The recent state of knowledge of global warming report released by the IPCC claims to have direct evidence of the enhanced greenhouse effect (EGE) responsible for global warming. In Chapter 3 Section 3.4 p40 of WG1 they make claims (1) the available data do not indicate a detectable trend in upper-tropospheric relative humidity, and (2) there is now evidence for global increases in upper-tropospheric specific humidity over the past two decades, which is consistent with the observed increases in tropospheric temperatures and the absence of any change in relative humidity.
Water in the upper atmosphere is important as, according to the theory, increases in greenhouse gases set off a positive feedback loop that amplifies the temperature increase, by increasing in water content in the warmer air and decreasing infared radiation is released to space. Notably, all global climate models (GCMs) show warming in the upper troposphere according to the EGE.
Now claims of direct evidence of the mechanism for global warming are particularly important, as they provide proof that increasing temperature is not due to some other mechanism. I have been looking into the veracity of this claim, here and here.
But despite the IPCC claims of no change to the relative humidity, the figure from the NCEP here shows it to be falling strongly, at all levels of the upper atmosphere and particularly in the upper troposphere (UT). A very readable paper by Minschwaner and Dessler (MD07) provides a clue. They show that while increasing temperature slightly increases water vapor and specific humidity (and thus a positive feedback increasing climate sensitivity to CO2 doubling), the amount is much less (-30%) that generally shown by the climate models that assume constant relative humidity. Their modelling shows that the increase in specific humidity is not enough to keep up with the amount needed to keep relative humidity constant for the increasing temperature, and so relative humidity falls. The paper says the models get it right about the positive feedback, but wrong about relative humidity, and so the effect is exaggerated.
While MD07 explains increasing specific humidity and falling relative humidity, and a positive feedback loop if temperature is increasing, does it provide evidence of the EGE as a cause of present day global warming? If temperatures and water vapor in the upper troposphere were actually increasing, while surface temperatures have been increasing, then this would provide the evidence for EGE they seek.
The problem is, temperatures in the upper tropospheres are not rising, as shown at the right hand side of the graph above, from Douglass et al. 2007. Despite claims by the IPCC of evidence of EGE, the only place where temperatures really rising in the upper troposphere is in the climate models themselves! The problem with rising temperatures with elevation, and erroneous constant humidity, is a double deviation from the actual observations!
Usually evidence of something means evidence in the real world, not evidence in a computer model.
Which brings me to another point — is there any evidence of EGE? While they claim small spectral differences demonstrate EGE, there are also other gaping inconsistencies between models, the literature and observations. For a start, the models assume constant relative humidity, MD07 claims this leads to exaggeration of CO2 sensitivity, and in the real world
relative humidity is decreasing. The same goes for UT temperature, and specific humidity. For there to be increased feedback, there has to be increased temperature. Reading the IPCC gives you the sort of uncomfortable feeling like a weather report that says it is sunny when its raining outside.
Two claims made in the IPCC Chapter 3 Section 3.4 p40 of WG1 are obviously false: (1) that the available data do not indicate a detectable trend in upper-tropospheric relative humidity, and (2) there is now evidence for global increases in upper-tropospheric specific humidity over the past two decades, which is consistent with the observed increases in tropospheric temperatures and the absence of any change in relative humidity.
Use of dubious evidence and false claims to support a theory indicates the degree of confirmation bias operating in global warming. Even though the science is contradictory, evidence of an enhanced EBE would be so convenient. It gives credibility to a whole raft of phenomena bundled up as anthropogenic global warming. It has the effect of leading the unwary to think that somehow the science is settled in this area, and there are no other possible explanations for recent warming. It may even lead to convictions! At least then it would go before people who differentiate models and reality.