I agree there is no EMP big enough to take out the whole world but its interesting that few people realize the HUGE EMP that can be created blowing up a nuclear bomb in the atmosphere.

"in July 1962 the Starfish Prime test damaged electronics in Honolulu and New Zealand (approximately 1,300 kilometers away), fused 300 street lights on Oahu (Hawaii), set off about 100 burglar alarms, and caused the failure of a microwave repeating station on Kauai, which cut off the sturdy telephone system from the other Hawaiian islands."

http://en.wikipedia.org/wiki/High-altitude_nuclear_explosion

Granted still not enough to take out the world but really interesting if you think of the economic shutdown effect this would have on a small countries economy. Which is why I am unsure why the US has never used an EMP in general (not just the nuke generated kind) to cripple a enemy nations economy, and then swoop in and complete a mission while the communications and infrastructure is down.