I was wondering if anyone had any insight into the recent paper by Stephens et al. on the albedo of the Earth. Judith Curry thinks the paper is profound. Andrew Montford and Roger Pielke Sr apparently think it is a landmark paper. Although it seems like an impressive piece of work, I’m not quite sure that I see what is so profound about it.
There seem to be two main results in the paper. A very accurate estimate of the Earth’s albedo (0.293) and that the two hemispheres have very similar albedos: the Northern and Southern Hemispheres (NH, SH) reflect the same amount of sunlight within ~ 0.2Wm-2. The latter result is interesting because it implies that increased reflection from SH clouds exactly balances the increased reflection from the NH land masses.
Something discussed in the paper is that climate models don’t produce this observed hemispheric symmetry. This, obviously, has some people excited: the models are wrong, again!!!!. Something else that seems to have got some excited is the idea that clouds act as some kind of buffer against large changes:
The albedo appears to be highly constrained on the hemispheric and global scale and overinterannual timescales. The hemispheric symmetry is an example of such a constraint, and the interannual variability of reflected energy is another example. The interannual variability is small, mostly regulated by the changes to clouds associated with the main modes of climate variability. Overall, these changes occur in a way that minimizes the global effects of clouds on the albedo, buffering the Earth system from large changes.
Now, I’m not quite sure I get the significance of this, or even quite what it means. If the NH albedo is dominated by land, and if the system will tend to be symmetric (through more reflection from clouds in the SH to balance the enhanced reflection from land in the NH) then I can see how this can act to prevent large swings, but I don’t think this means that clouds will act to oppose any large changes in external forcing. If anything, it might suggest constraints on internally forced changes, rather than on externally forced changes.
If you consider Soden & Held (2006), the albedo feedback is small, but positive (< 0.5 Wm-2K-1) which seems consistent with the albedo being reasonably constant, but is certainly not consistent with the albedo acting to buffer against large changes in external forcing.
So, it seems like an interesting and impressive piece of work, but apart from an accurate estimate of albedo, that there is hemispheric symmetry, and that climate models are not properly representing this symmetry, I’m don’t quite get what some are all excited about. If anyone else has any insights, maybe they could post them in the comments.