IARC-JAXA uses a variant of the Bootstrap Algorithm, their version, as designed by Comiso, is explained here, from page 50 onwards.
NSIDC offer three products, two use versions of the Bootstrap algorithm, one uses the NASA Team algorithm. The main public NSIDC presentation is on their Sea Ice News page, this uses the NASA Team Algorithm.
Bremen (University of Bremen) use an algorithm developed by Gunnar Spreen and colleagues, their most recent paper (Spreen 2008) is here. This is called the ASI algorithm. In Spreen 2008 it is noted that:
In the northern hemisphere the mean overall differenceThis might be taken to mean that Bremen (ASI) is an outlier that always reads low. However that conclusion, from the above quote, would be incorrect because we're considering 2011 as measured by ASI against 2007 as measured by ASI.
for ASI minus Bootstrap is -1.4% +/- 8.2% with a mean correlation
of 0.95. For ASI minus NASA-Team 2 the mean difference amounts to
-2.0% +/- 8.8% with a mean correlation of 0.93.
I read up on these algorithms earlier this year, to a point, that point being the stage at which I knew I'd need to learn an entire body of science to really get to grips with the algorithms' pros and cons. That said I can recommend those sources for anyone wanting to get a grasp of issues like the impact of weather on sea ice extent, like the transitory patches of unfeasibly low concentration we sometimes see in the middle of the ice-pack. That aside, some have suggested that the difference between the graphs from these three approaches is mainly due to periods of averaging. I doubt this. The differences between the algorithms are easily able to account for the differences between them.
I've tried to work out if it's possible to get a handle on the uncertainties involved, so as to be able to see at what point we'd know we're 95% likely below 2007. The problem is that for all three methods uncertainties vary with the level of concentration, weather and other factors. So the uncertainty changes with conditions, and it's beyond me to attempt to calculate the uncertainty.
Putting aside for the moment the area record, we're now in a position where it looks like only one out of three extent indices shows a record low for 2011, the other two are likely to be close, but not below 2007. The question remains, is 2011 really a new record year? In short nobody knows. None of these indices show the 'real' value of extent. All represent the best efforts to ascertain a figure for extent, each index having it's own pros and cons. All one can say is that according to the University of Bremen ASI algorithm there is a new record. Add in the area record back into the mix and you've got a 50/50 situation. But that's 2 different algorithms (indeed approaches) giving records on their own terms.
By the close of the season and a significant increase extent I expect that this year will in essence be so close that arguing about the differences is academic. What is for sure is that any talk of a recovery is not supported by the evidenced. We've had 4 years since 2007, ample time for the ice-pack to recover, yet there has been no recovery.
Some may point to Maslanik 2011 as evidence of a recovery, after all the authors themselves state:
The recovery in multiyear ice extent through MarchHowever this is to be expected. Given that the sea-ice is some way from all melting away to give a seasonally sea-ice free state, some ice is bound to survive from one year to the next. This means that there will be a residual of multi-year ice in the pack. However as the pack is thinner and more mobile the lifetime of ice in the pack should be lower*. What we are seeing is consistent with an ice-pack that has transitioned to a younger, thinner state. It is not consistent with a general recovery to a pre-2007 state. Were that the case we would need to see an increase in summer minimum extent, that is clearly not happening.
2011 from the extreme reductions in 2007 and 2008 along
with the continued aging of the surviving ice through multiple
melt seasons is consistent with an ice pack that has not passed
a tipping point across the Arctic Ocean as a whole, and reflects
favorable large‐scale ice transport patterns conducive to
retaining multiyear ice.
*If I find this ref, and remember this, I'll amend this post with a PS. I have read a paper which has a graphic showing increasing speeds hence reducing ages of ice in the Beaufort Gyre. The problem is with 200+ arctic related papers on my laptop, after an hour of searching I can't find it. Argh!
Graphs of the three extent indices.
University of Bremen.