The warming function of CO2 alone is pretty well understood; it expected to be a little less than a degree C for a doubling of CO2. However, global climate change models typically have a much higher result, on the order of 3 degrees C, the result of positive feedback from higher amounts of water vapor (a more important greenhouse gas than CO2) which are presumed to result from the higher temperature. While sensible at first glance, this assumption has been subject to considerable skepticism in the skeptical community, who offer the (also plausible) story that increased water vapor results in greater cloud cover, and increased albedo (reflection of sunlight off the cloud top back into space), reducing the climate sensitivity to CO2.
|Vonder Haar et al 2012, Fig. 4|
“Weather And Climate Analyses Using Improved Global Water Vapor Observations”
Vonder Haar, T. H., J. Bytheway, and J. M. Forsythe (2012), Weather and climate analyses using improved global water vapor observations,For a more detailed discussion of this paper by a qualified climate scientist, I suggest you read the discussion at Roger Pielke Sr.'s blog.
Geophys. Res. Lett.,doi:10.1029/2012GL052094, in press.
The NASA Water Vapor Project (NVAP) dataset is a global (land and ocean) water vapor dataset created by merging multiple sources of atmospheric water vapor to form a global data base of total and layered precipitable water vapor. Under the NASA Making Earth Science Data Records for Research Environments (MEaSUREs) program, NVAP is being reprocessed and extended, increasing its 14-year coverage to include 22 years of data. The NVAP-MEaSUREs (NVAP-M) dataset is geared towards varied user needs, and biases in the original dataset caused by algorithm and input changes were removed. This is accomplished by relying on peer reviewed algorithms and producing the data in multiple “streams” to create products geared towards studies of both climate and weather. We briefly discuss the need for reprocessing and extension, steps taken to improve the product, and provide some early science results highlighting the improvements and potential scientific uses of NVAP-M. The results of Figs. 1 and 4 have not been subjected to detailed global or regional trend analyses, which will be a topic for a forthcoming paper. Such analyses must account for the changes in satellite sampling discussed in the supplement. Therefore, at this time, we can neither prove nor disprove a robust trend in the global water vapor data. “The extended calculation using coupled runs confirms the earlier inference from the AMIP runs that underestimating the negative feedback from cloud albedo and overestimating the positive feedback from the greenhouse effect of water vapor over the tropical Pacific during ENSO is a prevalent problem of climate models.