Pathways Toward the Onset of Climate-Carbon Cycle Disruptions

Carbon cycle records exhibit short-lived bursts throughout Earth's history. It has been hypothesized that such disruptions are caused by rate-induced tipping events, or significant departures from steady state when a parameter changes faster than a critical rate. Empirical evidence of this is difficult to obtain given the system's complexity. Recent work provides a possible route forward by showing that one can treat the carbon cycle as a nonlinear, forced dynamical system–that is, a system displaying characteristic behavior in time, driven by time-dependent processes. Building on such observations, we ask how bursts are instigated. We first develop an excitable, two-component stochastic dynamical system of the carbon cycle that exhibits both noise- and rate-induced tipping. Our model partitions carbon between inorganic and organic reservoirs in the upper ocean. Bursts occur as excitations from a steady state to which the system eventually relaxes. We quantify the mean stochastic perturbation and rate that excites the system and determine the most probable excitation path. To investigate the extent to which such pathways are observable, we develop an algorithm to extract characteristic carbon cycle bursts from isotopic data spanning the last 66 Myr. Results indicate that premonitory triggers exist. To clarify their dynamical origin, we compare their characteristic features to predictions from distinct classes of excitable systems and stochastic differential equations.