[helloworld11]

If the magnetosphere disappeared tomorrow, completely, as it did on Mars at one point, it would take literally billions of years for our atmosphere to totally degrade away. It would take at least tens of millions for it to degrade appreciably. The magnetosphere of Mars failed just 500 million years after our red neighbor formed, and even after the several billion intervening years, and after having started with much less overall atmospheric bulk than Earth, Mars still has some atmosphere left (very little to be sure, but we're talking about billions of years of being bathed in solar wind and radiation). In other words, if our magnetosphere disappeared tomorrow, you likely wouldn't have to worry about major atmospheric failure for many generations of your family's lives.

Caveat: Even with the atmosphere fully present, the magnetosphere does indeed stop many charged particles that a gas barrier does not, and this would definitely be an immediate problem to surface life to some extent (how much is debatable however). We'd also be much more susceptible to electronic and electrical grid damage caused by a much larger percentage of solar storms that would have previously been too weak to do much because of our giant magnetic shield..

[htag]

Yes, our magnetosphere has had extended periods (~hundreds of years) of being in a weakened state without disrupting all life on Earth. Our empirical evidence on the size of the impact is limited, mostly due to how long ago it was. It was a big enough impact that it did leave evidence.

It's not an humanity destroying event, but it is a big stressor on a world already going through a lot.

[chasil]

It's actually measured in thousands of years.

"Other sources estimate that the time that it takes for a reversal to complete is on average around 7,000 years for the four most recent reversals."

A range of 2k-12k is also in the introduction.

https://en.wikipedia.org/wiki/Geomagnetic_reversal