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Geomagnetic Field Effects on Living Systems
buoyancy, which is called “MAC balance for Magnetic, Archimedean, and Coriolis,” with only minute
roles for viscous and inertial force (Yadav et al., 2016). A relevant geodynamo model has to be in such
a force balance (Yadav et al., 2016). Contemporaneous simulations invoke high viscosities to suppress
fow turbulence to keep the computational costs manageable. Te unrealistically large viscosity in these
simulations is a major concern (Yadav et al., 2016). Tey showed that the state-of-the-art simulations
with a viscosity that is lower than in most simulations, but still much larger than in the Earth’s core, can
approach a realistic force balance (Yadav et al., 2016). Teir simulations produce many properties that
have been theoretically predicted in the past (Yadav et al., 2016).
6.3.4 Decrease of Geomagnetic Field
Te GMF has changed on diferent time scales in geological history, and the present normal polarity
started around 774 thousand years ago (774 ka) during Chibanian (~774–129 ka). Te changes in the last
200 years have been potentially diminishing declines. An eminent GMF reversal would not be so unex
pected (De Santis et al., 2004). Te magnetic poles are also moving, and it is estimated that the N pole
has moved 1,100 km over 170 years. According to analysis of ESA (European Space Agency) on the distri
bution of GMF up to June 2014, it seems plausible that global GMF intensity is not weakened evenly, and
there is a great diference depending on the place (De Michelis et al., 2017). Tus, changes in the GMF
refect changes in the deep Earth, and there are two cases of extreme changes: reversals and excursions.
During the former event, the GMF strength decreases, the poles rapidly reverse polarity, and the polar
ity reverses as mentioned above (Brown et al., 2007; Valet and Plenier, 2008; Ferk and Leonhardt, 2009).
Meanwhile, in the case of excursions, the magnetic pole moves but eventually returns to the original
polarity (fip-fopping poles). Excursion generally refers to an event in which the position of the geo
magnetic pole is 45° or more away from the N pole or S pole. Excursion occurs in a very short geological
period, so it is difcult to fnd it from fragmentary paleomagnetic records, and it is still a mysterious
phenomenon (Suganuma, 2020). Many of the excursion records have been found in seafoor and lake
bottom sediments where estimates of continuous GMF fuctuations are possible, and fortunately, they
have also been found in paleomagnetic lava (Suganuma, 2020). So far, there is no consensus on the fre
quency of excursions, but it is estimated that it may have occurred 23 times in the ~800 kyr since the
M–B boundary (Oda, 2005).
Oda (2005) showed the excursions and relative MF intensity, which are compiled from Channell et al.
(1998), Channell (1999), Guyodo and Valet (1999), and Channell and Kleiven (2000). Te data of GMF
intensity variations were obtained for the last 800 kyr. Te data of relative GMF intensity variations were
obtained from Ocean Drilling Program (ODP) Site 983 in the northern North Atlantic Ocean. Te rela
tive GMF intensity was calculated by standardizing the natural residual magnetization with the isother
mal residual magnetization. Oda (2005) found that many excursions tend to occur during the period of
low GMF strength. Tis suggests that when the magnetic dipole component, which is the main driving
force of the GMF, weakens, the non-dipole component becomes relatively dominant, causing excursion.
Te GMF intensity over the past few centuries has been declining strongly, triggering a GMF reversal.
It can be inferred that by analyzing the past excursion events, the present GMF is not in the early stages
of reversals or excursions and will recover without an extreme event such as an excursion or reversal
(Brown et al., 2018). Brown et al. (2018) inferred that for excursions to occur, a weakening of the feld
across much of the globe spreading from multiple sources is required, and not just localized weaken
ing expanding from a South Atlantic Anomaly-like feature. With special reference to the geomagnetic
excursions, however, the N magnetic pole is heading from Canada into Siberia, and recently crossed
the International Date Line (Livermore et al., 2020). Tat is, since the frst in situ measurements in
1831 of its location in the Canadian arctic, the pole has drifed inexorably toward Siberia, accelerating
between 1990 and 2005 from its historic speed of 0–15 km/year to its present speed of 50–60 km/year. In
late October 2017, the N magnetic pole crossed the international date line, passing within 390 km of the
geographic pole, and is now moving southwards (Livermore et al., 2020). Its rapid motion, plus other