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Isaac Held's Blog

25. Relative humidity feedback

Some feedbacks in AR4 models, from Held and Shell 2012.  The three red columns on the right provide the traditional perspective:  the “Planck feedback”– the response to uniform warming of surface and troposphere with fixed specific humidity (\lambda_T), the lapse rate feedback at fixed specific humidity (\lambda_L), and the water vapor feedback (\lambda_Q).  The three blue columns on the left provide an alternative perspective — with the fixed relative humidity uniform warming feedback (\tilde{\lambda}_T), the fixed relative humidity lapse rate feedback, (\tilde{\lambda}_L), and the relative humidity feedback (\tilde{\lambda}_H).  The sum of the three terms, shown in the middle black column, is the same from either perspective.  Surface albedo and cloud feedbacks are omitted. Each model is a dot.

This is the continuation of the previous post, describing how we can try to simplify the analysis of climate feedbacks by taking advantage of the arbitrariness in the definition of our reference point, or equivalently, in the choice of variables that we use to describe the climate response. There is nothing fundamentally new here — it is just making explicit the way that many people in the field actually think, myself included.  And if  you don’t like this reformulation, that’s fine — it’s just an alternative language that you’re free to adopt or reject.

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