[alted]

The probability may actually be much higher: about 12% per decade for a -850mT event; 1.5% for -1700mT [1].

Wikipedia estimates the Carrington was anywhere from -800 to -1750 [2].

[1] "On the probability of occurrence of extreme space weather events", Pete Riley, 2012, doi:10.1029/2011SW000734

[2] https://en.wikipedia.org/wiki/Carrington_Event