Coming down to earth: What if a big geomagnetic storm does hit?

The fourth installment in my series on geomagnetic storms is now up on the GiveWell blog.

The first three posts were about the odds of a big storming hitting earth. This one shifts to the question of likely impacts. Mostly I think the evidence is reassuring. But given the stakes, I think we should not relax, and instead support more thorough research.