When the next big problem comes online, be it the Euro crisis, nuclear proliferation, an overstretched Internet, a killer flu, or any of the other possibilities I consider in X-Events, we will suffer a complexity overload.

To function effectively, the system scientist must know a considerable amount about the natural world AND about mathematics, without being an expert in either field. This is clearly a prescription for career disaster in today's world of ultra-high specialization.

In fact, the very nature of an X-event is that it is both rare and surprising. So I would not say that any specific X-event is likely. What I would say, though, is that some X-event is not only plausible, but very likely in a time scale of a few years.

If you ask which of the scenarios I think is most dangerous, though, I will give a different answer. In that form of the question, I regard a nuclear attack, terrorist-generated or otherwise, as the most threatening combination of likelihood and long-term damage to modern life today.

The process of globalization has now interconnected almost everything ranging from financial markets to transport networks to communication systems in a huge system that no one really understands.

What distinguishes a mathematical model from, say, a poem, a song, a portrait or any other kind of "model," is that the mathematical model is an image or picture of reality painted with logical symbols instead of with words, sounds or watercolors.

A good analogy is stretching a rubber band. You can stretch and stretch and even feel the tension increase in the muscles in your hands and arms as the gap from one end of the band to the other widens. But at some point you reach the limits of elasticity of the band and it snaps. The same thing happens with human systems.