The sensitivity of the climate system to radiative forcing from anthropogenic greenhouse gas emissions remains very uncertain. The IPCC best estimate for average surface warming in response to a doubling of atmospheric CO2 is about 3 deg. C. In recent years, Spencer & Braswell, Lindzen & Choi, and others, have argued that climate sensitivity has been overestimated, and that it might be only 1 deg. C, or even less. Clearly, an observational estimate of the sensitivity of the climate system would be beneficial, but there are practical difficulties in diagnosing feedbacks from satellite data, which I have described in previous seminars. In this talk, I will use a forcing-feedback-ocean diffusion model to explain global average ocean temperature variations since 1880, with a special emphasis on data since 1955 where we have temperature information to 700 m depth. A combination of assumed anthropogenic and volcanic radiative forcings from GISS, as well as internal forcing by ENSO, will be used to estimate what climate sensitivity best replicates both ocean temperature observations, and radiative flux measurements by the NASA Terra CERES instrument.