How does the collective wisdom of Arctic Sea Ice blog participants compare with expert scientific analysis in forecasting the September sea ice extent? This question seems worth exploring with a crowd-source experiment. You are all invited to submit, as comments to this post, your best guess for the mean September extent of Arctic sea ice (NSIDC). Your numerical prediction should be in the first line of the comment, followed by at least a sentence or two explaining the basis for your prediction — whether pure intuition, elaborate calculations, links to a web page, whatever you’ve got. General discussion is welcome too.