There are essentially two definitions of abrupt climate change:

• In terms of physics, it is a transition of the climate system into a different mode on a time scale that is faster than the responsible forcing.

• In terms of impacts, "an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it".

(National Research Council, 2002)

These definitions are complementary: the former gives some insight into how abrupt climate change comes about ; the latter explains why there is so much research devoted to it, why it inspires catastrophe movies, and may even be the reason why you are reading this page.