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.