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From the latest doomsday scenario:

At a U.S. House of Representatives hearing on October 12th, experts warned that the greatest existential threat to the country may come from the detonation of a nuclear EMP bomb. It could kill as many as 90 percent of all Americans within a year.

What would an EMP attack actually do? It could involve the detonation of a hydrogen bomb delivered by missile or even satellites at a high altitude of 30-400 km, creating an electromagnetic pulse that would knock out the electrical grid. But not only that - all electrical devices in the range of the blast could be fried. No lights, no computers, no phones, no internet, not even cars would work. The lack of refrigeration is likely to spoil food, causing mass starvation. Add to that lack of clean water, no air traffic control or any financial transactions taking place and you have widespread devastation in the U.S.

The casualties incurred would not be from the explosion, as it can happen too high for its nuclear effects to be felt strongly on the ground. But the loss of life-sustaining infrastructure could bring slow but sure disaster.

This kind of doomsday prediction comes courtesy of two members of the former congressional EMP commission - Dr. William R. Graham and Dr. Peter Vincent Pry. Dr. Graham is a physicist who was a science advisor to President Reagan and administrated NASA. Dr. Pry is a former CIA officer responsible for analyzing Soviet and Russian nuclear strategy, who has served on numerous congressional boards related to security.

Are nuclear/EMP experts in agreement that a single nuclear EMP can destroy the electronics of a whole country? What are the conditions for such an event, given that enough nuclear tests have been performed in the atmosphere in the past, yet disruption/destruction of electronics was never that large? Ok, some are mentioned in the snippet. Is that all it would take? Wikipedia does mention a HEMP being more destructive, but without citations or much detail.

The info/claims have also been reported in more mainstream media, but with less detail.

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    The claim is suspect from two standpoints. First, as an electrical engineer I have difficulty believing that the effects (though certainly serious) due to an EMP bomb would be as universal as suggested. In particular, metal buildings (and car bodies) would provide a lot of protection. Second, the resourcefulness of citizens would result in the invention of numerous techniques to bypass the damage. Eg, cars could be rigged to use old-fashioned "point" ignitions (and many small engines are already this way and would not require any changes). – Daniel R Hicks Nov 13 '17 at 1:26
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    HEMP could damage transformers and those take months to build. The grid going down will cause ripple effects - for example pipelines won't work (pumps need electricity) so fuel delivery will be affected. With no fuel and electricity, delivery of goods will be nearly impossible. How widespread will those effects be is hard to tell, but in theory a single HEMP event could affect the whole lower 49 states. – ventsyv Nov 13 '17 at 15:32
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    Wow. 90% of the population is unaware of canned and dried foods? I did not know that. – PoloHoleSet Nov 13 '17 at 18:24
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    @PoloHoleSet You and everyone you know store enough canned food to last you a year? Good for you. Most people don't. – ventsyv Nov 16 '17 at 18:17
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    @MartinSchröder - No, not correct. While an EMP could take down huge portions of the power grid, short-term, by wiping out certain areas, in a cascading effect, it wouldn't magically reach all corners of the nation, and would not take out all transportation, only stuff within the immediate range would be taken down. If you'd cause chaos and bedlam in metropolitan areas, then machinery in rural areas, out of range, aka food, would not be hit. You'd have to have a widely distributed, universal EMP delivery, and who has the capability for that? – PoloHoleSet Nov 20 '17 at 17:19
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That number was mentioned in the statement of Dr William R Graham and Dr Peter Vincent Pry, chairman and chief of staff of the Commission to Assess the Threat to the United States from EMP attack.

The statement quotes an article by ambassador Henry Cooper, former Director of the U.S. Strategic Defense Initiative:

The result could be to shut down the U.S. electric power grid for an indefinite period, leading to the death within a year of up to 90 percent of all Americans—as the EMP Commission testified over eight years ago.

With the following note below the line:

Ambassador Henry F. Cooper, “Whistling Past The Graveyard…” High Frontier (September 20, 2016) highfrontier.org/sept-20-2016-whistling-past-the-graveyard/ See also: highfrontier.org/category/fobs. On up to 90% U.S. fatalities from an EMP attack, during a congressional hearing, Rep. Roscoe Bartlett asked me if such high fatalities could result, and I responded: “We don’t have experience with losing the infrastructure in a country with 300 million people, most of whom don’t live in a way that provides for their own food and other needs. We can go back to an era when people did live like that. That would be—10 percent would be 30 million people, and that is probably the range where we could survive as a basically rural economy.” U.S. House of Representatives, Hearing, “Threat Posed By Electromagnetic Pulse (EMP) Attack” Committee on Armed Services (Washington, D.C.: July 10, 2008), p. 9.

Here is the full statement for the record. The 2008 EMP commission report does not give any casualty estimates.

In conclusion that 90% estimate is highly speculative estimate given by Dr Henry Cooper. While he is certainly a credible individual, it appears this is just a "gut" estimate that is not backed by any serious studies.

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    Note that it's the director of SDI trying to highlight the importance of SDI. Not that I doubt his credibility, but it is his interest. – ugoren Nov 13 '17 at 19:43
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    Apparently they're running a controlled experiment on this in Puerto Rico. – Daniel R Hicks Nov 14 '17 at 2:48
  • I gave this answer an accept even though it touches only one aspect of what turned out to be a rather broad/deep question. I think the rest would be too difficult to fit in SE format. – Fizz Nov 16 '17 at 17:05
  • @Fizz. I can expand on that, I only focused on the question in the title because I thought that's the main thing you wanted answered and didn't want to go in the weeds too much. – ventsyv Nov 16 '17 at 18:16
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    I would also point out that 10% does not refer to the number of actual survivors, it's his assessment of the number of people USA can sustain as a "rural economy" - without any bearing on whether a single EMP could reduce USA to that state. Which doesn't seem very likely, since, as mentioned in comments to the question, EMP effects on exposed targets can be very inconsistent. – Danila Smirnov Nov 20 '17 at 7:45

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