State-dependence of climate sensitivity: attractor constraints and palaeoclimate regimes

TL;DR

This paper investigates how climate sensitivity varies with climate regimes and internal variability using a conceptual Earth system model, emphasizing the importance of state-dependent analysis for understanding climate response.

Contribution

It introduces a novel approach to quantify climate sensitivity as a distribution within the climate attractor, highlighting state-dependence and regime-specific variability.

Findings

Climate sensitivity varies significantly between cold and warm regimes.

Sensitivity can be represented as a distribution reflecting internal variability.

State-dependent sensitivity is confirmed through model perturbations.

Abstract

Equilibrium climate sensitivity (ECS) is a key predictor of climate change. However, it is not very well constrained, either by climate models or by observational data. The reasons for this include strong internal variability and forcing on many time scales. In practise this means that the 'equilibrium' will only be relative to fixing the slow feedback processes before comparing palaeoclimate sensitivity estimates with estimates from model simulations. In addition, information from the late Pleistocene ice age cycles indicates that the climate cycles between cold and warm regimes, and the climate sensitivity varies considerably between regime because of fast feedback processes changing relative strength and time scales over one cycle. In this paper we consider climate sensitivity for quite general climate dynamics. Using a conceptual Earth system model of Gildor and Tziperman (2001) (with Milankovich forcing and dynamical ocean biogeochemistry) we explore various ways of quantifying the state-dependence of climate sensitivity from unperturbed and perturbed model time series. Even without considering any perturbations, we suggest that climate sensitivity can be usefully thought of as a distribution that quantifies variability within the 'climate attractor' and where there is a strong dependence on climate state and more specificially on the 'climate regime' where fast processes are approximately in equilibrium. We also consider perturbations by instantaneous doubling of CO$_2$ and similarly find a strong dependence on the climate state using our approach.