The issue of the relative roles of the human addition of CO2 and the resulting water vapor feedback remains an incompletely understood issue [and thanks to Tom Fuller for encouraging me to address this question].
As reported on Watts Up With That in a post titled NASA says AIRS satellite data shows positive water vapor feedback
“AIRS temperature and water vapor observations have corroborated climate model predictions that the warming of our climate produced as carbon dioxide levels rise will be greatly exacerbated — in fact, more than doubled — by water vapor,” said Andrew Dessler, a climate scientist at Texas A&M University, College Station, Texas.
Dessler explained that most of the warming caused by carbon dioxide does not come directly from carbon dioxide, but from effects known as feedbacks. Water vapor is a particularly important feedback. As the climate warms, the atmosphere becomes more humid. Since water is a greenhouse gas, it serves as a powerful positive feedback to the climate system, amplifying the initial warming. AIRS measurements of water vapor reveal that water greatly amplifies warming caused by increased levels of carbon dioxide. Comparisons of AIRS data with models and re-analyses are in excellent agreement.
For an observational proof of strong water vapor feedback Dressler recommends
Dessler, A.E., and Sherwood, S.C. A matter of humidity, Science, 323, 1020-1021, DOI: 10.1126/science.1171264, 2009
Dessler, A.E., Zhang, Z, and Yang, P. The water-vapor climate feedback inferred from climate fluctuations, 2003-2008, Geophys. Res. Lett., 35, L20704, DOI: 10.1029/2008GL035333, 2008
where the abstract reads
Between 2003 and 2008, the global-average surface temperature of the Earth varied by 0.6°C. We analyze here the response of tropospheric water vapor to these variations. Height-resolved measurements of specific humidity (q) and relative humidity (RH) are obtained from NASA’s satellite-borne Atmospheric Infrared Sounder (AIRS). Over most of the troposphere, q increased with increasing global-average surface temperature, although some regions showed the opposite response. RH increased in some regions and decreased in others, with the global average remaining nearly constant at most altitudes. The water-vapor feedback implied by these observations is strongly positive, with an average magnitude of λ q = 2.04 W/m2/K, similar to that simulated by climate models. The magnitude is similar to that obtained if the atmosphere maintained constant RH everywhere,
while their conclusion is
“The existence of a strong and positive water-vapor feedback means that projected business-as-usual greenhouse gas emissions over the next century are virtually guaranteed to produce warming of several degrees Celsius. The only way that will not happen is if a strong, negative, and currently unknown feedback is discovered somewhere in our climate system.”
The interesting statement that ”
Dessler explained that most of the warming caused by carbon dioxide does not come directly from carbon dioxide, but from effects known as feedbacks. Water vapor is a particularly important feedback”
illustrates why skillful climate prediction is such a difficult science issue. Unlike direct radiative forcings, such as a volcanic eruption, in which the diabatic cooling in the troposphere and diabatic heating in the stratospheric can be straightforwardly diagnosed from the amount of ejecta, the accurate long-term climate change from the human addition of CO2 is a much more complex problem. The water vapor feedback also involves clouds and precipitation in which phase changes into liquid water and ice occur.
There is a new paper under review that illustrates the inability of the IPCC type models to skillfully predict the water vapor feedback and thus raises questions on Dressler’s conclusion of model skill (e.g see also Roy Spencer’s research on this topic). This new paper is
Wu, C., T. Zhou, and D.-Z. Sun, 2009: Atmospheric Feedbacks over the Tropical Pacific in Observations and Atmospheric General Circulation Models: An Extended Assessment. J. Climate, Submitted.
The abstract of this paper reads
“The dynamical and radiative feedbacks from the deep convection over the tropical Pacific are quantified using ENSO signal in that region for both the observation and 16 climate models. Different from a previous analysis, we recognize the nonlinear relationship between deep convection and SST over that region, and perform the evaluation using the data from the warm phase and the cold phase separately. We also employ a much longer dataset than the previous analysis. While the results confirm the previous finding that most models underestimate the cloud albedo feedback and overestimate the water vapor feedback, we also show that the discrepancies mainly come from the warm phase, underscoring deep convection as a major source of error. In the cold phase, the models are found to have feedbacks of comparable magnitude and similar spatial pattern to the observations. Examination of the cause of the weaker feedback from cloud albedo in the models suggests that the bias is likely linked to a weaker relationship between the short-wave cloud forcing and the precipitation in the models. In addition, the analysis reveals a systematic feedback bias from the latent heat flux: the models tend to have a too strong positive feedback of latent heat flux over the central Pacific. The results suggest that the deficiency in the atmospheric feedbacks, particularly those from the deep convection, is a possible cause for the excessive cold-tongue in coupled models.”
Excerpts from the conclusion read
“…..our extended analysis further substantiates the suggestion that the excessive cold-tongue problem may have something to do with the weak regulating effect from the model atmosphere—the deep convection in particular. The analysis based on the data from the ENSO warm phase shows that all models — with no exception — have a net atmospheric feedback that is far weaker than that in the observation. While in the cold phase, some models replicate the observed ∂Fs/∂T feedback. Further more, this result underscores the relationship between the underestimate of feedbacks and deep convection.”
“……The results underscore the potentially critical role of deep convection in the large-scale tropical ocean-atmosphere interaction, and the continuing difficulty in capturing this role in the current state-of-the-art climate models.”
Other papers on this topic led by Dr. Sun include
Sun, D.-Z., Y. Yu, and T. Zhang, 2009: Tropical Water Vapor and Cloud Feedbacks in Climate Models: A Further Assessment Using Coupled Simulations. J. Climate, 22, 1287-1304.
Thus, while Andrew Dressler is correct that water vapor feedback is required to significantly amplify the warming effect of added CO2, the water vapor feedback itself is not as well understood as he has indicated. Moreover, their conclusions indicate that any warming of the atmosphere, such as from black carbon (soot) would also have such a strong positive water vapor feedback. The Dressler analysis should be generalized to include all positive and negative radiative forcings.