Doomsday Scenarios: What to Believe?

When the Soviet Union developed nuclear weapons, the magazine Bulletin of the Atomic Scientists created the "Doomsday Clock." As the prospect of nuclear war gets more likely, the minute hand gets closer to midnight. This week, the hand was moved from seven minutes to night to five--the closest it's been since the Cold War. But in a new twist, global warming has been added as an imminent threat to human kind. Are recent predictions about the end of our species any more credible than those we've already survived? Can we count on unforeseen circumstances and human ingenuity? Does popular culture convey the right message or promote unnecessary anxiety?