Localized ecological systems are known to shift abruptly and irreversibly from one state to another when they are forced across critical thresholds. Here we review evidence that the global ecosystem as a whole can react in the same way and is approaching a planetary-scale critical transition as a result of human influence. The plausibility of a planetary-scale ‘tipping point’ highlights the need to improve biological forecasting by detecting early warning signs of critical transitions on global as well as local scales, and by detecting feedbacks that promote such transitions.

Many large mammals became extinct worldwide at the end of the Pleistocene epoch, around 12,000 years ago. Here, it is shown that smaller mammals, which often provide much more comprehensive fossil records than large mammals, were much less likely to respond to the Pleistocene