Speaking of that ranch in Montana …

Dick Garwin tells Congress that, in his formidable opinion, the chance of a successful terrorist use of a nuclear weapon in the United States or Europe is twenty percent per year:

GARWIN: What we are missing is really the response to a terrorist nuclear explosion in a Western city. I think Senator Nunn alluded to this. We need to organize ourselves so that if we lose a couple hundred thousand people, which is less than a tenth percent of our population, it doesn’t destroy the country politically or economically.

But we need to have a way to survive such an attack, which I think is quite likely—maybe 20 percent per year probability, with American cities and European cities included. And we need to be able to survive that. We have no real planning to do it in the business community or in the government.

EDWARDS: I’m sorry. What did you say, Dr. Garwin, the probabilities were? Twenty percent?

GARWIN: Yes, to have a nuclear explosion—not just a contamination dirty bomb—in the next year, 20 percent in my estimation. Could be 10 percent, not 100 percent.

EDWARDS: If that doesn’t wake up this country, I don’t know what would.

[Full text in the comments]

As co-author (with Peter Zimmerman) of the “Bomb In the Backyard,” I am also worried, of course, about terrorists acquiring a nuclear device. But this is a bit much. Twenty percent change per year compounds to nearly 90 percent chance over ten years and 99 percent over twenty years.

In other words, a virtual certainty.

I would have said the probability was an order of magnitude lower—which is to unlikely but still very dangerous—given the evident difficulty of acquiring the fissile material.

My friend Matthew Bunn’s PhD dissertation, Guardians at the Gates of Hell: Estimating the Risk of Nuclear Theft and Terrorism, includes a detailed, plausible calculation that placed the annual risk at just over three percent. Although the model, as he concedes, isn’t definitive, it does “make explicit the assumptions about the key factors affecting the risk and provide a tool for assessing the effectiveness of alternative policies.”

The calculation also appears as an article, A Mathematical Model of the Risk of Nuclear Terrorism , in The Annals of the American Academy of Political Science 607, September 2006. I’ve stripped out the math, just to give you a little of the flavor:

Suppose, as one plausible estimate, that the factors in the equations for Pc and Rc have the following numerical values:

Number of plausible nuclear terrorist groups, Nn = 2
Yearly probability of an acquisition attempt by a particular group, Pa(j) = 0.3
Probability of choosing an acquisition attempt based on outsider theft, Po(j) = 0.2
Probability of choosing an acquisition attempt based on insider theft, Pi(j) = 0.3
Probability of choosing to attempt to purchase black market material, Pb(j) = 0.3
Probability of choosing to … convince a state to provide material, Ps(j) = 0.2
Probability that an outsider theft attempt will succeed, Pos(j,k) = 0.2
Probability that an insider theft attempt will succeed, Pis(j,k) = 0.3
Probability that a black market acquisition attempt will succeed, Pbs(j,k) = 0.2
Probability that an acquisition attempt from a state will succeed, Pss(j,k) = 0.05
Probability of … convert[ing] acquired items to nuclear capability, Pw(j,k) = 0.4
Probability of delivering and detonating bomb once acquired, Pd(j,k) = 0.7
Consequence of terrorist nuclear attack, Cc = $4 trillion

In this example, the number of plausible nuclear terrorist groups in the world is small, but greater than zero. For simplicity, assume for the sake of this example that the various probabilities are the same for all groups in the set Nn and for all acquisition attempts of a given type by those groups.

[snip]

With these values, one would expect a significant acquisition attempt roughly once every other year … The probability that such an acquisition attempt would be successful, and would lead to the detonation of a terrorist nuclear bomb somewhere in the world, would be in the range of 5 percent. … The yearly probability of nuclear terrorism would be just over 3 percent. … The probability of nuclear terrorism over a ten-year period, Pc(10), would be just under 30 percent.

Check out the real thing. The Σ won’t bite.