work out the details of a follow-up treaty . We must hope they will be able to agree to deep reductions . A recent study by Physicians for Social Responsibility showed that if only 300 of the weapons in the Russian arsenal attacked targets in American cities , 90 million people would die in the first half hour . A comparable U.S. attack on Russia would produce similar devastation . Further , these attacks would destroy the entire economic , communications and transportation infrastructure on which the rest of the population depends for survival . In the ensuing months the vast majority of people who survived the initial attacks in both countries would die of disease , exposure and starvation . The destruction of the United States and Russia would be only part of the story . An attack of this magnitude would lift millions of tons of soot and dust into the upper levels of the atmosphere , blocking out sunlight and dropping temperatures across the globe . In fact , if the entire Russian and U.S. strategic arsenals were involved in the fighting , average surface temperature worldwide would fall 10 degrees Centigrade to levels not seen on
-LRB- CNN -RRB- -- When President Obama called for a world free of nuclear weapons in Prague , Czech Republic , this spring , many dismissed this part of his speech as idealistic rhetoric . But the abolition of nuclear weapons is not an unrealistic fantasy . It is a practical necessity if the American people are to have a secure future . President Obama should use his Nobel speech this week to reaffirm his commitment to this essential and obtainable goal . It is essential because a world armed with nuclear weapons is simply too dangerous for us to countenance . Since the end of the Cold War we have tended to act as though the threat of nuclear war had gone away . It has n't . It is only our awareness of this danger that has faded . In fact , there are some 25,000 nuclear weapons in the world today ; 95 percent of them are in the arsenals of the United States and Russia . Just this past weekend , the START treaty limiting the number of U.S. and Russian warheads expired . Negotiators in Geneva , Switzerland , have not yet been able to