The poor man’s EMP nuke
This article is a great over view of the dangers of EMPs. Man made and natural. Both can be a civilization killer. The EMP Commission has stated that a major EMP events could result in a 90% kill rate of the American population within the first year.
This article also brings up other dangers. More localized EMP events using such devices as camera flashes and Tesla coils.
It is a long article but well worth a read.