The real question is why is the earth’s magnetosphere which protects us from solar radiation decreasing in strength.
It’s because the dipolar field of the earth is decreasing... But why is that happening?
Because in the alignment of the axis of the earth as the magnetic pole representing the axis of the spin of the earth’s core aligns with the axis of the crust or the geographic axis the dynamo output decreases. Take a look at the path of the magnetic pole and match it to a timeline for one complete cycle.
As the axis travel apart the dipolar field increases and thus the strength of the magnetosphere protecting us. Thus an ice age occurs at greatest polar opposition. When the two approach alignment the magnetosphere decreases and the increased solar absorption on earth causes a global warming.
The same thing happens on other planets but more frequently than earth as the spinning iron core of earth acts as a stabilizer. The sun, due to it’s lack of density reverses polarity frequently. The reverse is due to the external field environment in the galaxy.
Much like an AC generator, but on a cosmic scale. Earth on the other hand is more like a DC motor. Then, I guess, the atmosphere could be a blanket rectifier.