and how much of the planet's surface it covers.
24. michael sweet at 01:59 AM on 28 August, 2010
    CBW, I think that NSIDC and other scientists prefer to use ice extent because the errors in measuring it are lower. The sensors have difficulty telling apart melt ponds on top of ice and open ocean so that adds to the error of area measurements. They prefer to use extent data dealing with the public because they are more sure of those numbers. see this link:
    http://nsidc.org/arcticseaicenews/faq.html#area_extent
    
    In any case, all of the measures of ice are going down. It is just a question of how fast they are going down. Once the new ice volume satellite comes on line that data will be much more useful. That should be sometime this fall.
25. Ned at 02:04 AM on 28 August, 2010
    CBW writes: The only reason it is used is because it has value for navigation, and hence a longer historical record.
    
    Yes, for most physical processes ice area is probably more relevant than ice extent. But estimates of ice extent tend to be more reliable than ice area, at least in the non-winter months.
    
    For those of us who are just casual observers and aren't using sea ice data in actual quantitative models, the main thing is just to be consistent. The folks over at WUWT have shown a tendency to shift back and forth among different data sets (extent, area, NSIDC, JAXA, CT, PIPS, etc.) depending on which one is more exciting to them at any given time. That's a recipe for self-deception, IMHO.
    
    Pick one metric that you like to track, and stick with it.
26. Ned at 02:05 AM on 28 August, 2010
    Oops, Michael Sweet beat me to it...

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