The scientists concede there are "many uncertainties" in their findings, partly because it's impossible to predict just who is likely to go to war with whom, and how those wars will be fought. But they point out, as astronomer Carl Sagan did years ago, that during World War II the United States had only two nuclear bombs, and it dropped both of them on Japanese cities. So it's not unprecedented that other countries would also likely attack major cities.
And that's one reason the scientists are so alarmed. Urbanization has swept the planet, and today there are many cities with more than 10 million inhabitants, many of them in areas where the political climate is unstable and hostile. Even one nuclear weapon, they concluded, could kill more people than some countries have lost through war during their entire history.
That lead Turco to conclude that "human society is extremely vulnerable at this time," a modest statement considering these conclusions in the report:
"Thirty-two countries that do not now have nuclear weapons possess sufficient fissionable nuclear materials to construct weapons, some in a relatively short period of time."
In some cases, the casualties could "rival previous estimates for a limited strategic war between the superpowers involving thousands of weapons carrying several thousand megatons of yield," partly because more people live in concentrated areas, surrounded by more and more volatile materials.
"An individual in possession of one of the thousands of existing lightweight nuclear weapons could kill or injure a million people in a terrorist attack."
"Many nuclear weapons are small in size and light in weight and could easily be transported in a car or van." Some tactical nuclear weapons in the U.S. arsenal, for example, weigh only about 300 pounds.
The scientists admit that the lethality of a weapon is subject to many variables, even such things as local wind and whether it's raining, so their numbers should not be taken as absolutes.
But they insist that while many Americans may think the world is growing safer and the nuclear threat is easing, the opposite is true.
"We're on a trend toward a buildup (in nuclear weapons) around the world," Toon said.
And it wouldn't take a huge arsenal, or many weapons, to produce catastrophe. "Even a single surface nuclear explosion, or an air burst in rainy conditions, in a city center is likely to cause the entire metropolitan area to be abandoned at least for decades owing to infrastructure damage and radioactive contamination," the scientists say in the conclusion of their report. It would also leave at least a million dead, and a million more injured.
The danger from nuclear weapons is not less today than when two superpowers threatened each other just a few years ago. It is more, they said repeatedly. Much more.