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This paper considers approaches to estimating climate sensitivity involving the basic physics of the feedback processes rather than attempting to estimate climate sensitivity from time series of temperature. The latter have to assume a perfect knowledge of all sources of climate variability —something generally absent. The results of a variety of independent approaches all point to relatively low sensitivities. We also note that when climate change is due to regional and seasonal forcing, the concept of one dimensional climate sensitivity may, in fact, be inappropriate. Finally, it should be noted that I have not followed the common practice of considering the feedback factor to be the sum of separate feedback factors from water vapor, clouds, etc. The reason for this is that these feedback factors are not really independent. For example, in fig. 2, we refer to a characteristic emission level that is one optical depth into the atmosphere. For regions with upper level cirrus, this level is strongly related to the cloud optical depth (in the infrared), while for cloud-free regions the level is determined by water vapor. However, as shown by Rondanelli and Lindzen [30], and Horvath and Soden [31], the area covered by upper level cirrus is both highly variable and temperature dependent. The water vapor feedback is dependent not only on changes in water vapor but also on the area of cloud-free regions. It, therefore, cannot readily be disentangled from the cloud feedback.
