Among people who contemplate disasters and try to prepare for such mostr are what fall in the category of events known as a Low Probability/High Consequence Event. That is an event that has a low probability of occurring but has very high consequences if it does occur.
Such events include disasters such as EMP events, Supervolcano Eruptions, Nuclear War, Worldwide Pandemics and several other type of events. All of these have a few common elements. The question of the probability of these events is always an issue of debate.
The likelihood of a Nuclear EMP event was considered to be so minimal as to not be worth considering until several nonrational, politically unstable and unpredictable nations such as Iran, North Korea and Pakistan either developed or are in the process of developing types of nuclear weapons which seem ideally suited to such uses. Coupled with their development of ballistic missiles which are again ideally suited for such uses and the testing launching of such missiles off of freighters on the high seas, the “probability” of such events has risen quite dramatically in some people’s minds.
The difficulty of defending against such actions by hardening the power grid and protecting critical electronics from such an attack have apparently convinced the Federal government to not bother taking any action at all. The result is the United States and most of the Western Industrial world is susceptible to being virtually destroyed by a few rogue nations using technology not much different from that of the late 1940s.
Thus there exists the possibility of Asymmetrical Warfare where a country with relatively unsophisticated technology has the ability to exploit a weakness of a much more technologically sophisticated country and do extensive damage or even destroy or neutralize an enemy entirely.
Even more difficult to contemplate is the results of a Solar Flare comparable to the Carrington Event of 1859. If you have read William Forstchen’s One Second After book then you have taste of the possible results of such an attack. If your imagination is up to the task, try to imagine the scenario in that book occurring worldwide.
In all of these events the casualties of such disasters can be optimistically estimated to be the MAJORITY of the human race affected by the event. The survivors will be a minority of the people alive at the time of the event. Depending on the type of event and it’s area of effect the casualties could range from 50% up through as much as 90% of the people living at the time the event occurs. Some people consider these estimates to be optimistic.
One ancillary effect and more long term of such a disasters is the collapse of the technological and industrial infrastructure on which most of the world depends.