Short and long term water vapor feedback
Posted by Ari Jokimäki on May 17, 2011
There’s a lot of discussion on water vapor feedback in different media. Relating to this discussion, I’d like to note an interesting paper which seems to have gone largely unnoticed (also in scientific literature). The paper in question is “An Elicitation of the Dynamic Nature of Water Vapor Feedback in Climate Change Using a 1D Model” by Hallegatte et al. (2006).
Hallegatte et al. are studying the different response times of feedback processes and how they affect the overall water vapor feedback. They say:
Another difficulty in the interpretation of feedback processes arises from the speed of the different responses. Some processes participating in the feedback mechanism may be fast, others may be very slow. This problem is usually avoided by analyzing only the new equilibrium reached by the system after a perturbation has been applied. This current practice might, however, ignore important dynamical components of the response, especially when the forcing is varying.
They study this question with a simple 1D model for atmospheric water vapor. The simplicity of the model of course introduces some uncertainties (they can’t model the dynamic changes for example in Hadley circulation and convection is accounted for only indirectly).
The result of their analysis is shown in the following graph, which shows the evolution of water vapor feedback after a step change in the forcing (doubling of carbon dioxide concentration):
Feedback factor having value below 1 represents negative water vapor feedback. As we can see from the graph, the end result is strongly positive water vapor feedback after 10 years. However, especially noteworthy feature is the apparently negative short-term feedback. It seems that water vapor feedback turns positive only after 2-3 years.
Hallegatte et al. explain this fast negative feedback like this:
The fast pole corresponds to the lowering of latent heat flux due to rainfall decrease, which comes from the rising temperature (corresponding to a decrease in RH). This mechanism constitutes one path of the WV feedback: any transient trajectory showing an increase in atmospheric absolute humidity requires an imbalance between precipitation and evaporation, and hence necessitates an increase in atmospheric latent energy content compared with the equilibrium state. In consequence, the WV feedback process should involve a rapid atmospheric cooling, as formalized in our model, with a time response of about a few weeks.
They also study how the water vapor feedback works for more realistic change in the forcing instead of step change. In this case, the fast negative feedback is hardly detectable, but it only reduces the initial warming a bit.
In our model, the WV feedback is found to have a positive static gain of 36%, and a characteristic time longer than 4 yr, making the WV feedback fully active only in response to perturbations that last at least 10 yr.
It seems that based on this study, studying water vapor feedback in the context of anthropogenic global warming should be done in decadal time scales.
Reference: Hallegatte, Stéphane, Alain Lahellec, Jean-Yves Grandpeix, 2006: An Elicitation of the Dynamic Nature of Water Vapor Feedback in Climate Change Using a 1D Model. J. Atmos. Sci., 63, 1878–1894. [abstract, full text]
See also: On quick feedback determinations