Those of us who are fortunate enough to live in the developed world fret about myriad minor — or sometimes improbable — hazards: carcinogens in food, air crashes, and so forth.

But we are less secure than we think. We are in denial about scenarios that could cause such devastation that even one occurrence would be too many.

Much has been written about possible ecological shocks triggered by the impact of a growing human population's demands on the biosphere, and about the social and political tensions stemming from resource scarcity or climate change.