I don’t like to comment about the information stemming from “Watts up with That” because no one in their right mind gets information from such a source, and by doing so I’m only allowing nonsense to set the tone in the climate debate, but Anthony Watts has a recent post about the water vapor feedback which I felt compelled to elaborate on.
I have little to say about the graphical approach or the cited paper at the bottom by Paltridge et al. I have already expressed concern over the usage of Re-analysis data for Relative humidity information, especially for older data. I’m particularly skeptical of sharp decreases in RH closer to the surface where the physics of the water vapor budget of the lower troposphere involves a tight thermodynamic coupling of the surface and atmosphere.
[edit, see update] McIntyre has his own post
The bottom line is that there is much better evidence from other sources showing a powerful water vapor feedback, and even the paper by Platridge et al. warn to be cautious of the results. I have posted on the progress in understanding the WV feedback as it has evolved over the decades through improved theoretical, observational, and model-based approaches, and I see no reason why all that should be thrown out the window right now. There’s lots of literature providing good evidence of a strongly positive water vapor feedbacks (e.g., Soden et al 2005 and Dessler et al 2008) and more through treatment is given in the IPCC AR4 report and NAS report on feedbacks.
What if relative humidity were in fact on the decline?
Anthony Watts and CA make it out to be self-evident that this should mean a negative climate feedback, but it’s not self-evident. His citation to Miskolczi is pretty much a credibility-destroyer. Admittedly, some of this confusion may arise from a very poorly worded abstract by Paltridge et al. more than anything. But the very title of Watts’ post is therefore misleading (even though he says “then water vapor ‘may’ cause a negative feedback” later on).
Relative humidity is given as a percentage,
RH = 100*(Vp/SVp)
Which is a ratio of the total water vapor content to the saturation vapor pressure (the latter can be thought of as an “upper limit” to the possible water vapor concentration in the atmosphere, due to the limits imposed by temperature).
The water vapor feedback doesn’t happen because of changes in relative humidity, it happens because of changes in specific humidity (Vp). The reason relative humidity is important though is because it tells you how the change in water vapor content scales with the change in saturation vapor pressure at a higher temperature. It’s possible to have relative humidity drop but still have an increase in the saturation pressure, and depending on which one of those wins out tells you whether Vp is larger or smaller. So, if RH were actually declining, it could simply be that the water vapor feedback is positive but less powerful than the mainstream science suggests.
Earth’s paleoclimate history is incompatible with a negative water vapor feedback. Long-term outlooks over geologic time on climate sensitivity, the inferred sensitivity through looking at the Last Glacial Maximum and other observations, millennial scale outlooks or even the strength of the seasonal cycle show that the Earth’s equilibrium temperature is not insensitive to change (ironically, we are always reminded of this by those who argue global warming must be natural).
Update I’m withdrawing a particular comment I wrote in my thread on McIntyre and “iffy” data which is seen in the comments. My apologizies. I still think Watts implied such where he writes “…ignored for the most part by the mainstream climate community, even going so far as to having a debate over the paper’s source of data (NCEP reanalysis of radiosonde data) and declaring the data to be too ‘iffy’.” Really, I think this was the main concern, and there is no need to jump back to GISS “issues.” With a more clear reading, I think McIntyre has been a bit more sensible about this, at least up until the post gets into the “Paltridge viewpoint.”