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Geomagnetic Field Effects on Living Systems

6.5 Discussion and Conclusions

Te relationship between the GMF strength and UV shielding has been discussed and reviewed. Past

episodes with low GMF intensity afect the UV radiation levels reaching the Earth’s surface, as lowering

the magnetosphere shield results in lower stratospheric O3 levels, and thus lowers UV radiation shield

(Wei et al., 2014).

Tere are three potential sources of energetic particles that can interact with the atmosphere of the

Earth, i.e., (1) “solar particles (solar wind),” (2) “γ-ray burst,” and (3) “GCRs” (Wei et al., 2014). All these

three sources can reduce levels of O3 and O2 in the atmosphere. Te reduction is exacerbated when the

GMF strength is reduced or altered.

Te glacial-interglacial cycle is mainly due to changes in the amount of solar radiation due to changes

in the Earth’s orbit and axis of rotation. Te phenomenon is so-called “Milankovitch cycles,” which is a

theory that variations in eccentricity, axial tilt, and precession of the Earth resulted in cyclical variation

in the solar radiation reaching the Earth, and that this orbital forcing strongly infuenced the Earth’s cli­

matic patterns (Milanković, 1941; Knezevic, 2010). It occurs in the three basic cycles of about 21, 41, and

100 kyr, but recently even the shorter thousands to hundreds of years of climate change are supposed

to be afected by the GMF (Knezevic, 2010). By integrating these Milankovitch cycles with changes

in the oxygen isotope ratio of marine microfossils called foraminifera in seafoor sediments (glacial­

interglacial cycles), more detailed ages of seafoor sediments are determined, and consequently, the M–B

boundary is estimated to be dated to 772.9 ka as mentioned above (Haneda et al., 2020).

Te events supposed to be caused by the GMF reversal may be large-scale climate change. For many

years, Ueno et al. (2019) have been investigating to elucidate the mechanism by which the GMF could

afect climate during the M–B reversal transition. During the last GMF reversal transition, the geomag­

netic strength decreases greatly, so the GCRs increase and lower clouds increase, which is the so-called

“Svensmark efect,” and it is hypothesized that the GCRs-induced sunshade efect will cool the climate

(Ueno et al., 2019). Tey have examined the geological formations that recorded GMF reversals in vari­

ous places in East Asia (Ueno et al., 2019). As a result, they found that cooling occurred when the GCRs

increased by more than 40% from the present, and that the summer precipitation decreased and the

winter monsoon strengthened (Ueno et al., 2019). Tus, when GCRs increased during the last GMF

reversal transition, the umbrella efect of low-cloud cover led to high atmospheric pressure in Siberia,

causing the East Asian winter monsoon to become stronger (Ueno et al., 2019). Tis is the frst evidence

that GCRs infuence changes in the Earth’s climate (Ueno et al., 2019).

Te above-mentioned Svensmark efect is a hypothesis that the GCRs induce low cloud formation and

infuence the Earth’s climate (Ueno et al., 2019). Tis is because the GCRs ionize the atmosphere, increase

cloud condensation nuclei, and increase cloud cover (Molina-Cuberos et al., 2001). Te tests based on

recent meteorological observation data only show minute changes in the amount of GCRs and cloud

cover, making it hard to prove this theory (Ueno et al., 2019). However, during the last GMF reversal tran­

sition, when the amount of GCRs increased dramatically, there was also a large increase in cloud cover,

so it should be possible to detect the impact of GCRs on climate at higher sensitivity (Ueno et al., 2019).

Magnetic storms promoted the production of NOx in the stratosphere, resulting in O3 reduction of

more than 60% at high latitudes north in early 2004 (Randall et al., 2005). Similarly, atmospheric nitrous

oxide (N2O) concentrations increase when the GMF intensity is low and the shielding efect from mag­

netic storms and GCRs is reduced. Te Earth’s surface naturally emits N2O from the ocean and soil,

and the emission is increasing due to human practices. N2O emissions are enhanced in interglacial and

stadial (warm) climatic conditions, increasing by ~50% at the end of the last glacial period (Schilt et al.,

2013, 2014). Te increase in atmospheric N2O concentration during the Bølling-Allerød warm period

(15–13 ka) coincided with the apparent minimum intensity of the GMF at ~13 ka, during which period

the UV radiation levels reaching the Earth’s surface were elevated.

Roughly 550 Ma, the GMF rapidly fipped their orientations, swapping north and south to trigger the

massive extinction that ended the Ediacaran Period. Rapid GMF reversal destroyed a large part of the O3