The reason this tactic could be appealing for the reclusive North Korean regime is that it requires less accuracy. While North Korean ICBMs may not have guidance good enough to hit a target the size of city, an EMP only needs to hit a general area.
So, what would be the effects on the ground from a high-altitude nuclear blast? The short E1 pulse induces strong, transient electrical currents in conductors, and the longer the conductor, the more voltage it experiences. Small devices like smartphones and laptops may be unscathed, but the real problems happen in long cables that would experience surges of 10,000 volts or more.
That’s not going to harm a power line, but, as a 2010 report on EMP effects for Oak Ridge National Laboratory points out, it will affect the low voltage sensor and control lines connected to relays and control electronics. These lines usually only carry a few volts, and the surge will destroy computers, communication devices like routers, and safety relays.
The E2 pulse presents little danger, because it resembles lightning and most systems already have protection for this sort of surge. The long, slow E3 pulse is a threat, though, and may be strong enough to burn out transformers attached to long power cables.
While the generators themselves may be left intact, the damage inflicted by E1 and E3 pulses mean no electricity can reach people, so the lights will go out. What happens after that is speculation. The most extreme suggestion is that, pushed back to 19th century technology, America would starve, but likely it the situation wouldn’t be quite so dire.
On the Defense
Stopping an EMP attack isn’t easy. A high-altitude nuke is harder to stop than one coming down to low level. While the U.S. Ground-Based Midcourse Defense missiles can reach into space, shorter-range missile defenses like THAAD and Aegis may not be able to hit an ICBM at such high altitude.
Hardening electrical infrastructure against EMP is possible—in fact, some states have toyed with the idea of testing out such defenses. But an adequate EMP shield would cost billions and take years to implement. More than likely, it will likely take a major incident to convince people to make that kind of investment in the grid.
Deterrence has been the usual U.S. defense against nuclear attack, but that generally means retaliating in kind. Striking North Korea would be impossible to do without harming our ally South Korea, and in any case, the effects would hardly be equivalent. North Korea has one of the world’s most unreliable electricity supplies already. Even when it is working, it has less than one-tenth the capacity per capita of its southern neighbor. Power cuts are common, and many homes rely on batteries or cheap Chinese solar cells to power their few small appliances.
If Kim Jong-un chooses to fire a nuclear warning shot which causes damage but no direct loss of life, will the U.S. have an effective response? Of course, nobody wants to fire the first shot which might start a spiral of destruction, but a non-lethal attack that takes out electricity supplies would be a way of escalating without killing anyone.
Some have suggested that North Korea’s next move may be a massive cyberattack. But a nuclear EMP would prove North Korea’s nuclear capability and also cause billions of dollars of damage. The U.S. might shrug off yet another cyberattack, but an EMP is impossible to ignore.