Risk is the chance that something undesirable will happen. Everyone faces personal risks daily; we all have a chance of being struck by a car or by lightning or of catching a cold. None of these are certain to happen today, but they all can and do happen occasionally, some more frequently than others. Even though all risk is unpleasant, the consequences of being struck by an automobile are much more serious than those of catching a cold. Most people would do more and pay more to avoid the risks they consider most serious. Thus, risk has two important components: (1) the consequences of an event and (2) its probability. In addition, while the threat of lightning has always been present, the possibility of being struck by a car emerged only in the last century. Modern risks are constantly evolving.

Events that challenge the health of ecological systems...