“The palaeo-record shows that in the Earth System abrupt changes and surprises are a common feature, and that environmental extremes beyond those recorded during the period of instrumental record occur frequently. Especially large risks to the Earth System are associated with the ‘threshold-abrupt change’ behaviour that arises when a well-buffered system is forced beyond a certain limit. Until the time that the threshold is approached, it appears that the system is unresponsive to the forcing function. However, when the threshold is passed, the system can move to another state very quickly, a state that may prove to be difficult to reverse or may even be irreversible. Changes of this nature are especially dangerous in the context of global change. Societies can have little or no warning that a forcing factor is approaching such a threshold, and by the time that the change in Earth System functioning is observed, it will likely be too late to avert the major change.”