Update: Sandrine Bony confirms that there is nothing inconsistent between her 2006 review and the IPCC AR4 WG1 discussion on cloud feedbacks. See here
Bishop Hill accuses some IPCC scientists of ‘Charlatanry’ here.
The effect of low level clouds was touched on the Hockey Stick Illusion, where I briefly discussed a review paper by Bony et al (2006) looking at the low-level clouds (boundary layer clouds, in the jargon). I was struck by how different Bony’s story on the effects of low level clouds is to Tom Chivers’. Here is what she said in her paper:
Boundary layer clouds have a strongly negative [feedback effect] … and cover a very large fraction of the area of the Tropics … Understanding how they may change in a perturbed climate therefore constitutes a vital part of the cloud feedback problem.
So my understanding of the scientific literature is that low level clouds actually cool the planet. In Tom Chivers’ defence, it’s easy to get confused in this area because as readers of the Hockey Stick Illusion know, when the IPCC came to discuss boundary layer clouds in the Fourth Assessment Report, they lifted Bony’s text almost word for word, but making one rather important alteration:
Boundary-layer clouds have a strong impact … and cover a large fraction of the global ocean … . Understanding how they may change in a perturbed climate is thus a vital part of the cloud feedback problem.
Not all climate scientists are charlatans, and those who suggest they are are wrong. That said, I hope Tom will concede that there is a real problem with charlatanry among some scientists working on the IPCC assessments.
However Bishop Hill has made one rather important alteration of his own here. What Bony et al (2006) actually says is this:
Boundary layer clouds have a strongly negative CRF (Harrison et al. 1990; Hartmann et al. 1992) and cover a very large fraction of the area of the Tropics (e.g., Norris 1998b).
Bishop Hill paraphrases this as [negative feedback]. However CRF stands for Cloud Radiative Forcing, which is a forcing, not a feedback. Low level clouds would only represent a negative feedback if they tend to increase in a warming world, which Bony et al (2006) makes clear is debatable. This is rather the point of the phrase Understanding how they may change in a perturbed climate is thus a vital part of the cloud feedback problem.
What then of the IPCC’s wording, is it really misleading as to whether low level clouds could represent a negative feedback? No. It explicitly states that they could be.
Let’s look at more of the relevant paragraph from IPCC AR4 WG1, which Bishop Hill only partially quotes (my emphasis):
Boundary-layer clouds have a strong impact on the net radiation budget (e.g., Harrison et al., 1990; Hartmann et al., 1992) and cover a large fraction of the global ocean (e.g., Norris, 1998a,b). Understanding how they may change in a perturbed climate is thus a vital part of the cloud feedback problem. The observed relationship between low-level cloud amount and a particular measure of lower tropospheric stability (Klein and Hartmann, 1993), which has been used in some simple climate models and in some GCMs’ parametrizations of boundary-layer cloud amount (e.g., CCSM3, FGOALS), led to the suggestion that a global climate warming might be associated with an increased low-level cloud cover, which would produce a negative cloud feedback (e.g., Miller, 1997; Zhang, 2004).
That seems pretty clear. So, just who is the charlatan?
It’s also interesting to note that the case which Bony et al (2006) makes for low level clouds potentially representing a negative feedback is almost entirely based on models, which Bishop Hill normally poo poos, though oddly enough not on this occasion. Meanwhile empirical observations have since suggested that the net feedback of clouds is a warming feedback.
I wonder whether readers of The Hockey Stick Illusion will know that.