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.