The Earth’s magnetic field surrounds our planet like an invisible force field –
protecting life from harmful solar
radiation by deflecting charged particles
away. Far from being constant, this field
is continuously changing. Indeed, our
planet’s history includes at least several hundred global magnetic reversals,
where north and south magnetic poles
swap places. So when’s the next one
happening and how will it affect life on
Earth? During a reversal the magnetic field
won’t be zero, but will assume a weaker
and more complex form. It may fall to 10% of the present-day strength and
have magnetic poles at the equator or
even the simultaneous existence of
multiple “north” and “south” magnetic
poles. Geomagnetic reversals occur a few times
every million years on average.
However, the interval between reversals
is very irregular and can range up to tens
of millions of years. There can also be temporary and
incomplete reversals, known as events
and excursions, in which the magnetic
poles move away from the geographic
poles – perhaps even crossing the
equator – before returning back to their original locations. The last full reversal,
the Brunhes-Matuyama, occurred around
780,000 years ago. A temporary reversal, the Laschamp event, occurred around 41,000 years ago. It lasted less than
1,000 years with the actual change of
polarity lasting around 250 years. Power cut or mass extinction? The alteration in the magnetic field
during a reversal will weaken its
shielding effect, allowing heightened
levels of radiation on and above the
Earth’s surface. Were this to happen
today, the increase in charged particles reaching the Earth would result in
increased risks for satellites, aviation,
and ground-based electrical
infrastructure. Geomagnetic storms,
driven by the interaction of anomalously
large eruptions of solar energy with our magnetic field, give us a foretaste of
what we can expect with a weakened
magnetic shield. In 2003, the so-called Halloween storm caused local electricity-grid blackouts in
Sweden, required the rerouting of flights
to avoid communication blackout and
radiation risk, and disrupted satellites
and communication systems. But this
storm was minor in comparison with other storms of the recent past, such as
the 1859 Carrington event, which caused aurorae as far south as the Caribbean. The impact of a major storm on today’s
electronic infrastructure is not fully
known. Of course any time spent
without electricity, heating, air
conditioning, GPS or internet would have
a major impact; widespread blackouts could result in economic disruption measuring in tens of billions of dollars a
day. In terms of life on Earth and the direct
impact of a reversal on our species we
cannot definitively predict what will
happen as modern humans did not exist
at the time of the last full reversal.
Several studies have tried to link past reversals with mass extinctions – suggesting some reversals and episodes
of extended volcanism could be driven by a common cause. However, there is no evidence of any impending
cataclysmic volcanism and so we would
only likely have to contend with the
electromagnetic impact if the field does
reverse relatively soon. We do know that many animal species
have some form of magnetoreception that enables them to sense the Earth’s
magnetic field. They may use this to assist in long-distance navigation during
migration. But it is unclear what impact
a reversal might have on such species.
What is clear is that early humans did
manage to live through the Laschamp
event and life itself has survived the hundreds of full reversals evidenced in
the geologic record. Can we predict geomagnetic reversals? The simple fact that we are “overdue”
for a full reversal and the fact that the
Earth’s field is currently decreasing at a
rate of 5% per century, has led to suggestions that the field may reverse within the next 2,000 years. But pinning
down an exact date – at least for now –
will be difficult. The Earth’s magnetic field is generated
within the liquid core of our planet, by
the slow churning of molten iron. Like
the atmosphere and oceans, the way in
which it moves is governed by the laws
of physics. We should therefore be able to predict the “weather of the core” by
tracking this movement, just like we can
predict real weather by looking at the
atmosphere and ocean. A reversal can
then be likened to a particular type of
storm in the core, where the dynamics – and magnetic field – go haywire (at
least for a short while), before settling
down again. The difficulties of predicting the weather
beyond a few days are widely known,
despite us living within and directly
observing the atmosphere. Yet
predicting the Earth’s core is a far more
difficult prospect, principally because it is buried beneath 3,000km of rock such
that our observations are scant and
indirect. However, we are not
completely blind: we know the major
composition of the material inside the
core and that it is liquid. A global network of ground-based observatories
and orbiting satellites also measure how
the magnetic field is changing, which
gives us insight into how the liquid core
is moving. The recent discovery of a jet-stream within the core highlights our evolving
ingenuity and increasing ability to
measure and infer the dynamics of the
core. Coupled with numerical simulations
and laboratory experiments to study the
fluid dynamics of the planet’s interior, our understanding is developing at a
rapid rate. The prospect of being able to
forecast the Earth’s core is perhaps not
too far out of reach.