During the 50s, 60s and 70s, the clock on the cover of the Bulletin of the Atomic Scientists reminded us of how close we were to a holocaust. The clock got as close as 2 minutes to midnight, after the H-bomb was developed, and has never been set earlier than 11:43 (in 1991, at the end of the Cold War).

Today, it sits at 11:55. This partly is because the Bulletin now considers other threats, including global warming, but most of the changes since 1991 have come because of proliferation and the threat of a nuclear terrorist attack. Even when the clock was set at 11:43, though, it recognized a substantial danger of catastrophe because of a mistake.

The real point of the clock is to remind us of the fragility and instability of our current world. Before the 1950s, it was inconceivable that people could do anything that would, in essence, end the world. Nuclear weapons made us realize that it was possible, perhaps even probable. And, oddly, that realization, along with works like Silent Spring, may have made it possible for us to understand longer-term threats, like global warming.

Of course, people react in different ways to the uncertainty of the modern world. Vladimir Putin and, I fear, some of our leaders, may think that security lies in making it dangerous for others to threaten you, rather than in making it more difficult to threaten others in the first place. These things seem to run in cycles, though, so perhaps cooler heads will prevail. I just hope it happens before the clock strikes twelve.