  Most black holes are known to be unstable to emitting Hawking radiation (in
asymptotically flat spacetime). If the black holes are non-extreme, they have
positive temperature and emit thermally. If they are extremal rotating black
holes, they still spontaneously emit particles like gravitons and photons. If
they are extremal electrically charged black holes, they are unstable to
emitting electrons or positrons. The only exception would be extreme
magnetically charged black holes if there do not exist any magnetic monopoles
for them to emit. However, here we show that even in this case, vacuum
polarization causes all magnetic black holes to be unstable to emitting smaller
magnetic black holes.
