Lessons on Climate Sensitivity From Past Climate Changes

Over the last decade, our understanding of climate sensitivityhas improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equilibrium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geological recordsas feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved differently. Do these insights constrain the climate sensitivityrelevant for the present day? In this paper, we review the progress made in theoretical understanding of climate sensitivityand on the estimation of climate sensitivityfrom proxy records. Particular focus lies on the background state dependence of feedback processes and on the impact of tipping points on the climate system. We suggest how to further use palaeodata to advance our understanding of the currently ongoing climate change.