Comment By Chris Colose On Water Vapor Feedback

There is a posting  on the weblog Climate Change An Analysis of Key Questions entitled “How not to discuss the Water Vapor feedback” by Chris Colose with respect to the Climate Science set of weblogs on the subject  Climate Metric Reality Check #3 – Evidence For A Lack Of Water Vapor Feedback On The Regional Scale.

Chris Colose has the following issues with the Climate Science weblog:

1.  He states that the roughly constant relative humidity of the atmosphere “is an emergent property in models, not a built-in assumption“.

This is certainly true (the models do predict this response), but the models themselves are hypothesis, constructed with parametrization of physical processes. Thus, the emergence of a roughly constant relative humidity in a model needs to compared (tested) with real world data, and this is where the assumption of a nearly constant relative humidity does not appear to be occurring recently, at least for regions where the data is available to compare.

2. He concludes, with citations of two papers (see and see)  that a nearly constant relative humidity with an increase in temperature is “confirmed by real world observations..”

However, these two papers do not confirm this. The first paper only deals with surface humidity (which as shown on Climate Science is very significantly altered by land use ; e.g. see), while the second paper shows no trend in absolute humidity since 1998 (see their Figure 1) despite ssurface temperature increasing since then! Indeed, plotting the model data since 1900 is disingenuous, since there is no data to compare with. 

3.  He reports that “[t] he point about relative humidity remaining constant applies mainly to global scale averages.”

This is an important admission, and he needs to elaborate on this statement.  The second paper that he lists as supporting his conclusion on a roughly constant relative humidity is for the oceans from 50N to 50S, so that already conflicts with his statement about applying mainly to global scale averages. In fact, to construct a global average, we need to obtain the data and regional and smaller scales, which is what we have done in our paper

Wang, J.-W., K. Wang, R.A. Pielke, J.C. Lin, and T. Matsui, 2007: Does an atmospheric warming trend lead to a moistening trend over North America?Geophys. Res. Letts., in revision.

4.  He states that it is the “upper levels  [of the atmosphere] which are radiatively significant”. He also writes “[i]n fact, as specific humidity changes in a warming climate, around 90% of the radiative response is dominated by the mid to upper atmosphere (above 800 millibars), with more effect in tropical latitudes.”

It is true that the upper levels are radiatively important. However, so are the lower levels, as Chris acknowledged where he wrote “[s]urface radiative fluxes are most sensitive to specific humidity variations in the lower troposphere”.

Climate Science Conclusion

Thus, while we agree that it is important to monitor the changes in water vapor  in the upper atmosphere, it is also as  important to monitor the depth-integrated water vapor (precipitable water).  Our weblogs on the subject of regional trends in precipitable water are a part of this discussion, and we welcome Chris Colose joining  in this important scientific debate.

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