How about more fun:
A. reversal of earth's magnetic poles.
Nobody thinks much about the effects from that, although it has happened numerous times in earths geologic history---and without, apparently,
disturbing the Van Allen belts and exposing the earth to cosmic radiation... But I bet it would mess with electrical utilities, computers, and all
radio communication or indeed electrical fields of any type, from grandpa's pacemaker in his heart to every cellphone and aircraft beacon. Hmm.
B. A virus like Tobacco Mosaic, only in maize.
An already-real world threat, this plant virus originated in the
solanaceae family that includes tomatoes, potatoes, and tobacco. The virus
itself is genetically unstable, and has branched out beyond its original host. If it were to attack domestic corn/maize....most of which is
hybridized for monoculture with most year's seed being genetic sisters...this could prove even more devastating than the European corn borer moth,
which seriously damaged global production in the early 1990s. No more corn syrup. Meaning, no soft drinks, practically no processed food of any kind
until sugar beet production can be increased, which might take a decade. In the mean time, no cow or chicken feed for much of earth's
livestock.....
C. A Coronal Mass Ejection, a repeat of the Carrington Event
A much more violent version of scenario "A." Read
the wikipedia article: Carrington Event
. When this happened in 1859, it was so violent that telegraph towers emitted sparks(!) Picture if you will, every utility pole near you showering a
cloud of sparks. How would transformers react? How about your cell phone, your car's computer, hell, even
your own nervous system...