Another look at climate sensitivity

I. Zaliapin (1) , M. Ghil (2, 3, 4, 5, 6)

1 University of Nevada [Reno]
2 Département des Géosciences - ENS Paris
3 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
4 AOS - Department of Atmospheric and Oceanic Sciences [Los Angeles]
5 IGPP - Institute of Geophysics and Planetary Physics [Los Angeles]
6 IPSL (FR_636) - Institut Pierre-Simon-Laplace

We revisit a recent claim that the Earth's climate system is characterized by sensitive dependence to parameters; in particular, that the system exhibits an asymmetric, large-amplitude response to normally distributed feedback forcing. Such a response would imply irreducible uncertainty in climate change predictions and thus have notable implications for climate science and climate-related policy making. We show that equilibrium climate sensitivity in all generality does not support such an intrinsic indeterminacy; the latter appears only in essentially linear systems. The main flaw in the analysis that led to this claim is inappropriate linearization of an intrinsically nonlinear model; there is no room for physical interpretations or policy conclusions based on this mathematical error. Sensitive dependence nonetheless does exist in the climate system, as well as in climate models - albeit in a very different sense from the one claimed in the linear work under scrutiny - and we illustrate it using a classical energy balance model (EBM) with nonlinear feedbacks. EBMs exhibit two saddle-node bifurcations, more recently called 'tipping points,' which give rise to three distinct steady-state climates, two of which are stable. Such bistable behavior is, furthermore, supported by results from more realistic, nonequilibrium climate models. In a truly nonlinear setting, indeterminacy in the size of the response is observed only in the vicinity of tipping points. We show, in fact, that small disturbances cannot result in a large-amplitude response, unless the system is at or near such a point. We discuss briefly how the distance to the bifurcation may be related to the strength of Earth's ice-albedo feedback.

Domaines: Climatologie

I. Zaliapin, M. Ghil. Another look at climate sensitivity. Nonlinear Processes in Geophysics, 2010, 17 (2), pp.113-122. ⟨10.5194/npg-17-113-2010⟩. ⟨hal-01136720⟩