Is the astronomical forcing a reliable and unique pacemaker for climate? A conceptual model study

TL;DR

This study uses a conceptual climate model to analyze the stability and uniqueness of astronomical forcing as a pacemaker for ice age cycles, revealing multistability and episodes of temporary desynchronization.

Contribution

It introduces a novel approach to identify and analyze multistable synchronization in paleoclimate models under complex astronomical forcing.

Findings

Multistable synchronization occurs at low forcing strength.

Strong forcing leads to unique, monostable synchronization.

Temporary desynchronization episodes can last about 50 kyr.

Abstract

There is evidence that ice age cycles are paced by astronomical forcing, suggesting some kind of synchronisation phenomenon. Here, we identify the type of such synchronisation and explore systematically its uniqueness and robustness using a simple paleoclimate model akin to the van der Pol relaxation oscillator and dynamical system theory. As the insolation is quite a complex quasiperiodic signal involving different frequencies, the traditional concepts used to define synchronisation to periodic forcing are no longer applicable. Instead, we explore a different concept of generalised synchronisation in terms of (coexisting) synchronised solutions for the forced system, their basins of attraction and instabilities. We propose a clustering technique to compute the number of synchronised solutions, each of which corresponds to a different paleoclimate history. In this way, we uncover multistable synchronisation (reminiscent of phase- or frequency-locking to individual periodic components of astronomical forcing) at low forcing strength, and monostable or unique synchronisation at stronger forcing. In the multistable regime, different initial conditions may lead to different paleoclimate histories. To study their robustness, we analyse Lyapunov exponents that quantify the rate of convergence towards each synchronised solution (local stability), and basins of attraction that indicate critical levels of external perturbations (global stability). We find that even though synchronised solutions are stable on a long term, there exist short episodes of desynchronisation where nearby climate trajectories diverge temporarily (for about 50 kyr). (...)