Download Are there connections between the Earth`s magnetic field and climate?

Earth and Planetary Science Letters 253 (2007) 328 – 339
www.elsevier.com/locate/epsl
Are there connections between the Earth's magnetic
field and climate?
Vincent Courtillot a,⁎, Yves Gallet a , Jean-Louis Le Mouël a ,
Frédéric Fluteau a , Agnès Genevey b
a
Paléomagnétisme et Géomagnétisme, Institut de Physique du Globe de Paris, Institut de recherches associé au CNRS,
4 place Jussieu, 75231 Paris Cedex 05, France
b
Centre de Recherche et de Restauration des Musées de France, France
Received 11 July 2006; received in revised form 19 October 2006; accepted 19 October 2006
Available online 20 December 2006
Editor: R.D. van der Hilst
Abstract
Understanding climate change is an active topic of research. Much of the observed increase in global surface temperature over the
past 150 years occurred prior to the 1940s and after the 1980s. The main causes invoked are solar variability, changes in atmospheric
greenhouse gas content or sulfur due to natural or anthropogenic action, or internal variability of the coupled ocean–atmosphere
system. Magnetism has seldom been invoked, and evidence for connections between climate and magnetic field variations have
received little attention. We review evidence for correlations which could suggest such (causal or non-causal) connections at various
time scales (recent secular variation ∼10–100 yr, historical and archeomagnetic change ∼ 100–5000 yr, and excursions and reversals
∼ 103–106 yr), and attempt to suggest mechanisms. Evidence for correlations, which invoke Milankovic forcing in the core, either
directly or through changes in ice distribution and moments of inertia of the Earth, is still tenuous. Correlation between decadal
changes in amplitude of geomagnetic variations of external origin, solar irradiance and global temperature is stronger. It suggests that
solar irradiance could have been a major forcing function of climate until the mid-1980s, when “anomalous” warming becomes
apparent. The most intriguing feature may be the recently proposed archeomagnetic jerks, i.e. fairly abrupt (∼100 yr long)
geomagnetic field variations found at irregular intervals over the past few millennia, using the archeological record from Europe to the
Middle East. These seem to correlate with significant climatic events in the eastern North Atlantic region. A proposed mechanism
involves variations in the geometry of the geomagnetic field (f.i. tilt of the dipole to lower latitudes), resulting in enhanced cosmic-ray
induced nucleation of clouds. No forcing factor, be it changes in CO2 concentration in the atmosphere or changes in cosmic ray flux
modulated by solar activity and geomagnetism, or possibly other factors, can at present be neglected or shown to be the overwhelming
single driver of climate change in past centuries. Intensive data acquisition is required to further probe indications that the Earth's and
Sun's magnetic fields may have significant bearing on climate change at certain time scales.
© 2006 Elsevier B.V. All rights reserved.
Keywords: geomagnetism; archeomagnetism; paleomagnetism; climate change
1. Introduction
⁎ Corresponding author.
E-mail address: [email protected] (V. Courtillot).
0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2006.10.032
Paleomagnetic and climate research have a longstanding association. Recovering the fossil memory of the
V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
magnetic field in ancient rocks has allowed retracing the
evolution of the main tectonic plates, placing them in the
proper paleogeographical context. Joint use of paleogeographic reconstructions (provided by paleomagnetists and marine geoscientists), paleo-sea level
reconstructions (provided by stratigraphers and sedimentologists) and paleo-topographic reconstructions of
orogens (provided by structural geologists), together
with global climate modeling (GCM) has illuminated a
number of issues: e.g. the climate of Pangea in the
Permo–Triassic or the evolution of the monsoon as the
India–Asia collision progressed, the Paratethys Sea
disappeared and the Tibetan plateau rose [1–3]. The
measurement of certain rock magnetic properties down
well-dated stratigraphic cores (where the dating itself is
often provided by paleomagnetism through the geomagnetic polarity time-scale) has produced a number of
climate-related indicators. For instance, the direction of
the principal axes of the magnetic susceptibility tensor
can allow one to recover past wind or current directions
[4,5]. But in all these instances, paleo- and rock magnetism provide tracers but do not imply any causal
connection between climate and the geomagnetic field.
In this paper, we wish to summarize a number of
recent studies which have identified potential correlations between the two over a range of time scales from
decades to hundreds of thousands of years. We first
briefly set the stage by recalling the current state of
understanding of the main agents forcing climate over
these time scales. We then discuss evidence for
correlations between magnetic and climate variations
over the 10–100 yr, 103–104 yr and 105–106 yr time
scales. Whenever a correlation is suggested, we discuss
whether a causal connection can be invoked and in
which direction it might operate. This paper offers an
“outside” perspective by a team of scientists mainly
involved in geo-, archeo- and paleomagnetism, hoping
to underline intriguing observations and to establish new
links with the community of climate research.
2. What are the main contenders for driving
climate change?
The equilibrium temperature close to the surface of
the Earth is determined primarily by electromagnetic
radiations from the Sun, covering a broad range of
wavelengths, which presently amount to some 342 W
m− 2 at the top of the atmosphere [6]. The amounts of
energy reflected from the top of clouds, aerosols and
atmosphere (∼ 77 W m− 2) and from the Earth's surface
(∼ 30 W m− 2) define the Earth's albedo. The atmosphere
absorbs 67 W m− 2 and 168 W m− 2 reach the Earth's
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surface, where they are also absorbed. Based on Stefan's
law, one can estimate that the resulting present-day
equilibrium temperature of the Earth should be on the
order of − 18 °C. However, 390 W m− 2 are re-emitted at
IR wavelengths by the Earth's surface towards the
atmosphere, of which 155 W m− 2 are again emitted back
towards the Earth's surface due to the presence of greenhouse gases (GHG), raising the equilibrium temperature
to ∼ + 15 °C, and making, in particular, the existence of
liquid water and life possible. The main greenhouse gas
is water vapor (H2O), accompanied by CO2, CH4, and
other more minor constituents. The generally accepted
view is that changes in total solar irradiance (TSI = the
total amount of energy coming from the Sun at all
wavelengths), in atmospheric water vapor and carbon
dioxide content (and also aerosols emitted by volcanoes,
notably SO2, see below) are the main potential agents of
climate change at various timescales. Defining the
appropriate observables that should be used to describe
climate is not so straightforward. Among many indicators, the mean global temperature of the Earth's surface
is generally favored. However, its definition is not as
clear-cut as may seem, and measuring it properly is a
daunting task, particularly as one moves further back in
time [7]. Uncertainties are much larger in the southern
than in the northern hemisphere, and increase rather fast
(as one goes back in the past) prior to 1950.
The main features of temperature variations over the
past 100 yr or so include warming from the end of the
19th century to the early 1940s, followed by little change
or even cooling until ∼ 1970, and then warming since
then, the trend becoming steeper after the mid-1980s.
Warming before 1950 and after 1980 is generally
believed to be due to increases in the concentration of
greenhouse gases, lack of volcanic activity, enhanced
solar irradiance and internal variability of the coupled
atmosphere–ocean system [8]. Cooling from 1940 to
1970 is often disregarded as being part of the noise, or
variability. Exponential rise of GHGs due to human
activity in the past 150 yr is well documented. But it is
interesting to note also that the last 60 yr are a period of
unusually high solar activity (possibly unique in the past
8000 yr: [9–11]). Solanki [12] and Foukal [13] find a
good correlation between solar irradiance and global
temperature until at least 1980 (see also [14]). Scafetta
and West [15] calculate, based on an empirical model
with four timescale-dependant climate sensitivities to
solar variation, that ∼ 75% of the 1900–1980 global
warming has a solar origin, whereas the figure drops to
∼ 30% for the period 1980–2000. If only to make matters
a bit more complex, peaking of global sulfur emissions in
the 1980s and rapid decline since [16] could account for
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part of the warming in the past two or three decades. It is
therefore understandable that determining the relative
contributions of CO2 and solar irradiance (and other
potential forcing factors) to climate change over the past
(and future) century is a matter of active debate (see e.g.
[17]; also, for “minority views” [18,19]). Some revealing
quotes from a recent review by Bard and Frank [20]:
“Until the beginning of the 1980s, the relation between
the Sun and climate change were still viewed with
suspicion by the wider climate community”; “…the
matter remains controversial because most of these
records are influenced by other factors in addition to
solar activity. Moreover, we still lack a fundamental
understanding of all causal relationships between solar
activity and climate”.
Being able to determine the respective influences of
the main candidates as forcing factors for earlier periods
depends in large part on the (paleo) climatologists' ability
to actually evaluate indicators depending on each factor
over a broad range of time scales. This has been done
for pre-instrumental times by using proxies (e.g. [21]):
for global temperature one can use changes in oxygen
isotopic composition of rocks, ice or fossils (noted δ18O);
for solar irradiance, concentrations in isotopes such as
10
Be and 14C which form under the influence of solar and
galactic cosmic rays (see discussions f.i. in [14], and in
[20]); and for the concentration in carbon dioxide, the
partial pressure pCO2 in air bubbles trapped in ice (e.g.
[22]). As an example, Fig. 1 shows the evolution over the
past two millennia of (1) a proxy for temperature (δ18O
from a stalagmite in the central Alps), (2) a proxy for
solar irradiance (Δ14C) (see [20]) and (3) CO2 concentration from ice cores complemented with instrumental
measurements for the recent decades ([18]; data from
[23,24,17]): until ∼1900, solar irradiance appears to have
been the prime forcing factor, at a time when pCO2 did
not change much yet.
Crowley [25] has attempted to unravel the causes of
climate change over the past 1000 yr by comparing the
simulated global surface temperature induced by
different forcing factors with the evolution of “actual”
temperature reconstructed from proxies and instrumental data. More precisely, Crowley [25] calculates the
individual temperature responses to changes in solar
variability, CO2 and volcanism using a linear upwelling/
diffusion energy balance model (this EBM “calculates
the temperature of a vertically averaged mixed-layer
ocean/atmosphere as a function of forcing changes and
radiative damping”; see [25], p. 272). Volcanic forcing
displays random-like spikes of short duration (∼a year),
up to 20 W m− 2 in amplitude. Solar variability results in
forcing with decadal to millennial fluctuations with an
amplitude ∼ 1–2 W m− 2. The range for CO2, which
becomes significant mainly after 1800, is ∼ 2 W m−2.
Fig. 2 from Crowley [25] displays the resulting individual temperature responses. Crowley [25] concludes
that as much as 41 to 64% of pre-anthropogenic (pre1850) decadal scale temperature variations were due to
Fig. 1. The δ18O record of a stalagmite from the Spannagel cave in the central Alps (dashed line) covering the last 2000 yr, compared to 14C
production rate (Δ14C) (full line with reversed scale), a proxy for solar irradiance ([23]; see also proxies for sunspot numbers and reconstructions in
Solanki et al. [12]). CO2 concentration— from ice cores and instrumental measurements from [24] and [17]. MCO is the warm Medieval Climate
Optimum and LIA stands for Little Ice Age. Figure from Veizer [18].
V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
Fig. 2. Response of a linear upwelling/diffusion energy balance model
(EBM) to different forcings, calculated at a sensitivity of 2.0 °C for a
doubling of CO2. Temperature variations in °C. Forcings are (1)
Volcanic; (2) Solar, based on a reconstruction of solar variability by
Bard et al. [26]; (3) Greenhouse gases; and (4) Tropospheric aerosols
(figure adapted from Crowley [25]).
changes in solar irradiance and volcanism. Changes
caused by CO2 and volcanism are responsible for the
simulated temperature increase found by Crowley [25]
from the mid-19th century to the early 20th century. Yet,
there is a large discrepancy between model and instrumental data from 1850 to 1950, when data indicate
much cooler temperatures (see Fig. 4 in [25]). Another
temperature reconstruction by Zorita et al. [27] for the
last 500 yr using a forced AOGCM (i.e. with
introduction of atmosphere–ocean coupling) also fails
to reproduce accurately changes in trends of 20th
century (northern hemisphere) temperature (amplitudes
also differ by a factor up to 3). Scafetta and West [28]
find that “the amplitude of the 11-year solar signature on
the temperature record seems to be up to 3 times larger
than in the theoretical predictions”.
3. Correlations between magnetism and climate at
the 10–100 yr scales
Now, does geomagnetism have relevant data or
evidence to contribute? Le Mouël et al. [29] recently
proposed to introduce some simple, non-linear measures
(or indices) of high frequency variations in the
geomagnetic field, linked to external currents in the
ionosphere and magnetosphere forced by the solar wind
and electro-magnetic radiations. These indices are
defined as the range (maximum minus minimum) of
hourly mean values of each component of the
geomagnetic vector taken over one day. An alternate
index, actually yielding the same results, is the sum of
the squares of differences in successive hourly values
taken over one day. A complete time series of these
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indices can be reconstructed over most of the past
century in a few observatories. The resulting time series
are low-pass filtered, first on an annual basis, clearly
revealing the ∼ 11 yr solar cycle and its harmonics, then
with an 11-yr filter to reveal smaller-amplitude longerterm trends [29]. The main result of this study is that a
largely common “overall magnetic trend” emerges,
which is very similar regardless of which index,
geomagnetic vector component or observatory is used
(Fig. 3; in which all data are represented in a normalized
way, with the mean value over the entire time interval of
definition removed, and then divided by the root-mean
square amplitude over the interval). The trend rises from
1910 to 1955, decreases until 1968, rises again to 1988
and has been decreasing since then. The turn points are
rather sharp. The “overall magnetic trend” correlates
well with the evolution of solar irradiance (as reconstructed by [12,13]). It also correlates with the magnetic
aa index (e.g. [31]) and the Wolf index, i.e. the number
of sunspots (see also [32]). The aa magnetic index is
constructed in such a way as to measure the irregular
magnetic variations, after carefully removing the regular
daily variation SR [31]. The SR variation is attributed to
the atmospheric dynamo in the E layer of the
ionosphere, which is ionized by the UV-X radiations
from the Sun. On the other hand, the irregular variations
comprise a number of different components, including
the solar wind and its changes among their primary
sources. Our magnetic indices do not sort or separate
these variations; but when we retain only the five
quietest days of each month (a way to isolate the
contribution of SR), all essential features of the curves
remain the same [29].
We view the fact that the long-term “overall magnetic
trend” is essentially common to all these indices (aa, W,
full new indices regardless of which component or
which observatory is used, indices reduced to five
quietest days of each month) as evidence that the entire
system of ionospheric and magnetospheric currents,
despite all their complexity, pulses in rough unison with
the Sun on a decadal scale (and that this also applies to
the main spectral components of total solar irradiance –
i.e. photons – and also to the solar wind — i.e.
particles). None of this was a priori obvious. Note that
magnetic variations revealed by the new indices, which
reflect precisely the changes in the UV-X component of
the irradiance, are a far more sensitive indicator (by
several orders of magnitude!) than the total solar
irradiance which varies by only ∼ 1‰.
If solar activity is correlated to climate over much of
historical times, it might be expected that the “overall
magnetic trend” would correlate with the recent
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evolution of global temperature, and this is indeed the
case up to the mid-1980s, but not since then (Fig. 3). Of
course, the relation is this case does not imply a causal
link from Earth's magnetism towards climate, but from
the Sun to both climate and magnetic changes. Clearly,
100 yr is not enough to ascertain that such a correlation
is robust, but it is as impressive as many of the correlations of time series proposed over this time range.
Le Mouël et al. [29] note that global temperature
departs from all other indicators (solar and magnetic) in
the late 1980s and suggest that this is when the signal
(possibly corresponding to anthropogenic warming?)
starts emerging from noise. This is in agreement with the
estimates of Scafetta and West [15], in which the solar
contribution drops by a factor 2 (or more) after 1980.
Similar results are found when temperature evolution is
compared to that of solar cycle length or cosmic ray flux
[33–35]. Note that the leveling or drop in temperature
from ∼ 1940 to ∼1970 matches solar and magnetic
series, and not the monotonous accelerated rise in CO2.
And the period from ∼1850 to ∼ 1950 is the one over
which the modeling results of Crowley [25] are well
above the data, with significant discrepancy between
∼ 1880 and ∼1910, and a serious deviation in the
decades around 1970. There are therefore good indications of a significant contribution from solar irradiance
to climate change over at least the first 3/4 of the 20th
century, with the anthropogenic CO2 contribution
possibly becoming significant only after the mid1980s [12], although the origin of this “anomalous
temperature” cannot be considered as demonstrated.
4. On cosmic rays, clouds and climate
Bard and Frank [20] conclude in their recent review
that “for the moment, the exact mechanisms by which
cosmic radiation and solar forcing may affect cloud
formation remain very poorly understood and clearly
require future research efforts”. This rather pessimistic
view seems to be changing quickly. Three mechanisms
are thought to link solar variability with climate (e.g.
[36]): (1) changes in solar irradiance leading to changes
in heat input to the lower atmosphere; (2) solar
ultraviolet radiation coupled to changes in ozone
concentration heating the stratosphere; and (3) galactic
cosmic rays (the impact of GCR on climate was first
proposed by Ney [37]). These are modulated by longterm solar magnetic activity [38], by changes of the
source of GCRs [39] as well as by changes of the Earth's
magnetic field. Cosmic rays could in turn act on climate
in three ways [36]: (1) through changes in the
concentration of cloud condensation nuclei; (2) thunderstorm electrification; and (3) ice formation in
cyclones. A correlation between cosmic ray flux and
cloud cover was first noted by Svensmark and FriisChristensen [40] over one solar cycle, and linked to
Fig. 3. Time evolution over the 20th century of the eleven-year running averages of magnetic indices based on modulus of the geomagnetic field at the
Eskdalemuir and Sitka observatories (ESK and SIT) compared to solar irradiance S(t) and global mean temperature T globe (figure from Le Mouël
et al. [29]). Magnetic indices are from Le Mouël et al. [29] and their definition is recalled in the text. Temperature is from Jones et al. [30]. Irradiance is
from Solanki [12]. All curves have had their mean over the time interval of definition removed and have been divided by their root-mean square
amplitude over this interval for normalization. The vertical axis is therefore dimensionless and directly comparable.
V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
low-altitude clouds by Marsh and Svensmark [41]:
cosmic ray flux varies by 15% on average (and up to
50% at the poles) over this time period. The Earth's
magnetic field acts as a more or less efficient (timevarying) shield on these high-energy charged particles.
The relation between cosmic rays and clouds shows a
geographical pattern, with areas of highly significant
correlation and almost no correlation in other areas
([42]; see below). Higher cosmic ray flux would lead to
more low clouds and thus higher albedo and lower
Earth surface temperatures. The cosmic ray variation
over one solar cycle translates as a change of energy
input to the atmosphere on the order of 1.5 W m− 2 ,
which is not negligible compared for instance to the
estimated radiative forcing from anthropogenic CO2
emissions (∼ 2 W m − 2 ). And cosmic ray intensity has
varied (in the past millennia) by as much as a factor of 4
compared to recent solar cycles [36]. However, the
GCR flux–cloud correlation has been criticized. Sun
and Bradley [43] did not find any evidence at a global
scale over the longer time period from 1950 to 1995.
Laut [44] noted inconsistencies in Svensmark and
Friis-Christiansen's [40] paper and considered that
they largely overestimated the relation between solar
activity and Earth's climate (but see also [45]). The
difficulty in obtaining uncontroversial evidence for a
GCR flux–cloud correlation over several decades
could be due either to difficulties in intercomparing
ship observations with satellite data, or to an overprint
resulting from other mechanisms, such as ENSO-like
atmosphere–ocean coupling [45].
Despite these criticisms, the GCR–climate relation is
now better accepted because physico-chemical mechanisms are emerging [36]. The energy input to the
atmospheric system from cosmic rays is only a billionth
of solar irradiance, yet it may have profound effects on
several atmospheric processes. Carslaw et al. [36] have
reviewed the cloud properties which are influenced by
“microphysical processes” and discuss two ways in
which cosmic rays may affect cloud droplet number: the
ion-aerosol “clear-air” and “near-cloud” mechanisms.
The clear air mechanism relies on the production of
ultrafine sulfate aerosols from ions, which act as cloud
condensation nuclei. Atmospheric measurements support such a mechanism [46,45]. The near cloud
mechanism relies on the production of ice nuclei in
the vicinity of clouds (even if these are not thunderstorm
clouds) induced by the perturbation in the global
atmospheric electrical circuit due to the ionization of
the atmosphere by the GCR flux [47,48,45]. The
amplitudes of these effects are still uncertain and at
the frontier of cloud physics research. Carslaw et al. [36]
333
conclude “It will be difficult to separate solar and
cosmic ray effects (…) Geomagnetic field variations
could in principle untangle this ambiguity because they
affect cosmic rays but not solar irradiance, but these
variations occur on much longer time scales than the
solar variations”. We will see below that this may not
always be the case when one turns to the record of the
past millennia, over which the internal geomagnetic
field has varied significantly on time scales even shorter
than some solar time-scales…
Also, we now have more robust direct and indirect
evidence of a GCR–climate relation. Very recently,
Vieira and da Silva [49] argue that in the southern
Pacific Ocean variations in cloud cover are related to the
presence of the Southern Hemisphere magnetic anomaly, and that the causal mechanism involves stronger
cosmic ray/cloud interaction in the lower field region.
Even more recently, Svensmark et al. [50] report on a
laboratory experiment where a gas mixture attempting
to represent the chemical composition of the lower
atmosphere was subjected to UV light and cosmic rays.
These authors find that released electrons promote fast
and efficient formation of the building blocks for cloud
condensation nuclei. Moreover, GCR are well correlated
with continuous satellite data measuring low cloud
cover over much of this period [45]. Using a global
numerical model of ion production, Usoskin et al. [42]
calculated the expected distribution of correlation
between cosmic ray ionization and low-cloud amount
for the years 1984–2000. A significant correlation,
higher than 90%, was simulated, mainly over oceans (in
particular above the North Atlantic). Conversely,
correlation is zero at low latitudes, as expected.
In order to check the GCR-climate hypothesis, comparisons have been made over a longer time interval. For
instance, Wagner et al. [51] have compared the 10Be and
36
Cl records (GCR proxies) with a climate proxy record
(δ18O) between 20 and 50 kyr BP. But because production of cosmonuclides is in part controlled by the
strength of the geomagnetic field (e.g. [52]) and because
the geomagnetic field can vary quite rapidly and in an as
yet insufficiently constrained way [53], it may not be
possible to use these cosmonuclides in a robust way as
proxies of climate on a time scales from 103 to 105 years
(Bard and Frank, [20]: “…with the currently available
reconstructions of field intensity and cosmogenic nuclide
production over the past 200 kyr, it is not possible to
extract a solar component with the precision required to
draw meaningful conclusions.”). However, based on a
very careful analysis of correlations between solar
activity, cosmic rays and Earth's temperature over the
last millennium, Usoskin et al. [14] find that “periods of
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V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
higher solar activity and lower cosmic ray flux tend to be
associated with warmer climate, and vice-versa”. They
also show that cosmic ray flux correlates with temperature better than with sunspot numbers and that a positive
correlation between geomagnetic dipole moment and
temperature further supports the role of cosmic rays.
Important changes have come from evolution of
magnetospheric configuration: as geomagnetic activity
(of external origin) increases, the auroral oval expands
equatorward and the subauroral region moves to lower
geomagnetic latitudes. Feynman and Ruzmaikin [54]
have shown that, from 1890 to 1985, the area of the
auroral oval has evolved in parallel with the aa index and
Earth's global temperature. The trend follows the “overall
magnetic trend” which Le Mouël et al. [29] have
identified in all magnetic observatories with long enough
records, and again the correlation fails only after ∼1985.
So far, observed correlations between Earth's climate
and geomagnetism have involved mostly that part of
magnetic changes which is controlled by external
(solar), not internal (core) variations. Yet, moving on
to longer time scales may have uncovered links between
the internal field and climate.
correspond to previously unrecognized abrupt geomagnetic features of internal origin with rather low spherical
harmonic degree.
Archeomagnetic jerks are found around 1400, 800,
200 AD and 800 BC. Subsequent work [61] has added an
event around 1600, with other less robust possibilities at
1800, 600 AD and 350 BC. This implies 4 rather clear
events in the past two millennia (i.e. a “repeat time” on
the order of 500 yr, a time-constant characteristic of
secular variation of the equatorial dipole field — e.g.
[57]) if only the more robust events are included. The
number goes to 7 if all suggested events are included,
implying a time constant of 200–300 yr, not very
different from the duration of the events themselves and
more characteristic of non-dipole secular variation
(though see [53]). In this paper, we restrict ourselves to
the better-identified events, which remain short and
rather rare on the time scales considered.
Gallet et al. [61] have compared the occurrences of
archeomagnetic jerks with paleo-climate indicators,
5. Moving on to the archeomagnetic time scale
(103–104 yr)…
Based on newly acquired archeo-intensity data from
archeological and historically dated material recovered
from western Europe and the Middle-East, Genevey and
Gallet [55] and Gallet et al. [56] observed rather sharp
maxima in time variations of the intensity of the ancient
field, associated with sharp curvature changes in
direction. These features, which are stronger and faster
than previously realized [57,58,53] have been called
“archeomagnetic jerks” (Fig. 4). The name may be a bit
misleading, as these are rapid, ∼100-yr long increases
in field intensity by 15–30%. But the idea behind the
term was to note previously unrecognized sharp features
at a time-scale intermediate between geomagnetic jerks
(1–2 yr in a centennial time series) and excursions or
reversals (103–104 yr in 105–106 yr time series). Note
that these archeomagnetic jerks are being observed over
a widening area, first centered in Europe and then
extending to the Middle East. Similar observations are
mentioned by Snowball and Sandgren [59] in Northern
Europe and Stoner et al. [60] in Northern Canada. The
global extent and time coincidence of some of these
features is a matter of debate and ongoing research
(begging for a much larger database). Gallet et al. [61]
argue that several archeomagnetic jerks are indisputable,
being observed at a continental scale, and hence must
Fig. 4. Geomagnetic field intensity variations in Western Europe
during the past 1300 yr determined from archeomagnetic analyses.
Vertical and horizontal error bars correspond to standard deviations of
intensity means and age brackets of dated sites, respectively. The
geomagnetic field intensity variations deduced from geomagnetic field
models from 1850 onwards are indicated by small crosses. Climatic
variations during the past millennium are deduced from retreats and
advances of Alpine glaciers. Cooling periods are indicated by shaded
bands. Figure from Gallet et al. [61].
V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
Fig. 5. Geomagnetic field intensity variations in Mesopotamia during
the four millennia BC determined from archeomagnetic analyses,
compared to climate change determined in the North Atlantic from
Bond et al. [65]. On top are indicated the main societal changes in the
Middle East. Figure from Gallet et al. [64].
such as advances and retreats of glaciers in the Swiss
Alps [62,63], which are good indicators of climate
change in Europe, at least during the past few millennia,
when there are data. There is a good correspondence
between jerk times (particularly their rising period) and
advances in glaciers. The correlation between cold
events and archeomagnetic jerks is significant at
∼ 1600, ∼ 1350, ∼ 800 AD and ∼ 800 BC. Very recently,
Gallet et al. [64] have extended the database to 3000 BC
in the Middle East, and found older jerk events at
∼ 1600, ∼ 2100 and ∼ 2700 BC (Fig. 5; see also [59]).
They found that these geomagnetic variations coincide
in time with cooling periods detected in the North
Atlantic from ice-rafted debris [65].
6. …and to the paleomagnetic time scale (excursions
and reversals)?
As one attempts to go back further in the past, data
become scarce and interpretations more speculative.
Fuller [66] argues that the strongest minima in paleointensity within the Brunhes chron (i.e. the past ∼800 ka; cf.
the paleointensity stack of Guyodo and Valet [67])
correlate with obliquity minima, and that reversals
preferentially occur when the average amplitude of the
335
obliquity signal is low. This interpretation is at odds with
the analysis of Kent and Carlut [68], who find no
discernible obliquity modulation (41 ka) of the geomagnetic field in sedimentary paleointensity records (neither
those of reversals in the past 5.5 Ma nor those of
excursions in the past 800 ka). Other authors claim that a
number of paleomagnetic field excursions occurring in
the past 800 ka correlate with minima in precession and
warmer interglacial episodes, hence with climate change
as seen in the series of δ18O measurements. Carcaillet
et al. [69,70] find that over the past ∼1 Ma geomagnetic
dipole moment variations exert the main control on 10Be
production variations. They also find a correlation (less
significant but intriguing) between these records and δ18O
variations measured on benthic foraminifera, which
would indicate that geomagnetic excursions occur
preferentially during interglacial/glacial transitions or
deglaciation events. A similar claim has recently been
put forward by Acton [71]. Some authors also find a
spectral peak at ∼100 ka period in both the 10Be/9Be ratio
and paleointensity and paleoinclination variations
[70,72,73]. A possible connection between Earth's
magnetic field and Milankovic orbital cycles is therefore
presently hotly debated. In this debate, a critical aspect
concerns the fact that part of the observed correlations
between magnetic intensity and climate could be due to
changing carbonate content of the sediments, itself
controlled by climate fluctuations. As a consequence,
current opinions range from meaningful correlation to
artefact and lack of significance (e.g. [74,67,75]).
7. Are there possible mechanisms?
We acknowledge the fact that the correlations we have
outlined between Earth's mean temperature and variations in the geomagnetic field are tentative, and become
better grounded in observations as one moves from the
distant past to the present. And of course, a correlation
does not in itself suffice to demonstrate a causal
connection. Nevertheless, we feel the community should
be on the lookout for more data to test these proposals.
We now briefly attempt to outline possible mechanisms,
going from the longer to the shorter time scales.
First, as far as Milankovic frequency forcing is
concerned, there can be no doubt on the sense of the
causal relationship. Orbital changes are the main cause
of temperature and CO2 variations in the atmosphere.
Could they also generate the observed magnetic changes,
through direct destabilization of convection due to the
very tiny and regular changes in the distribution of forces
acting on the fluid core? This is reminiscent of forcing
of the geodynamo at precessional frequencies [76].
336
V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
Precession was abandoned for decades as a potential
mechanism to drive a dynamo, but Tilgner [77] has
recently found that precession-driven dynamos can exist
at magnetic Reynolds numbers characteristic of the
Earth's core. Another possibility invokes an indirect
mechanism [78], in which magnetic instabilities would
be driven by variations in polar ice caps, leading
(through loading and unloading) to variations in the
Earth's rotation speed and then to changes in core flow.
This mechanism is highly speculative and not yet borne
out by modeling or dynamo theory.
Then comes the correlation of archeomagnetic jerks
with cold episodes (at least in western Europe). Intensity
changes may either reflect changes in axial dipole
strength, changes in dipole tilt, or else changes in low
order components of the non-dipole field. New data
seem to confirm the existence of several “archeomagnetic jerks” [59,60]. There is ongoing debate as to the
geographical extent of the events, for which the most
reliable data come from a large but not global area:
Gallet et al. [56,61,64] argue for a minimum extent
ranging from the Eastern Atlantic to Central Asia,
implying low degree and order features, whereas
Gomez-Paccard et al. [79] do not find significant
geomagnetic features in the (far lower quality) data
from Japan and the South-western USA. The resolution
in either space or time of current global models [53,58]
is not sufficient to identify such features.
Should intensity changes be related to the axial dipole
(and the causal relationship occur through modulation of
cosmic rays and ensuing modifications in low-cloud
cover), one would expect paleointensity maxima to
correlate with maxima in shielding, hence minima in
cosmic rays reaching low clouds, hence lesser cloud
cover and albedo and higher temperatures, the opposite
of what is apparently observed at millennial and
multimillenial scales. Biblical accounts of exceptional
luminosity events in the first millennium BC have been
interpreted by Siscoe et al. [80] as showing that coronal
auroras can occur at low latitudes under the hypothesis of
a much reinforced geomagnetic dipole intensity. On the
contrary, Raspopov et al. [81] and Dergachev et al. [82]
suggested that these luminosity events could be linked
with a “Sterno-Etrussia” excursion, implying very low
dipole field intensity (which is not observed in archeomagnetic data of Genevey et al. [83]). But Gallet et al.
[56,61,64] have noted that if archeomagnetic jerks
correspond to extrema in dipole tilt, this could drag
the auroral oval and the subauroral regions to lower
(geographical) latitudes, where cosmic rays could
interact with a more humid troposphere, causing more
intense cloud condensation and therefore cooling: this is
how the internal geomagnetic field could in part control
galactic cosmic rays impinging on the troposphere, in the
cloud-effective energy range [54]. If archeomagnetic
jerks involve low degree non-dipole components of the
field, the geometry of the cosmic-ray/troposphere interaction can be more complex but the effects could be
similar to those due to a strongly inclined dipole. Resolving this would require far better global coverage of
archeointensity data than are presently available.
The third observation set is the correlation of decade
long magnetic trends with recent cooling and warming
episodes over much of the past century: Solanki [12]
and Le Mouël et al. [29] claim a good correlation
between magnetic field changes, solar irradiance and
global temperature from the late 19th century to the mid1980s. Causality in that case of course places the
primary agent in the Sun. Solar variability at these timescales affects in parallel the part of total solar irradiance
which interacts with the atmosphere, causing temperature changes, and the part of TSI which (in addition to
the solar wind) interacts with the magnetosphere and
ionosphere, generating the observed geomagnetic variations. Over this period, the “overall magnetic trend” is
therefore essentially at the same time the “ solar evolution (irradiance) trend” and the “global temperature
Fig. 6. Schematic representation of sources of forcing of climate change.
The roles of the Sun's and Earth's magnetic fields in modulating
incoming cosmic ray flux is emphasized. Adapted from Veizer [84].
V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339
trend”, but it is far easier to extract because of the
enhanced sensitivity discussed above.
Fig. 6 shows a simplified model of the relations
between the various forcing functions and envelopes in
which climate develops. Solar irradiance S(t) summarizes the time-varying input from the Sun over a wide
range of frequencies, and intimately implies the covariant solar magnetic field SMF(t). Cosmic-ray flux
CRF(t) is another time-varying forcing function of
extra-solar and solar origins. These are modulated by
both the Solar and Terrestrial magnetic fields SMF(t)
and EMF(t).
The resulting time-varying forcing function, related
to changes in solar photon and particle fluxes but also to
the variations of the magnetic fields of Sun and Earth,
acts on the Earth's fluid envelopes, being involved (with
varying degrees of certainty and of understanding of
causative mechanisms) in cloud nucleation, 14C and
10
Be generation, and changes in the water cycle (the
main greenhouse gas). This complex series of processes
finally result (among many other variables) in timevarying temperature T(t) at lower tropospheric levels,
considered as one aspect of “climate”. Of course, the
amount of CO2 in the atmosphere is also a time-function
with feedbacks on the climate system itself. The
problem is to determine how much of the climate
change signal stems from pCO2 changes.
The observed correlation between temperature and
magnetism fails after the mid-1980s, when solar
irradiance and magnetic activity drop, whereas temperature continues an accelerated rise [12]. This is when
anthropogenically-induced global warming might first
become apparent. Having lost the “Sun–Magnetism–
Climate connection”, which seems to have prevailed
over geological until very recent times, may be a
worrying loss…
8. Conclusion
In conclusion, correlations between magnetic variations and climate may be more significant than
previously realized. We see that no forcing factor, be it
changes in CO2 concentration in the atmosphere or
changes in cosmic ray flux modulated by solar activity
and geomagnetism, or possibly other factors, can at
present be neglected or shown to be the overwhelming
single driver of climate change in the past century. Most
of the time, the prime, joint forcing factor is in solar
variations (at the decadal time scale) or orbital forcing
(at the Milankovic scale). The Sun is clearly a significant driver of changes not only in climate but in the
overall behavior of the ionosphere and magnetosphere,
337
and external geomagnetic field; this modulates incoming fluxes of cosmic rays which are increasingly recognized as potential drivers of changes in cloud cover
and albedo. The work of Le Mouël et al. [29], based on
very sensitive yet robust magnetic indices, shows that
this situation may have prevailed until the mid-1980s.
At longer time scales, we have seen that changes in the
internal geomagnetic field itself might somewhat
unexpectedly trigger significant climate change: archeomagnetic jerks may be the only evidence that changes
in the internal magnetic field itself can at times have a
significant influence on climate, possibly through the
cosmic-ray/low-cloud connection at times of extremal
tilt of the dipole. Although still in need of confirmation,
their detection is therefore particularly exciting: Gallet
et al. [64] have recently underlined a potential connection between these geomagnetic events and some
major societal changes in the Middle East through
climatically driven environmental fluctuations (Fig. 5).
A correlation at the longer time scales of Milankovic
cycles remains very speculative at this time.
Acknowledgements
We thank five anonymous reviewers for helping us to
improve our manuscript. IPGP Contribution NS 2175.
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