Today, the best that can be said is that the extent and duration of future EMP-induced blackouts would depend on whether the emitter generates a small or large pulse, and whether it is detonated at ground-level or from high altitudes.

CENTERFORSECURITYPOLICY: The Blackout Next Time

Another team of US physicists recently published a paper suggesting that an unusually large solar flare from the Sun could have caused the pulse of energy.

BBC: Gamma-ray burst 'hit Earth in 8th Century'

Large sun-grazing comets could bring on the sort of global electronics meltdown usually associated with electromagnetic pulse weapons or a full-scale nuclear exchange.

FORBES: Sun-Grazing Comets As Triggers For Electromagnetic Armageddon

While it's always hard to pin down the exact origins of an idea -- especially one like pulse-code modulation, where a number of parties were involved -- sifting out the large and significant contributions from the gold pan of history tends to bring up the same names time and again.

ENGADGET: Primed: digital audio basics Mobile