L.F. Richardson was a British meteorologist interested in war and wanted to understand its causes. In the years between 1820 and 1945, he collected data on the hundreds of wars that had then been fought on our planet. Richardson's results were published posthumously in a book called 'The Statistics of Deadly Quarrels'.

He found that the more people killed in a war, the less likely it would occur, and the longer before you would witness it, just as violent storms occur less frequently than cloudbursts. His results can be graphed and simple extrapolation suggests that, a war in which most of the world population is killed will not be reached for about a thousand years (1820 + 1000=2820).

However, the proliferation of nuclear weapons has very likely moved the curve down and the waiting time to Doomsday may be ominously short. The shape of Richardson's curve is within our control, but only if humans are willing to embrace nuclear disarmament and dramatically restructure the planetary community. (taken directly from Carl Sagan's "Cosmos")