There are two very big magnets in our lives: the Earth's and the sun's. They both have a tendency to flip -- or reverse polarity -- over time. The sun does it very frequently -- once every 11 years or so. Earth, on the other hand, flips polarity very irregularly often taking millions of years between switches.
Why the difference? It has to do with how rowdy and dynamic the two bodies are.
The sun's flipping is caused by it being a gigantic spinning ball of conductive super-hot plasma. Or, as Discovery News' resident astrophysicist Dr. Ian O'Neill explains it: "The solar cycle ebbs and flows over an approximate 11 year period. From 'solar minimum' to 'solar maximum,' our nearest star’s internal magnetic field gets wound up by the sun’s differential rotation. Differential rotation means that the sun rotates faster at the equator than it does at the poles, dragging the magnetic field -- like an elastic band -- that is embedded in the superheated plasma. As the sun approaches solar max (as it is now) the magnetic field is at its most stressed, causing magnetic arcs to be forced from the solar interior and into the lower corona."
The Earth's magnetic field, on the other hand, is created in the outer core of the planet. This is a fluid part of the core, made mostly of iron and nickel, and is about 2,000 kilometers thick.
Temperature-driven convection currents in that metal fluid, as well as the Earth's spin, causes the fluid to spin and create electric currents. That creates magnetic fields, which are made even more powerful by the charged metals passing through them, creating electric currents of their own, and so on and so forth.
This vicious electromagnetic cycle is called the geodynamo. When these flows reverse, which can only happen under rare and complex conditions, the Earth's magnetic field reverses.
Now there's always someone who wants to believe that a solar magnetic reversal will cause the end of the world. The matter has been settled regarding the imminent magnetic field switch by the sun: it will not wipe out Earth, civilization, or even afternoon tea. But what about the switch in the Earth's polarity?
These are thought to include transition periods when Earth's magnetic field is temporarily very weak, which could expose us to a lot of harmful charged particles from the sun that are currently are deflected.
"If the Earth's magnetic field is weakened during a reversal, more of these particles will get through to the upper atmosphere. This could be a problem, but most likely the atmosphere is thick enough to protect the Earth's surface," explained Cathy Jordan, an Earth science educator affiliated with Cornell University.
The fact that the Earth's magnetic field switches is only known because geologists have detected it in rocks, especially volcanic rocks that are high in magnetic mineral content. When these rocks flow out onto the Earth's surface they cool and solidify, but not before they "feel" the Earth's magnetic field and line up their magnetic minerals with that field -- like a zillion little compasses.
Since lavas have been flowing out onto Earth's surface for all of Earth's history, they have preserved a very long and detailed history of Earth's magnetic field. What this shows us is that life on Earth has survived ancient switches just fine: there is no connection in the geological record of massive extinction events and magnetic reversals.
Still, there might be effects on wildlife and technology, which would be interesting to observe. Sadly, the window of likely magnetic reversal is anytime in the next few thousand years, so no one living today is likely to have the chance.