Threshold phenomena in the marine carbon cycle*

The history of the marine carbon cycle is punctuated by transient
disruptions. These events are typically attributed to anomalous
inputs of carbon from external sources or changes in the rates at
which carbon is buried in sediments. The magnitude of the events is
commonly assumed to scale proportionately to the size of the
disturbance. Here I show, via construction and analysis of a
2-dimensional dynamical system, how carbon-cycle disruptions can
instead result from nonlinear amplification of relatively small
perturbations that exceed a threshold. When the solitary fixed point
is stable, small but finite perturbations of the steady state can
result in a large-amplitude excitation that precedes the return to the
fixed point. The excitation represents runaway ocean acidification.
Similar events are observed in the paleoclimate record. In
particular, the model's disruptions exhibit a characteristic size and
rate of growth consistent with observations. The threshold for
initiating an excitation is also consistent with results inferred from
previous disruptions. The modern oceanic uptake of carbon will likely
exceed this threshold by the end of the present century.