The five great mass extinctions of the geologic past are each associated with significant perturbations of Earth’s carbon cycle. But many past environmental events are not associated with mass extinction. What makes them different? By transforming geochemical signals to physical variables, we find that mass extinctions are associated with rates of environmental change that exceed a limit imposed by mass conservation in a normal carbon cycle. We suggest that external perturbations of the carbon cycle, such as extensive volcanism, can excite responses that breach this threshold. We conclude with brief remarks on the relevance of these findings to modern environmental change and a potential sixth extinction.