Answer: The molten outer core of the earth generates a magnetic field that shields most of our planet from charged particles speeding toward us, mostly from the sun. We see this field in action when we look at a compass — the needle always swings toward the magnetic north pole, which is close to, but not the same as the geographic North Pole.
By analyzing the “paleomagnetic record”—a record of the magnetic field preserved in lava flows and sedimentary rocks—scientists have shown that Earth’s magnetic field changes over time. The position of the magnetic north pole moves, sometimes so much that the pole can end up on the opposite side of the globe. The most recent reversal took place just 780,000 years ago. When this happens, the strength of the magnetic field is gradually reduced to between 10 and 20 percent of its normal strength. We would notice this by looking at a compass — the needle wouldn't point strongly toward the north, and it would change direction as we moved around the world.
During a reversal, compasses could show incorrect directions and people could have difficulty navigating. The same goes for those fishes, birds and mammals that sense the magnetic field during migrations as well as certain types of bacteria that use it for sensing which way is up. The decrease in strength also reduces the protection that the magnetic field provides against cosmic rays and particles coming from the sun, and this could disrupt low-earth orbiting satellites as well as some communication and power grid systems. Also, the aurora, a phenomenon of light in the sky known as the Northern Lights, might be visible at much lower latitudes.
On the bright side, we know that people, animals, and bacteria have all survived previous geomagnetic reversals. Even during the current normal polarity interval, there have been large fluctuations in magnetic field strength.
—Cathy Constable, director of Earth Sciences and professor of geophysics, Institute of Geophysics and Planetary Physics