Many, many popular science articles claim that if the Earth didn't have a magnetic field, then the much higher concentration of cosmic rays that reached the surface would cause health damage to humans. (It's easy to find a zillion such articles with a simple Google search, so I won't bother linking to them.) Some of these article just say that the higher concentration of radiation would be "dangerous", while others use stronger language like "catastrophic". But I couldn't find any articles that give any hard numbers, or even qualitative language more specific than broad terms.
If the Earth's magnetic field vanished overnight, then how much more radiation (in grays or sievert or whatever quantity this can be most easily answered) would Earth receive at sea level?
My guess is that the health issues wouldn't be that serious, since the Earth's magnetic field has changed orientation many times and left the Earth with a negligible magnetic field for thousands of years - and as far as I know, those events didn't cause mass extinctions or anything. (Although I believe that the last flip occurred before the evolution of anatomically modern humans, so we don't have any direct evidence about human life in the long-term absence of a geomagnetic field.)
A related question is about the claim that without a geomagnetic field, the stronger solar wind "would" or "could" or "might" (depending on the pop-sci article) strip away the Earth's atmosphere. If this is true, then I'd be curious what the time scale would be. It must be geologically long, given the many periods of thousands of years without a geomagnetic field during which the atmosphere was apparently negligibly affected.