You worry about your heating bill this winter. You worry about your cholesterol. And the West Nile virus. And avian flu. Now and then, you may worry about a repeat of 9/11 in some diabolical new form. But you probably don't spend much time thinking about the threat of neos (near earth objects, such as asteroids, which could strike the planet with catastrophic impact) or the accidental release of deadly nerve gas at the facility near Toole, Utah, where the U.S. Army is disposing of chemical weapons by burning them.

Lee Clarke wants to change that. He wants ordinary citizens, planners, and policymakers to pay more attention to potential events that are admittedly improbable but possible nonetheless—events with consequences so severe that we ignore them at our peril.

Not that he supposes we'd be better off in a constant state of anxiety. On the contrary. Thinking ahead in this way may have a practical payoff (consider the preemptive response to Hurricane Rita versus the lack of adequate preparation for Katrina), but there is more at stake. By acknowledging that such terrible things could happen, we admit there is much that remains beyond our control.

Clarke makes his case in a flawed, occasionally maddening, but timely book, Worst Cases: Terror and Catastrophe in the Popular Imagination. "Worst cases," in Clarke's fuzzy usage, can mean anything from the 1937 explosion of the zeppelin Hindenburg or the 1940 failure of the Tacoma Narrows Bridge to the extinction of the human race. A notion so elastic has little purchase. And the subtitle seems to belong to a different book, perhaps one concerned with disaster in movies, novels, and so on.

Worst Cases differs from Richard Posner's book of 2004, Catastrophe, which is more systematic ...