Paltridge et al. (2009) write, “water vapor feedback in climate models is large and positive” and “the various model representations and parameterizations of convection, turbulent transfer, and deposition of latent heat generally maintain a more-or-less constant relative humidity (i.e., an increasing specific humidity q) at all levels in the troposphere as the planet warms,” and that this “increasing q amplifies the response of surface temperature to increasing CO2 by a factor or 2 or more.” Consequently, knowledge of how q responds to atmospheric warming is of paramount importance to the task of correctly predicting how air temperatures respond to increasing CO2 concentrations. Paltridge et al. explored this important subject by determining trends in relative and specific humidity at various levels in the atmosphere based on reanalysis data of the National Centers for Environmental Prediction (NCEP) for the period 1973–2007.
The three researchers report, “the face-value 35-year trend in zonal-average annual-average specific humidity q is significantly negative at all altitudes above 850 hPa (roughly the top of the convective boundary layer) in the tropics and southern midlatitudes and at altitudes above 600 hPa in the northern midlatitudes.” Given these findings, Paltridge et al. conclude “negative trends in q as found in the NCEP data would imply that long-term water vapor feedback is negative—that it would reduce rather than amplify the response of the climate system to external forcing such as that from increasing atmospheric CO2.”
Paltridge, G., Arking, A. and Pook, M. 2009. Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data. Theoretical and Applied Climatology 98: 351–359.