Sky and Telescope - March 2018 - 19

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t WANDERING POLE Lakebed sediments reveal
that Earth's virtual geomagnetic pole wandered all
over the planet during the Gauss-Matuyama field
reversal, 2.58 million years ago. The initial and final
stages appear in orange, rapid oscillations in dark
gray, and the main reversing event - including
two wide swings in latitude - in blue.

Scientists have never directly observed a reversal in action,
so it's not entirely clear what happens. Still, they generally
agree that the ﬁeld strength doesn't just drop to zero and then
ramp back up. Instead, the dipolar ﬁeld will probably break
down into many small-scale localized patches. Bundles of
magnetic ﬁeld lines should intersect the surface in numerous
locations, including at mid-latitudes and the equator. As Peter
Driscoll (Carnegie Institution for Science) says, "You'd see
lots of South Atlantic Anomalies all over the place." After a
few thousand years, the dipole ﬁeld would eventually recover
its strength and turn back on with the opposite polarity.
Despite sensationalized media reports that our home
planet could be heading for a magnetic reversal "soon,"
researchers have no such short-term concerns. "The soonest
one might begin is probably a couple thousand years from
now," says Gary Glatzmaier (University of California, Santa
Cruz), "and if it does, then the reversal itself would likely last
about 5,000 years. But it's just as likely that one won't occur
that soon."
It's possible, however, that we could instead be heading
toward a period of signiﬁcantly reduced ﬁeld intensity. The
last of these "excursions" was the Laschamp
event of 41,000 years ago, when the magnetic poles moved substantially but didn't
Paleomagnetic Excursions
actually switch polarity. "The ﬁeld strength
12
has ﬂuctuated by 10% many times in the
past without resulting in a reversal," Finlay
10
says, "and the presently observed decrease
in the dipole strength does not seem to be
8
unusually large."
In fact, Earth's magnetic ﬁeld is so chaotic
6
that whether it's strengthening or weakening depends on your time frame. "If you back
Relative field strength

WA NDERING POLE: SOURCE: J. G LEN / R. COE / J. LIDDICOAT / JOUR NAL OF GEOPHYSICAL
R ESE AR CH; PREHISTORIC RE V ERSA L: YOH A N GU YODO & JE A N-PIERRE VA LE T / NATUR E

out to a 10,000-year time scale, we're near
an uptick," says Driscoll.
Although the details of what triggers ﬁeld
reversals remain unknown, Driscoll and other
geophysicists have successfully produced them in
computer simulations of ﬂuid motions in the outer core. As
he explains, "Your model can be a stable dipole ﬂuctuating
happily in its initial orientation for a long time, and then it
reverses randomly, and then it will be happy in its new orientation for a while."
A geomagnetic reversal would be very bad news, but it
would not spell the end of humankind. There is little or no
correlation in the fossil record between reversals and major
extinctions of life, so apparently the biosphere can withstand
these ﬂip-ﬂops without too much disruption.
But the negative impact on our society would probably be
huge. "People don't realize how much of our technological
advances rely on the Earth's magnetic ﬁeld," says Stanley.
"Our magnetic ﬁeld lines protect all of our electronics from
high-energy particles from the Sun."
With that natural shield greatly reduced, satellites would
be exposed to intense particle bombardment. Worse, we'd
lose the magnetic ﬁeld's ability to fend off solar outbursts.
Even a relatively modest solar ﬂare or coronal mass ejection could pump enough electrical energy into the lower
atmosphere to blow transmission lines over wide regions of
the planet, leading to long-lasting power outages affecting
hundreds of millions of people.

u PREHISTORIC REVERSAL Earth's last field flip,
the Brunhes-Matuyama reversal, occurred 780,000
years ago. Since then, the field strength has varied
wildly. Gray denotes measurement uncertainty; dots
at far right show recent data from volcanic flows.

Standard error
Volcanic data

4
2

Calabrian Ridge 2
La Palma
West Eifel
Blake
Brunhes- Big Lost
Jamaica/Pringle Falls
Laschamp
Matuyama Emperor

Delta

0
800,000

600,000

400,000

200,000

Time before present (years)
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Sky and Telescope - March 2018

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