[ebishop]

This is very true, but contrary to what most people think the real danger isn't nuclear fallout but nuclear winter. Nuclear blasts kick up a lot of dust into the atmosphere, and even a few blasts can drastically alter the climate. A study a few years ago found that a nuclear war in which only 50 Hiroshima sized bombs were detonated could wreak havoc. See the wikipedia article on this topic: http://en.wikipedia.org/wiki/Nuclear_winter

The danger of nuclear winter has actually become worse, and it is not dependent on the size of the bombs. The problem is that urban areas are becoming more and more densely built up, and petroleum products (e.g. plastics) are more frequently being used as building materials. You don't need a truly huge bomb -- just one big enough to set fire to everything, so that it can't be extinguished. The energy released from burning all the plastic and wood and other flammable materials found in a modern city can be even greater than the bomb itself. Further, when plastic burns, it's very dirty, and all that smoke gets carried far into the atmosphere. It's the dust and debris and smoke that gets thrown into the upper atmosphere that does the real damage, blocking the sun and changing the climate.