In the case of the Younger Dryas, the jeopardy that humanity faced was not from nuclear missiles but from the incoming fragments of a disintegrating giant comet, traveling at tens of kilometers per second, with the larger fragments as deadly as hundreds of nuclear warheads. Indeed, it is estimated that the total explosive power of the comet fragments that struck the earth in repeated episodes over a period of 21 years some 12,800 years ago would have been of the order of 10 million megatons--1,000 times greater than all the nuclear devices stockpiled in the world today.