Characteristic Excitations of Earth's Carbon Cycle*

Over geologic time, Earth's carbon cycle has been intermittently
disrupted by transient changes in the oceans' store of carbon. Each
of these events co-occurs with significant climate change; moreover,
mass extinctions are always accompanied by such disruptions. Yet the
cause of all of these events remains mysterious. Disruptions of the
carbon cycle are typically assumed to represent a proportionate
response to an external stressor, such as enhanced volcanism or the
release of methane. This talk reviews a combination of empirical
evidence and physical theory that suggests instead that many of these
events are characteristic nonlinear responses of the Earth system to
relatively minor perturbations. Two kinds of observations support
this conclusion: the existence of a characteristic carbon flux in the
surges that mark these events, and a tendency of minor events to occur
in pairs. Both observations are consistent with the predictions of a
model of an excitable carbon cycle.