Professor Andrew Dessler of the Department of Atmospheric Sciences of Texas A&M University requested the opportunity to respond to my post
I welcome his openess to discuss this issue, and am glad to post his guest weblog. We need more such collegial debate on these topics within the climate community. I will respond in an upcoming post.
Guest Weblog By Andrew Dressler
In a recent post, Prof. Pielke emphasized the uncertainties in our knowledge of the water vapor feedback. In doing so, he failed to recognize the many things that are confidently known about the water vapor feedback.
There are really two questions here: 1) do observations indicate that the water vapor feedback strong and positive, and 2) do models adequately reproduce the observed feedback?
For the first question, the evidence of a strong and positive water vapor feedback is overwhelming. Observations of the response of the atmosphere to events like the eruption of Mount Pinatubo and El Niño cycles show quite clearly that changes in water vapor lead to enhanced trapping of infrared radiation when the climate warms [Soden et al., 2002; Soden et al., 2005; Forster and Collins, 2004; Dessler et al., 2008]. For a more complete summary of why we’re so confident, see Dessler et al. 
It is particularly worth noting that the papers that Prof. Pielke referenced by Dr. Sun and colleagues (which he says casts doubt on models’ ability to simulate the feedback) clearly confirm with observations that the water vapor feedback is strong and positive.
Given the strong water vapor feedback seen in observations (~2 W/m2/K), combined with estimates of the smaller ice-albedo and lapse rate feedbacks, we can estimate warming over the next century will be several degrees Celsius. You do not need a climate model to reach this conclusion — you can do a simple estimate using the observed estimates of the feedbacks along with an expectation that increases in carbon dioxide will result in an increase in radiative forcing of a few watts per square meter.
The only way that a large warming will not occur in the face of these radiative forcing is if some presently unknown negative feedback that cancels the water vapor feedback. My opinion is that the cloud feedback is the only place where such a large negative feedback can lurk. If it is not there, and the planet does not reduce emissions, then get ready for a much warmer climate.
This brings us to the second question, whether models adequately simulate the feedback.
To investigate this, I have recently compared the global-average radiative response to changes in water vapor during El Niño cycles in climate models to that in reanalyses [Dessler and Wong, 2009]. While the details of the comparison are rich, it’s clear that climate models are doing a good job reproducing the radiative response of changes in water vapor to changes in the tropical surface temperature.
Prof. Pielke points to some Sun et al. papers to argue that the models are overestimating the feedback. What he fails to mention is that these papers only analyzed a small region of the planet (e.g., the Wu et al. paper looked at 5°N-5°S, 150°E-110°W, corresponding to about 2.4% of the surface area of the globe) and the “overestimate” they found was quite small.
Thus, it is a stretch to view the Sun et al. papers as demonstrating some pathological problem with the models’ water vapor feedback, or that this contradicts my global analysis.
Thus, we can conclude with extremely high confidence that the water vapor feedback is strong and positive (I would categorize it, in the IPCC’s parlance, as being unequivocal). And I would categorize it as very likely that models are accurately simulating this phenomenon. While uncertainties do exist (as Prof. Pielke pointed out), those uncertainties are small (which Prof. Pielke fails to point out). Given this, the most likely outcome of a business-as-usual emissions scenario is significant warming of several degrees Celsius.
Finally, some frequently asked questions about the water vapor feedback:
Didn’t a recent paper show that the water vapor feedback is negative?
There is a recent paper by Paltridge et al.  that shows that water vapor in the tropical upper troposphere in the NCEP/NCAR reanalysis decreased over the past few decades. I have repeated this calculation with more modern and sophisticated reanalysis data sets (ECMWF interim reanalysis and MERRA reanalysis) and this result does not hold in those data sets. Given all of the other evidence that the water vapor feedback is positive, all of the ways that long-term trends in reanalyses can be wrong, and lack of verification in more reliable reanalysis data sets, I conclude that the Paltridge et al. result is almost certainly wrong.
Models have biases in their water vapor fields. Doesn’t this mean their feedbacks are unreliable?
The models do indeed have biases in their predictions of the water vapor base state (it varies from model to model and regionally within a model, but is generally of order 10%) [John and Soden, 2007]. Yet they all simulate about the same water vapor feedback. How can that be? It turns out that the water vapor feedback is determined by the fractional change in water vapor, primarily in the tropical upper troposphere. And the models all calculate the same fractional change in water per degree of surface warming [John and Soden, 2007]. This is why they all get basically the same water vapor feedback, despite biases in the predicted base state.
Why is the tropical upper troposphere so important for the water vapor feedback?
It is the changes in water vapor in the tropical upper troposphere that plays the major role in the water vapor feedback. While photons from these water vapor molecules do not directly heat the surface, they do primarily regulate emission of energy to space. Because the troposphere is rapidly mixed by convection at a rate much faster than radiation, the effect of changes due to radiation fluxes that are entirely internal to the troposphere (e.g., due to changes in lower tropospheric water) will be rapidly wiped out by convection and have a small net impact on surface temperature. The tropics dominate the effect because of the smaller temperature difference between the surface and the upper troposphere in the mid-latitudes combined with smaller column abundances of water vapor there.
Dessler, A. E., and S. C. Sherwood (2009), A matter of humidity, Science, 323, doi: 10.1126/Science.1171264, 1020-1021. http://geotest.tamu.edu/userfiles/216/dessler09.pdf
Dessler, A. E., and S. Wong (2009), Estimates of the water vapor climate feedback during the El Niño Southern Oscillation, J. Climate, 22, doi: 10.1175/2009JCLI3052.1, 6404-6412. http://geotest.tamu.edu/userfiles/216/dessler09b.pdf
Dessler, A. E., P. Yang, and Z. Zhang (2008), The water-vapor climate feedback inferred from climate fluctuations, 2003-2008, Geophys. Res. Lett., 35, L20704, doi: 10.1029/2008GL035333. http://geotest.tamu.edu/userfiles/216/Dessler2008b.pdf
Forster, P. M. D., and M. Collins (2004), Quantifying the water vapour feedback associated with post-Pinatubo global cooling, Climate Dynamics, 23, 207-214.
John, V. O., and B. J. Soden (2007), Temperature and humidity biases in global climate models and their impact on climate feedbacks, Geophys. Res. Lett., 34, L18704, doi: 10.1029/2007GL030429.
Paltridge, G., A. Arking, and M. Pook (2009), Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data, Theor. Appl. Climatol., doi: 10.1007/s00704-009-0117-x, 351-359.
Soden, B. J., R. T. Wetherald, G. L. Stenchikov, and A. Robock (2002), Global cooling after the eruption of Mount Pinatubo: A test of climate feedback by water vapor, Science, 296, 727-730.
Soden, B. J., D. L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, and X. Huang (2005), The radiative signature of upper tropospheric moistening, Science, 310, 841-844.