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Abstract

Models indicate a time-varying radiative response of the Earth system to CO2 forcing. This variation implies a significant uncertainty in the estimates of climate sensitivity to increasing atmospheric CO2 concentration. In energy-balance models, the temporal variation is represented as an additional feedback mechanism, which also depends on the ocean temperature change. Models and observations also indicate that a spatio-temporal pattern in surface warming controls this additional contribution to the radiative response by modulating the tropospheric instability. Some authors focus on the atmospheric mechanisms that drive the feedback change, reducing the role of the ocean’s energy uptake variations. For the first time, I derive, using a linearized conceptual energy-balance model, an explicit mathematical expression of the radiative response and its temporal evolution. This expression connects the spatio-temporal warming pattern to a dynamical thermal capacity, stemming from changes in the ocean energy uptake. In comparison with more realistic energy-balance frameworks, and unlike the notion of additional feedback mechanisms, I show that an expanded effective thermal capacity better explains the variation of the radiative response, naturally connects with the spatio-temporal surface warming pattern and provides a non-circular framework to explain the variation of the climate feedback parameter.