The concept of climate sensitivity: History and development

The climate sensitivity concept (CSC) has more than a century of history. It is closely related to the concept of “climate forcing” or “radiative forcing,” which was fully presented and discussed by successive IPCC Assessment Reports (e.g. see Chapter 6 Houghton et al., 2001). According to CSC, a change in the equilibrium global near-surface air temperature (NST) of the Earth, ΔT, due to an external disturbance of the Earth’s energy balance (radiative forcing), can be linearly related to a change in the net radiation at some level in the atmosphere, ΔF. Thus, where λ is the climate sensitivity, which characterizes the ability of the climate system to amplify or reduce the initial temperature change initiated by the external forcing. The climate sensitivity has been estimated using Eq. (1.1) most frequently from the NST change, ΔT2 x, resulting from the radiative forcing due to a doubling of atmospheric carbon dioxide concentration from pre-industrial levels, ΔF has become a surrogate for λ and has played a central role throughout the history of IPCC in interpreting the output of numerical models, in evaluating future climate changes from various scenarios, and in attributing the causes of observed temperature changes. Between the 1960s and 1980s various types of deterministic models were used to estimate climate sensitivity, leading to a wide range of results.