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Note: I wrote this post well before the current crisis in North Korea, but this is where it came in the sequence, so I just haven’t gotten around to posting it until now.

Q: How many nuclear missiles would have to be launched at the United States to turn it into a complete wasteland?

Randall’s response: No response.

My response: Just one. Everyone else will take care of the rest.

But actually, no, that’s not true, as I will endeavor to show here. The real answer is that it’s surprisingly many. Let’s take a “best” case scenario, in which large, megaton warheads are dropped evenly over the continental United States in an effort to render all of the land area uninhabitable. (We’ll also ignore the ambiguity that many nuclear missiles carry multiple warheads.)

There are a number of websites that calculate the effects of nuclear explosions (and asteroids, if you’re interested) and draw them on a map. I used this one. If you type in 1000 kilotons, you will see that the bomb will kill most people with radiation within a radius of 2.5 km (1.5 miles); it will level most buildings within 4.6 km (2.9 miles), and it will cause third-degree burns within 10.7 km (6.6 miles). If dropped on downtown Manhattan, it would kill an estimated 1.3 million people.

But the most widespread effect of the bomb will be nuclear fallout, which will blow downwind as far as Maine. The calculator says that 34,000 square kilometers (13,000 square miles) will be affected by dangerous levels of fallout (although it does not clarify how long it will remain highly radioactive).

The contiguous United States has an area of 8.08 million square kilometers. Divide that by the area of effect of one bomb, and you get a minimum of 238 large nuclear bombs needed to turn the country into a complete wasteland. (Boy that number’s an eerie coincidence.)

Now, in an actual nuclear war, the Russians (or whoever has enough bombs) will have a much harder task because they won’t be dropped evenly over the United States. They’ll be clustered around strategic assets—military bases, industrial facilities, and the like, many of them doubled up in case of duds or missile defense systems. If they start at the top and go down the priority list, it would take well into the thousands or even tens of thousands of bombs to do the job.

In 1979, the U.S. Office of Technology Assessment undertook what probably is still the most comprehensive official study on the effects of a nuclear war. In this study, perhaps the most surprising thing about a nuclear war is that a lot of people survive! Even in the event of a full-scale war, the OTA estimated that “only” 35%-77% of the U.S. population would die in the first 30 days and “only” 20%-40% of the U.S.S.R. population, with perhaps 5 million more cancer deaths and a similar number of miscarriages later on.

Let me put that more lucidly: the average American has about a fifty-fifty chance of surviving a full-scale nuclear war.

(At least until plague and famine set in. Those could be a problem.)

So all those years when commentators said we had enough bombs to destroy the world ten times over, they were wrong. We didn’t even have enough to destroy it once. Or maybe we did if we did the “dumb” (non-strategic) thing and spread them out evenly over the planet. But in a “realistic” nuclear war, the world would recover—for a certain value of “recover”—within a few decades.

“But what about nuclear winter?” you ask. After all, the threat of a nuclear winter was not proposed until 1983, four years after the OTA study.

Robock et al. (2007) undertook a study of the perils of a nuclear winter resulting from a full-scale war. Such a war would kick up enough dust into the atmosphere to block out 20% of the sunlight for several months, sending global temperatures plunging by 10 degrees Celsius—down to ice age levels. This could easily result in a drop of 20 degrees Celsius (36 degrees Fahrenheit) in the United States.

But still, an ice age does not equal a complete wasteland. Humans survived the last ice age in North America with stone axes and bows and arrows, and we would survive a nuclear winter, too, at least a few of us. And parts of the southern United States would probably still qualify as hospitable. Thus, nuclear winter does not count as turning the country into “a complete wasteland” for the purposes of this question.

But wait! There’s one more caveat to this analysis, and that is the cobalt bomb, or “salted” bomb, also known as the “Doomsday Device”. You may have heard of the Doomsday Device from Dr. Strangelove. The idea is that you fill a nuclear bomb partially with cobalt, and in the explosion, it becomes highly-radioactive cobalt-60 fallout, which has a half-life of 5 years, much longer than normal fallout. This would act as a greater deterrent because an enemy would know they will suffer from much worse fallout if they attack, possibly enough to render the whole world uninhabitable.

I can’t give good numbers for this possibility. Not many studies have been done on the effects of cobalt bombs, and as far as we know, no actual cobalt bomb has ever been built. However, given some reasonable assumptions about the mass and distribution of cobalt, if you really tried, you might be able to get the number you need to wipe out the United States down to a few dozen.

Still, you may rest assured that no realistic scenario for nuclear war would turn the United States into a complete wasteland…just mostly one.