Our friends have become quite focused on one particular aspect of Arctic ice – the September minimum. The claimed rationale is that it affects the Earth’s energy balance, by reducing albedo.
That argument is very weak – because very little sunshine reaches the Arctic atmosphere in September. Most of that is reflected by clouds, and the light which does get through is mainly reflected off the water surface, due to the high angle of incidence.
http://earthobservatory.nasa.gov/Features/EnergyBalance/page3.php
By contrast, open water in September makes it easier for longwave radiation to escape from the oceans. So ice loss in September is probably a negative feedback – rather than the positive one which is normally claimed.
So what would be a better measure of albedo effects? How about average extent during the whole year to date?
It turns out that 2010 is higher than 2006 and about the same as 2005.
The Arctic minimum makes for good press and gives Joe Romm something to hyperventilate over, but really has very little meaning in the physical world. As we have seen, ice can grow back very quickly.
