Department of Earth, Atmospheric and Planetary Sciences
Title: Biosphere-geosphere instability and mass extinction
Abstract: Earth's carbon cycle, a loop between photosynthesis and respiration, represents metabolism at a global scale. Mass extinction is always accompanied by disruption of the loop. Yet the geochemical record also exhibits many such disruptions without mass extinction. What separates extinctions from benign events? At geologic timescales, mass extinctions are associated with perturbations of the carbon cycle that exceed a critical rate related to incomplete respiration. At short time scales, mass extinction instead occurs when a perturbation exceeds a critical mass related to other biogeochemical processes. A simple calculation suggests that the mass of carbon added to the oceans by human activities will exceed the critical mass by the year 2100. What happens next? Analysis of a two-dimensional dynamical system reveals two routes to runaway ocean acidification: via bifurcation of a stable fixed point to a stable limit cycle, or by the transient excitation of a stable fixed point (as in biophysical models of action potentials). The latter route, which is consistent with observations, shows how modest perturbations can result in catastrophic excitations of an otherwise stable carbon cycle.