Jim D
Regarding clouds, another Judy (Collins) sang “I really don’t know clouds at all” – and this could well have been the theme song for climate science today.
The Chief has given you some thoughts on the impact of changes in SW reaching the surface due to clouds when he wrote: “I’ll give you a clue – it all happened in the SW” (and he most likely knows much more about this subject than you or I do)
You certainly must know that it is estimated that the reflection of incoming SW radiation represents around -79 W/m^2 in the so-called “global energy balance”. A 5% change in reflected incoming SW radiation (-4.0 W/m^2) would have a slightly greater impact than a doubling of atmospheric CO2 (+3.7 W/m^2).
IPCC concedes in AR4, “cloud feedbacks remain the largest source of uncertainty” but, based on model simulations, concludes nevertheless that the net feedback from all clouds is strongly positive.
Spencer + Braswell have shown us subsequently, based on CERES satellite observations that, over the tropics, the net overall feedback from all clouds is strongly negative with warming.
http://blog.acton.org/uploads/Spencer_07GRL.pdf
S+B conclude that this is because the added reflection of incoming SW radiation from lower altitude clouds with warming is greater than the added absorption of outgoing LW radiation from high altitude clouds (IPCC models had previously estimated just the opposite net effect).
We do not know from this study whether or not this net negative feedback also occurs over regions outside the tropics (which only cover around 40% of Earth’s surface, but account for most of the incoming solar warming), nor do we know whether or not this negative feedback will operate over multi-decadal and longer time periods.
So is this simply a negative feedback or a separate forcing?
We know from ISCCP observations (Pallé et al.) that the global monthly mean cloud cover decreased by around 4.5% between 1985 and 2000. As a result the Earth’s global albedo decreased by the equivalent of around –5 W/m^2, i.e. decrease of reflected SW radiation (= heating of our planet). Over the period after 2000 the cloud cover recovered by around 2.5%, with an increase in reflected SW radiation of around +3 W/m^2 (= cooling).
http://bbso.njit.edu/Research/EarthShine/literature/Palle_etal_2006_EOS.pdf
Interestingly, these periods coincide well with a period of rapid global atmospheric warming followed by a period of no warming, as measured both at the surface and in the troposphere.
Spencer has since written a paper showing a correlation between cloud cover and the PDO, and the Chief has written here on apparent correlation with ENSO. This work would indicate that clouds do not only act as a feedback (to warming from human GHGs, for example) but represent an independent forcing factor in themselves, possibly driven by ocean current oscillations or whatever has caused these oscillations.
And then there is the observed long-term correlation between global temperature and solar activity/cosmic rays reported by Henrik Svensmark et al. This has led to the CLOUD experiment work at CERN, which has recently reported an experimentally observed link between cosmic rays and cloud nucleation in a controlled experiment, but more work is still required know to what extent this will play out in our atmosphere, IOW to validate this mechanism experimentally.
So I’d say it’s an exciting time.
We may soon know more about clouds than we do today.
But to me it appears likely that there is a connection between clouds and cyclical ocean currents as well as the sun – but we still do not know for sure how this works.
It seems less likely to me that clouds only act as a positive feedback to GHG warming, as was assumed by IPCC.
Max