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Kevin McKinney

"Some people maintain that it wouldn't matter if the Arctic becomes ice-free because it receives sunlight at such an oblique angle that most of it gets reflected back. I don't think this is true. . ."

It's not--and once again as in how many previous instances, this is not such a subtle point it would have been overlooked by the pros (only to be discovered by the wonders of "blogscience!") See:

http://en.wikipedia.org/wiki/File:Water_reflectivity.jpg

Andy

Since summer solstice is on June 21st, I would think that the ice-albedo effect would be of the most importance on that specific day (in theory). Not only is the angle of the sun optimal but also the increase in daylight hours in the arctic. My thought that the ice anomaly between 5/21 and 7/21 should be considered more important than the minimum area in September in regards to the ice-albedo feedback.

Neven

I see your point, Andy, but right now there is still a relatively big amount of ice to reflect all that sunlight.

Kevin McKinney

Andy does have a point, but if you refer to the graph I posted, it suggests that the "greater area" effect may be a pretty good counterweight to the "angle of incidence" effect--depending upon polarization. The "length of day" factor certainly plays in there, too; right now many of the communities in the Canadian archipelago are getting 20 hours of sunlight or more each day, and I'm fairly sure it's been a while since the Pole itself had so much as a good twilight. By contrast, "Equinox"--equal day and night hours, by definition--happens not too terribly long after minimum, usually.

To do a good estimate of total energy absorbed (and its distribution over the melt season) I suppose you'd need a good function for the angles of incidence over the season in various relevant latitude bands--surely available, to the knowledgeable at least--and some function describing albedo for said bands, also characterizing that out over the course of the season. Then you'd need to multiply out and do your sums, somehow; all I know is that my math won't do it!

Hmm, wonder if Tamino would find that an interesting thing to model for our edification?

Neven
Then you'd need to multiply out and do your sums, somehow; all I know is that my math won't do it!

As the 'owner' of this blog I should be doing the math and present it in an elucidating blog post, but I cannot stress enough how little I know! I wouldn't know where to start.

Should I go and ask Tamino if he wants to write a guest blog post?
He has used my work too (without asking). ;-)

Lou Grinzo

Neven:

Just the other day I stumbled across a reference to a paper that addresses exactly this issue (how much forcing do we get from albedo flip):

http://www.atmos-chem-phys.net/10/777/2010/acp-10-777-2010.pdf

From the paper:

---------------------
A disappearance of the Arctic ice cap during the sunlit period of the year would radically reduce the local albedo and cause an annually averaged 19.7 Wm−2 increase in absorbed solar flux at the Arctic Ocean surface, or equivalently an annually averaged 0.55 Wm−2 increase on the planetary scale. In the clear-sky scenario these numbers increase to 34.9 and 0.97 Wm−2 , respectively.
---------------------

The forcing from CO2 alone (not including aerosols other GHG, etc.) is around 1.6 Wm-2, if I remember the detail from IPCC WG1 Chapter 2 it correctly, so a boost of 0.55Wm-2 planet-wide is very significant.

Neven

Wow, that's a great reference, Lou! I have it bookmarked and will be sure to use it when I write a more extensive blog post on the ice albedo effect.

I'll go read and try to find this myself, but do you have any idea what their definition of 'the sunlit period of the year' is? Would this include for instance the period between 5/21 and 7/21 that Andy alludes to in his comment? That would be the time when the sun is in an 'optimal' angle and also the time with the most sun hours.

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