Physicist, MIT; Researcher, Precision Cosmology; Scientific Director, Foundational Questions Institute; Author, Our Mathematical Universe

A serial killer is on the loose! A suicide bomber! Beware the West Nile Virus! Although headline-grabbing scares are better at generating fear, boring old cancer is more likely to do you in. Although you have less than a 1% chance per year to get it, live long enough, and it has a good chance of getting you in the end. As does accidental nuclear war.

During the half-century that we humans have been tooled up for nuclear Armageddon, there has been a steady stream of false alarms that could have triggered all-out war, with causes ranging from computer malfunction, power failure and faulty intelligence to to navigation error, bomber crash and satellite explosion. Gradual declassification of records has revealed that some of them carried greater risk than was appreciated at the time. For examples, it became clear only in 2002 that during the Cuban Missile Crisis, the USS Beale had depth-charged an unidentified submarine which was in fact Soviet and armed with nuclear weapons, and whose commanders argued over whether to retaliate with a nuclear torpedo.

Despite the end of the Cold War, the risk has arguably grown in recent years. Inaccurate but powerful ICBMs undergirded the "mutual assured destruction" stability, because a first strike could not prevent massive retaliation. The shift toward more accurate missile navigation, shorter flight times and better enemy submarine tracking erodes this stability. A successful missile defense system would complete this erosion process. Both Russia and the US retain their "launch-on-warning" strategy, requiring launch decisions to be made on 5-15 minute timescales where complete information may be unavailable. On January 25 1995, Russian President Boris Yeltsin came within minutes of initiating a full nuclear strike on the United States because of an unidentified Norwegian scientific rocket. Concern has been raised over a recent US project to replace the nuclear warheads on 2 of the 24 D5 ICBMs carried by Trident Submarines by conventional warheads, for possible use against Iran or North Korea: Russian early warning systems would be unable to distinguish them from nuclear missiles, expanding the possibilities for unfortunate misunderstandings. Other worrisome scenarios include deliberate malfeasance by military commanders triggered by mental instability and/or fringe political/religious agendas.

But why worry? Surely, if push came to shove, reasonable people would step in and do the right thing, just like they have in the past?

Nuclear nations do indeed have elaborate countermeasures in place, just like our body does against cancer. Our body can normally deal with isolated deleterious mutations, and it appears that fluke coincidences of as many as four mutations may be required to trigger certain cancers. Yet if we roll the dice enough times, shit happens — Stanley Kubrick's dark nuclear comedy "Dr. Strangelove" illustrates this with a triple coincidence.

Accidental nuclear war between two superpowers may or may not happen in my lifetime, but if it does, it will obviously change everything. The climate change we are currently discussing pales in comparison with nuclear winter, and the current economic turmoil is of course nothing compared to the resulting global crop failures, infrastructure collapse and mass starvation, with survivors succumbing to hungry armed gangs systematically pillaging from house to house. Do I expect to see this in my lifetime? I'd give it about 30%, putting it roughly on par with me getting cancer. Yet we devote way less attention and resources to reducing this risk than we do for cancer.