In climate science, the concept of equilibrium climate sensitivity is widely used, but quite limited when making adequate projections of global mean surface temperature for the end of this century. The climate system has a strong internal variability on many time scales, is subject to a non-stationary forcing and mostly out of equilibrium with the changes in the radiative forcing. Also, the presence of large-scale transitions, for example due to the existence of tipping points, is difficult to capture within traditional measures of climate sensitivity. The role of small-scale processes (in many models represented as noise) to induce these transitions is not well known for many important tipping elements.

This short course will specifically address the problem of connecting fluctuations and response for the climate system, touching topics like climate sensitivity, climate variability, extreme events and tipping points. As is well known, in the case of non-equilibrium systems, the fluctuation-dissipation theorem cannot establish a comprehensive link between fluctuations and response, so that additional mathematical ideas must be introduced and heuristic approaches must be explored. In particular, this session will highlight several alternative concepts of climate sensitivity that have been developed more recently.