The following is from Wikipedia in October 2012:

A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth’s magnetic field. Unlike reversals however, an excursion does not permanently change the large-scale orientation of the field, but rather represents a dramatic, typically short-lived decrease in field intensity, with a variation in pole orientation of up to 45 degrees from the previous position. These events, which typically last a few thousand to a few tens of thousands of years, often involve declines in field strength to between 0 and 20% of normal. Excursions, unlike reversals, are generally not recorded across the entire globe. This is partially due to them not being recorded well within the sedimentary record, but also because they likely do not extend through the entire geomagnetic field. One of the first excursions to be studied was the Laschamp event, dated at around 40 kyr ago. Since this event has also been seen in sites across the globe, it is suggested as one of the few examples of a truly global excursion.

There is no agreed-upon mechanism for a geomagnetic excursions, but there are some theories:

• it is an inherent aspect of the Earth’s inner dynamo processes, because magnetic field lines sometimes become tangled and disorganized
• magnetic field is reversal within the liquid outer core (and not not the inner core)
• triggered by impact events (comet/asteroid) affecting plate tectonics

Excursions are less likely to leave evidence that is identifiable in geological records – they can easily be too small to be noticed. Consequently scientists are unsure how frequently they occur. So far 12 have been documented as occurring in the last 780,000 years, which means they happen (on average) at least every 65,000 years.

Laschamp Event

The most recent (that we are aware of) is the Laschamp Event. According to Wikipedia:

The Laschamp event was a short reversal of the Earth’s magnetic field. It occurred 41000 years ago during the ice age. The period of reversed magnetic field was 440 years, with the transition from the normal field lasting 250 years. The reversed field was 75% weaker whereas the strength dropped to only 5% of the current strength during the transition. This resulted in greater radiation reaching the Earth, causing greater production of beryllium 10. The magnetic excursion has been demonstrated in sediment cores from the Black Sea, and in a lava flow at Laschamp in the Clermont-Ferrand district. Higher levels of carbon 14 would also have been produced during the low field times.

The Laschamp Event is truly global in nature, with reversal evidence being discovered in the following locations:

• Black Sea
• Laschamp, France
• North Atlantic
• South Pacific
• Hawaii

The poles completely switched, taking approximately 250 years to do so, and remained in switched for 440 years. During the process, the Earth’s magnetic field was substantially weaker, with only 5% of today’s field strength.The most recent data was presented in a paper published in Earth and Planetary Science Letters in 2012.

Most orthodox scientists believe that a true magnetic reversal takes thousands of years in transition, with the reversal lasting many tens of thousands of years. The Laschamp Event is a true magnetic reversal in all but name, yet scientists continue to refer to it as a mere excursion.

Potential Harm

Just like a major magnetic reversal, there are several ways in which harm may come to humans and planet Earth:

• GPS (satellites failing) and compass malfunctions
• destruction of the ozone layer
• power grid collapses
• influx of cosmic rays

It is actually traces of cosmic ray influxes that help identify geomagnetic excursions, so that is certainly a risk.

Interestingly, because such excursions may be local events only, some places might be affected while others aren’t. We could see a mass exodus from the affected regions.