Another Look at Climate Sensitivity

We analyze here a classical energy balance model (EBM) with nonlinear feedbacks and revisit in this context a recent claim that — in a model-independent sense — the climate system may exhibit an asymmetric, large-amplitude response to normally distributed forcing; such a response would imply irreducible uncertainty in climate change predictions. We show that equilibrium climate sensitivity in all generality does not support such an intrinsic indeterminacy. EBMs, though, do exhibit a saddle-node bifurcation that gives rise to several steady-state climates; this 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 associated with the vicinity to such a bifurcation. We discuss this bifurcation in the EBM context and recall some well-known results about the stability and equilibrium sensitivity of such models. It is shown here, moreover, that small disturbances cannot result in a large-amplitude response, unless the system is at or near the bifurcation point. Our EBM suggests that the current Earth climate is relatively close to this bifurcation, and a moderate decrease of the global radiative input may bring the Earth to a “snowball” state; results of general circulation models point in the same direction. We discuss briefly how the distance to the bifurcation may be related to the Earth’s ice-albedo feedback.