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What happens if Earth's magnetic field shuts down? How long does it take for damage to occur?

What happens if Earth's magnetic field shuts down?
How exactly does this impact humans, and how long before we need to make significant changes for survival?

Does it make any difference if Earth's magnetic field shuts down "rapidly" vs., say, slowly decreasing by 2% a week for a decade?

My understanding is that, as Earth's interior cools, we expect that in the distant future *eventually* Earth's magnetic field will shut down.
For story / plot reasons, my story has this occurring near the present day plus or minus a hundred years (perhaps during WWII, roughly the same time the two magnetic north "local poles" were located by Project Nanook [(a)](https://en.wikipedia.org/wiki/North_magnetic_pole) ).

Some people imply that if the magnetic field of the Earth is not really that significant to human life, at least in the "near future" of few thousand years.
[(a)](https://worldbuilding.stackexchange.com/questions/245157/how-would-a-future-humanity-terraform-the-moon)
[(b)](https://scientific-speculation.codidact.com/posts/210827)

Other people claim that the magnetic field of the Earth is absolutely necessary for human life
(
[(a)](https://scientific-speculation.codidact.com/posts/244998)
[(b)](https://scientific-speculation.codidact.com/posts/237909)
[(c)](https://scientific-speculation.codidact.com/posts/208890)
[(d)](https://scientific-speculation.codidact.com/posts/255680)
[(e)](https://scientific-speculation.codidact.com/posts/210658)
[(f)](https://scientific-speculation.codidact.com/posts/245992)
[(g)](https://scientific-speculation.codidact.com/posts/214283)
[(h)](https://www.livescience.com/39780-magnetic-field-pushes-earth-core.html)
[(i)](https://worldbuilding.stackexchange.com/questions/154523/how-could-you-build-an-artificial-planet-sized-magnetic-field)
[(j)](https://worldbuilding.stackexchange.com/questions/190847/could-a-terraforming-project-use-a-metallic-asteroid-based-magnet-to-protect-a-p)
)
or for human-like life (the [Laschamp event](https://en.wikipedia.org/wiki/Laschamp_event) may have led to the the extinction of the Neanderthals).

Related:
* ["What Effect Does Moving the Magnetic Poles of a Planet Have?"](https://scientific-speculation.codidact.com/posts/206348)