I didn’t think it was sudden or abrupt, but it’s been a while since I read about it. You’re right- nothing we can do, anyway. :-) Doomers gotta doooom. I’d put this on the least of our worries.
The earth’s magnetic field is far from uniform anyway. The south Atlantic anomaly is just one big example with similar weak spots all over the globe.
http://en.wikipedia.org/wiki/South_Atlantic_Anomaly