The Sea Ice Outlook does accept estimates from outside the professional community. Maybe not everybody involved is thrilled by this, but I do think it's a good idea from my distant vantage. And Jim Overland, one of the people behind the SIO, is strongly in favor of it. (I had a chance to talk with him about the outlook and other ice matters a few weeks ago.)
In the most recent report, there are 3 submissions from 'outsiders' -- Chris Randles (you've seen him comment here as crandles) and Larry Hamilton, both at Neven's Arctic Sea Ice Blog, and one from Wattsupwiththat.
Watts' entry was a poll of readers. While perfectly legitimate as an entry, it's also perfectly useless scientifically. One goal of science is to gain understanding of the system in order to spread the knowledge around. Polling can't be spread.
Much more interesting are Chris's and Larry's methods. Both are obviously methods of great brilliance, as they currently have the same estimate as I do from my statistical method -- 4.4 million km^2 for this September. Beyond that, you can read their method descriptions in the Sea Ice Outlook report and start constructing your own method by not making the mistake they and I have made. Whatever those turn out to be. Larry Hamilton's write ups (one for ice extent, one for volume) are:
And you can also examine model output from the PIPS replacement model ACNFS, PIOMAS, and CFS as a basis for making your own estimates. (And please do cite others that you know of.)
All are welcome to post your methods here in addition to (or instead of) at the SIO.
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