Climate shocks that come out of the blue

Donald Rumsfeld, the former US Secretary of Defense, liked to distinguish between "known unknowns" and "unknown unknowns", and for this he was widely ridiculed. But Rumsfeld had a point. We all know, for instance, that global warming is harming cold-adapted species on mountain tops, even if we can't predict the ultimate magnitude of the damage. Yet the really alarming changes are those that come completely out of the blue - the unknown unknowns that we never even imagined.