Update April 28 2012: On Tamino in a new post titled Let’s do the math! there is a question by Ron Broberg as to why I consider single-year sea ice to have inertia. The reason is that, even with a single year of ice, since it has mass, its heat can be expressed in Joules. The mass of sea ice [its coverage and depth] can be expressed in terms of its heat content in Joules [i.e. the Joules required to warm and melt]. Multi-year sea ice, presumably would be thicker and thus more heat would be required to melt. However, just to illustrate, IF, for example, the area coverage and depth returned in a single year to their values years ago, the use of linear trends over decades would be meaningless as the “clock” would have reset. It is the mass of the ice that matters in terms of heat, not its age (although other aspects of ice are affected such as its albedo, density, etc).
In looking at the Cryosphere Today analysis below (with my eyeometer), it appeared that the areal coverage had stopped decreasing when averaged from 2006 to the present, although there are large variations between summer and winter. My “eye” still sees the change in the character of the analysis, but I have become convinced by the statistical trend analysis by Grant Foster and dana1981 (and several of the commenters) that this is not a significant change in the long term trend, nor can one even say the decline has stopped (due, presumably, to the large intraannual variations between anomalies in the winter and summer).
Also Al Rodger at Tamino’s Let’s do the math! , despite the same type of insults as Grant Foster is spewing, has a very informative plot of arctic sea ice. The portion of the time period that I focused on in my original post was since 2006 [using an eyeometer which saw a flattening of the anomalies, as I mentioned above]. Unfortunately, his time series is not up to the present (although this would make little difference in the mean that are shown). I have reproduced his excellent figure below.
It does confirm what I saw visually that there was a visual change in the character of the slope in ~2006. From a statistical perspective, as discussed on Tamino and Skeptical Science, I am now convinced it is too short a time to determine if there is a real change in the character of arctic sea ice decline. It certainly could just be a short-term hiatus in the decline, that does not significantly affect the longer term trend. Only time will tell if this is a correct interpretation.
My final comment is why I do not permit comments on my weblog. The reason is straightforward. If you read the posts by Grant Foster and the response by Grant Foster to a comment by michel, you will see the bitterness of his posts and of a number of his commenters. Readers of his weblog (and of Skeptical Science which often has the same tone) with questions are welcome to e-mail me directly at firstname.lastname@example.org. I will post, with permission, substantive questions, and my answers, on my weblog.
I posted an update of the predictions of Arctic sea ice in my post of April 20 2012 titled Sea Ice Prediction – Update To 2012. After an exchange of posts with Tamino (Grant Foster) and Skeptical Science (dana1981), I have become convinced that I made several methodological errors as well as did not properly explain my perspective on the analysis I presented. I did not
i) clearly state why I chose a start date of 2006,
ii) describe why I chose a relatively short time to compare the trends,
iii) why I compared the Cryosphere Today anomalies in Arctic sea ice area with the Vinnikov et al sea ice extent values.
First, my use of their long-term trend values to compare with a trend since 2006 assumes that short-term assessments have value for quantities which involve inertia (mass) such as heat and ice. If the sea ice area were to recover to its original area and thickness (for whatever reason), for example, it does not matter what its long-term trend was. The long-term trend (if there is one) would be reset. I have made this point often with respect to ocean heat content (e.g. see). It also applies to sea ice (although area is only one part of it). I chose 2006 for this reason to see if the long-term trend provided by Levitus et al has been interrupted (as it visually appears to be on the Cryosphere Today website).
Second, I compared anomalies of sea ice extent and of sea ice area, assuming they would be very close to each other. I have been convinced based on the analysis by Grant Foster and dana1981 that this is not correct [although see http://goo.gl/5rX5O from Zach on Tamino]. This was my more serious mistake.
What they report on Tamino and Skeptical Science is “that the decline in Arctic sea ice extent has actually occurred much faster than climate models were predicting 13 years ago.” and include the figure (from Skeptical Science)
I have no reason to question their finding with respect to sea ice extent. I am requesting that, since they appear to have the statistical analysis program and data readily available that Grant Foster and/or dana1981
i) perform the same analysis for sea ice area that they have done for sea ice extent
ii) perform the analysis of insolation-weighted sea ice trends; e.g. see
Pielke Sr., R.A., G.E. Liston, and A. Robock, 2000: Insolation-weighted assessment of Northern Hemisphere snow-cover and sea-ice variability. J. Geophys. Res. Lett., 27, 3061-3064
Pielke Sr., R.A., G.E. Liston, W.L. Chapman, and D.A. Robinson, 2004: Actual and insolation-weighted Northern Hemisphere snow cover and sea ice — 1974-2002. Climate Dynamics, 22, 591-595 DOI10.1007/s00382-004-0401-5.
If these two metrics also show that the model predictions are too conservative, than the Skeptical Science conclusion that “This rapid rate is precisely why the Arctic sea ice decline is described as a death spiral”
will have more evidence. Regardless, they would be adding to the discussion. I urge them to also present these same analyses (area anomalies and insolation-weighted) for the Antarctic sea ice.