Below the threshold there may be some impacts, but they will be smoothly varying with the change in the climate. Above the threshold something really nasty may happen. Examples that have been given include the instability of the thermohaline circulation that drives warmer water to the North Atlantic and the collapse of the west Antarctic ice sheet. If the warm waters of the Gulf Stream ceased to flow, the British Isles would be plunged into severe winters. They go out of their way to warn that unpredictable impacts of climate change could have unforeseeable consequences.
Surprises can make even the most careful calculation of impacts way off the mark.
And they say the speed at which the climate heats up is crucial. Non-linear systems, when rapidly forced, are particularly subject to unexpected behaviour. A fast rate of change is likely to generate more 'surprises' which inhibit the effective adaptation of both natural and managed systems. Soil degradation could be worsened by increased erosion caused by higher rainfall, while elsewhere more land could become desert. Existing stresses on fish stocks could be worsened as water temperature, salinity and entire ecosystems change.