tters 253 (2007) 328–339www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Are there connections between the Earth's magneticfield 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

Editor: R.D. van der Hilst

20 December 2006

Abstract

Understanding climate change is an active topic of research. Much of the observed increase in global surface temperature over thepast 150 years occurred prior to the 1940s and after the 1980s. The main causes invoked are solar variability, changes in atmosphericgreenhouse gas content or sulfur due to natural or anthropogenic action, or internal variability of the coupled ocean–atmospheresystem. Magnetism has seldom been invoked, and evidence for connections between climate and magnetic field variations havereceived little attention. We review evidence for correlations which could suggest such (causal or non-causal) connections at varioustime 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, eitherdirectly or through changes in ice distribution and moments of inertia of the Earth, is still tenuous. Correlation between decadalchanges in amplitude of geomagnetic variations of external origin, solar irradiance and global temperature is stronger. It suggests thatsolar irradiance could have been a major forcing function of climate until the mid-1980s, when “anomalous” warming becomesapparent. 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 fewmillennia, using the archeological record from Europe to theMiddle East. These seem to correlate with significant climatic events in the eastern North Atlantic region. A proposed mechanisminvolves variations in the geometry of the geomagnetic field (f.i. tilt of the dipole to lower latitudes), resulting in enhanced cosmic-rayinduced nucleation of clouds. No forcing factor, be it changes in CO2 concentration in the atmosphere or changes in cosmic ray fluxmodulated by solar activity and geomagnetism, or possibly other factors, can at present be neglected or shown to be the overwhelmingsingle driver of climate change in past centuries. Intensive data acquisition is required to further probe indications that the Earth's andSun'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

⁎ Corresponding author.E-mail address: courtil@ipgp.jussieu.fr (V. Courtillot).

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.10.032

1. Introduction

Paleomagnetic and climate research have a longstand-ing association. Recovering the fossil memory of the

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magnetic field in ancient rocks has allowed retracing theevolution of the main tectonic plates, placing them in theproper paleogeographical context. Joint use of paleo-geographic reconstructions (provided by paleomagne-tists and marine geoscientists), paleo-sea levelreconstructions (provided by stratigraphers and sedi-mentologists) and paleo-topographic reconstructions oforogens (provided by structural geologists), togetherwith global climate modeling (GCM) has illuminated anumber of issues: e.g. the climate of Pangea in thePermo–Triassic or the evolution of the monsoon as theIndia–Asia collision progressed, the Paratethys Seadisappeared and the Tibetan plateau rose [1–3]. Themeasurement of certain rock magnetic properties downwell-dated stratigraphic cores (where the dating itself isoften provided by paleomagnetism through the geomag-netic polarity time-scale) has produced a number ofclimate-related indicators. For instance, the direction ofthe principal axes of the magnetic susceptibility tensorcan allow one to recover past wind or current directions[4,5]. But in all these instances, paleo- and rock mag-netism provide tracers but do not imply any causalconnection between climate and the geomagnetic field.

In this paper, we wish to summarize a number ofrecent studies which have identified potential correla-tions between the two over a range of time scales fromdecades to hundreds of thousands of years. We firstbriefly set the stage by recalling the current state ofunderstanding of the main agents forcing climate overthese time scales. We then discuss evidence forcorrelations between magnetic and climate variationsover the 10–100 yr, 103–104 yr and 105–106 yr timescales. Whenever a correlation is suggested, we discusswhether a causal connection can be invoked and inwhich direction it might operate. This paper offers an“outside” perspective by a team of scientists mainlyinvolved in geo-, archeo- and paleomagnetism, hopingto underline intriguing observations and to establish newlinks with the community of climate research.

2. What are the main contenders for drivingclimate change?

The equilibrium temperature close to the surface ofthe Earth is determined primarily by electromagneticradiations from the Sun, covering a broad range ofwavelengths, which presently amount to some 342 Wm−2 at the top of the atmosphere [6]. The amounts ofenergy reflected from the top of clouds, aerosols andatmosphere (∼77 W m−2) and from the Earth's surface(∼30Wm−2) define the Earth's albedo. The atmosphereabsorbs 67 W m−2 and 168 W m−2 reach the Earth's

surface, where they are also absorbed. Based on Stefan'slaw, one can estimate that the resulting present-dayequilibrium temperature of the Earth should be on theorder of −18 °C. However, 390 W m−2 are re-emitted atIR wavelengths by the Earth's surface towards theatmosphere, of which 155Wm−2 are again emitted backtowards the Earth's surface due to the presence of green-house gases (GHG), raising the equilibrium temperatureto ∼+15 °C, and making, in particular, the existence ofliquid water and life possible. The main greenhouse gasis water vapor (H2O), accompanied by CO2, CH4, andother more minor constituents. The generally acceptedview is that changes in total solar irradiance (TSI= thetotal amount of energy coming from the Sun at allwavelengths), in atmospheric water vapor and carbondioxide content (and also aerosols emitted by volcanoes,notably SO2, see below) are the main potential agents ofclimate change at various timescales. Defining theappropriate observables that should be used to describeclimate is not so straightforward. Among many indica-tors, the mean global temperature of the Earth's surfaceis generally favored. However, its definition is not asclear-cut as may seem, and measuring it properly is adaunting task, particularly as one moves further back intime [7]. Uncertainties are much larger in the southernthan 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 thepast 100 yr or so include warming from the end of the19th century to the early 1940s, followed by little changeor even cooling until ∼1970, and then warming sincethen, the trend becoming steeper after the mid-1980s.Warming before 1950 and after 1980 is generallybelieved to be due to increases in the concentration ofgreenhouse gases, lack of volcanic activity, enhancedsolar irradiance and internal variability of the coupledatmosphere–ocean system [8]. Cooling from 1940 to1970 is often disregarded as being part of the noise, orvariability. Exponential rise of GHGs due to humanactivity in the past 150 yr is well documented. But it isinteresting to note also that the last 60 yr are a period ofunusually high solar activity (possibly unique in the past8000 yr: [9–11]). Solanki [12] and Foukal [13] find agood correlation between solar irradiance and globaltemperature until at least 1980 (see also [14]). Scafettaand West [15] calculate, based on an empirical modelwith four timescale-dependant climate sensitivities tosolar variation, that ∼75% of the 1900–1980 globalwarming has a solar origin, whereas the figure drops to∼30% for the period 1980–2000. If only tomakemattersa bit more complex, peaking of global sulfur emissions inthe 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 istherefore understandable that determining the relativecontributions of CO2 and solar irradiance (and otherpotential 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 revealingquotes from a recent review by Bard and Frank [20]:“Until the beginning of the 1980s, the relation betweenthe Sun and climate change were still viewed withsuspicion by the wider climate community”; “…thematter remains controversial because most of theserecords are influenced by other factors in addition tosolar activity. Moreover, we still lack a fundamentalunderstanding of all causal relationships between solaractivity and climate”.

Being able to determine the respective influences ofthe main candidates as forcing factors for earlier periodsdepends in large part on the (paleo) climatologists' abilityto actually evaluate indicators depending on each factorover a broad range of time scales. This has been donefor pre-instrumental times by using proxies (e.g. [21]):for global temperature one can use changes in oxygenisotopic composition of rocks, ice or fossils (noted δ18O);for solar irradiance, concentrations in isotopes such as10Be and 14C which form under the influence of solar andgalactic cosmic rays (see discussions f.i. in [14], and in[20]); and for the concentration in carbon dioxide, thepartial pressure pCO2 in air bubbles trapped in ice (e.g.[22]). As an example, Fig. 1 shows the evolution over the

Fig. 1. The δ18O record of a stalagmite from the Spannagel cave in the ceproduction rate (Δ14C) (full line with reversed scale), a proxy for solar irradiaSolanki et al. [12]). CO2 concentration— from ice cores and instrumental mOptimum and LIA stands for Little Ice Age. Figure from Veizer [18].

past two millennia of (1) a proxy for temperature (δ18Ofrom a stalagmite in the central Alps), (2) a proxy forsolar irradiance (Δ14C) (see [20]) and (3) CO2 concen-tration from ice cores complemented with instrumentalmeasurements for the recent decades ([18]; data from[23,24,17]): until∼1900, solar irradiance appears to havebeen the prime forcing factor, at a time when pCO2 didnot change much yet.

Crowley [25] has attempted to unravel the causes ofclimate change over the past 1000 yr by comparing thesimulated global surface temperature induced bydifferent forcing factors with the evolution of “actual”temperature reconstructed from proxies and instrumen-tal data. More precisely, Crowley [25] calculates theindividual temperature responses to changes in solarvariability, CO2 and volcanism using a linear upwelling/diffusion energy balance model (this EBM “calculatesthe temperature of a vertically averaged mixed-layerocean/atmosphere as a function of forcing changes andradiative damping”; see [25], p. 272). Volcanic forcingdisplays random-like spikes of short duration (∼a year),up to 20 W m−2 in amplitude. Solar variability results inforcing with decadal to millennial fluctuations with anamplitude ∼1–2 W m−2. The range for CO2, whichbecomes significant mainly after 1800, is ∼2 W m−2.Fig. 2 from Crowley [25] displays the resulting in-dividual temperature responses. Crowley [25] concludesthat as much as 41 to 64% of pre-anthropogenic (pre-1850) decadal scale temperature variations were due to

ntral Alps (dashed line) covering the last 2000 yr, compared to 14Cnce ([23]; see also proxies for sunspot numbers and reconstructions ineasurements from [24] and [17]. MCO is the warm Medieval Climate

Fig. 2. Response of a linear upwelling/diffusion energy balance model(EBM) to different forcings, calculated at a sensitivity of 2.0 °C for adoubling of CO2. Temperature variations in °C. Forcings are (1)Volcanic; (2) Solar, based on a reconstruction of solar variability byBard et al. [26]; (3) Greenhouse gases; and (4) Tropospheric aerosols(figure adapted from Crowley [25]).

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changes in solar irradiance and volcanism. Changescaused by CO2 and volcanism are responsible for thesimulated 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 in-strumental data from 1850 to 1950, when data indicatemuch cooler temperatures (see Fig. 4 in [25]). Anothertemperature reconstruction by Zorita et al. [27] for thelast 500 yr using a forced AOGCM (i.e. withintroduction of atmosphere–ocean coupling) also failsto reproduce accurately changes in trends of 20thcentury (northern hemisphere) temperature (amplitudesalso differ by a factor up to 3). Scafetta and West [28]find that “the amplitude of the 11-year solar signature onthe temperature record seems to be up to 3 times largerthan in the theoretical predictions”.

3. Correlations between magnetism and climate atthe 10–100 yr scales

Now, does geomagnetism have relevant data orevidence to contribute? Le Mouël et al. [29] recentlyproposed to introduce some simple, non-linear measures(or indices) of high frequency variations in thegeomagnetic field, linked to external currents in theionosphere and magnetosphere forced by the solar windand electro-magnetic radiations. These indices aredefined as the range (maximum minus minimum) ofhourly mean values of each component of thegeomagnetic vector taken over one day. An alternateindex, actually yielding the same results, is the sum ofthe squares of differences in successive hourly valuestaken over one day. A complete time series of these

indices can be reconstructed over most of the pastcentury in a few observatories. The resulting time seriesare low-pass filtered, first on an annual basis, clearlyrevealing the ∼11 yr solar cycle and its harmonics, thenwith an 11-yr filter to reveal smaller-amplitude longer-term trends [29]. The main result of this study is that alargely 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 normalizedway, with the mean value over the entire time interval ofdefinition removed, and then divided by the root-meansquare amplitude over the interval). The trend rises from1910 to 1955, decreases until 1968, rises again to 1988and has been decreasing since then. The turn points arerather sharp. The “overall magnetic trend” correlateswell with the evolution of solar irradiance (as recon-structed by [12,13]). It also correlates with the magneticaa index (e.g. [31]) and the Wolf index, i.e. the numberof sunspots (see also [32]). The aa magnetic index isconstructed in such a way as to measure the irregularmagnetic variations, after carefully removing the regulardaily variation SR [31]. The SR variation is attributed tothe atmospheric dynamo in the E layer of theionosphere, which is ionized by the UV-X radiationsfrom the Sun. On the other hand, the irregular variationscomprise a number of different components, includingthe solar wind and its changes among their primarysources. Our magnetic indices do not sort or separatethese variations; but when we retain only the fivequietest days of each month (a way to isolate thecontribution of SR), all essential features of the curvesremain the same [29].

We view the fact that the long-term “overall magnetictrend” is essentially common to all these indices (aa, W,full new indices regardless of which component orwhich observatory is used, indices reduced to fivequietest days of each month) as evidence that the entiresystem of ionospheric and magnetospheric currents,despite all their complexity, pulses in rough unison withthe Sun on a decadal scale (and that this also applies tothe 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 thatmagnetic variations revealed by the new indices, whichreflect precisely the changes in the UV-X component ofthe irradiance, are a far more sensitive indicator (byseveral orders of magnitude!) than the total solarirradiance which varies by only ∼1‰.

If solar activity is correlated to climate over much ofhistorical times, it might be expected that the “overallmagnetic trend” would correlate with the recent

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evolution of global temperature, and this is indeed thecase up to the mid-1980s, but not since then (Fig. 3). Ofcourse, the relation is this case does not imply a causallink from Earth's magnetism towards climate, but fromthe Sun to both climate and magnetic changes. Clearly,100 yr is not enough to ascertain that such a correlationis robust, but it is as impressive as many of the cor-relations of time series proposed over this time range.

Le Mouël et al. [29] note that global temperaturedeparts from all other indicators (solar and magnetic) inthe late 1980s and suggest that this is when the signal(possibly corresponding to anthropogenic warming?)starts emerging from noise. This is in agreement with theestimates of Scafetta and West [15], in which the solarcontribution drops by a factor 2 (or more) after 1980.Similar results are found when temperature evolution iscompared to that of solar cycle length or cosmic ray flux[33–35]. Note that the leveling or drop in temperaturefrom ∼1940 to ∼1970 matches solar and magneticseries, and not the monotonous accelerated rise in CO2.And the period from ∼1850 to ∼1950 is the one overwhich the modeling results of Crowley [25] are wellabove the data, with significant discrepancy between∼1880 and ∼1910, and a serious deviation in thedecades around 1970. There are therefore good indica-tions of a significant contribution from solar irradianceto climate change over at least the first 3/4 of the 20thcentury, with the anthropogenic CO2 contribution

Fig. 3. Time evolution over the 20th century of the eleven-year running averagEskdalemuir and Sitka observatories (ESK and SIT) compared to solar irradiet al. [29]). Magnetic indices are from LeMouël et al. [29] and their definitionfrom Solanki [12]. All curves have had their mean over the time interval ofamplitude over this interval for normalization. The vertical axis is therefore

possibly becoming significant only after the mid-1980s [12], although the origin of this “anomaloustemperature” cannot be considered as demonstrated.

4. On cosmic rays, clouds and climate

Bard and Frank [20] conclude in their recent reviewthat “for the moment, the exact mechanisms by whichcosmic radiation and solar forcing may affect cloudformation remain very poorly understood and clearlyrequire future research efforts”. This rather pessimisticview seems to be changing quickly. Three mechanismsare thought to link solar variability with climate (e.g.[36]): (1) changes in solar irradiance leading to changesin heat input to the lower atmosphere; (2) solarultraviolet radiation coupled to changes in ozoneconcentration heating the stratosphere; and (3) galacticcosmic rays (the impact of GCR on climate was firstproposed by Ney [37]). These are modulated by long-term solar magnetic activity [38], by changes of thesource of GCRs [39] as well as by changes of the Earth'smagnetic field. Cosmic rays could in turn act on climatein three ways [36]: (1) through changes in theconcentration of cloud condensation nuclei; (2) thun-derstorm electrification; and (3) ice formation incyclones. A correlation between cosmic ray flux andcloud cover was first noted by Svensmark and Friis-Christensen [40] over one solar cycle, and linked to

es of magnetic indices based on modulus of the geomagnetic field at theance S(t) and global mean temperature T globe (figure from Le Mouëlis recalled in the text. Temperature is from Jones et al. [30]. Irradiance isdefinition removed and have been divided by their root-mean squaredimensionless and directly comparable.

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low-altitude clouds by Marsh and Svensmark [41]:cosmic ray flux varies by 15% on average (and up to50% at the poles) over this time period. The Earth'smagnetic field acts as a more or less efficient (time-varying) shield on these high-energy charged particles.The relation between cosmic rays and clouds shows ageographical pattern, with areas of highly significantcorrelation and almost no correlation in other areas([42]; see below). Higher cosmic ray flux would lead tomore low clouds and thus higher albedo and lowerEarth surface temperatures. The cosmic ray variationover one solar cycle translates as a change of energyinput to the atmosphere on the order of 1.5 W m−2,which is not negligible compared for instance to theestimated radiative forcing from anthropogenic CO2

emissions (∼2 W m−2). And cosmic ray intensity hasvaried (in the past millennia) by as much as a factor of 4compared to recent solar cycles [36]. However, theGCR flux–cloud correlation has been criticized. Sunand Bradley [43] did not find any evidence at a globalscale over the longer time period from 1950 to 1995.Laut [44] noted inconsistencies in Svensmark andFriis-Christiansen's [40] paper and considered thatthey largely overestimated the relation between solaractivity and Earth's climate (but see also [45]). Thedifficulty in obtaining uncontroversial evidence for aGCR flux–cloud correlation over several decadescould be due either to difficulties in intercomparingship observations with satellite data, or to an overprintresulting from other mechanisms, such as ENSO-likeatmosphere–ocean coupling [45].

Despite these criticisms, the GCR–climate relation isnow better accepted because physico-chemical mechan-isms are emerging [36]. The energy input to theatmospheric system from cosmic rays is only a billionthof solar irradiance, yet it may have profound effects onseveral atmospheric processes. Carslaw et al. [36] havereviewed the cloud properties which are influenced by“microphysical processes” and discuss two ways inwhich cosmic rays may affect cloud droplet number: theion-aerosol “clear-air” and “near-cloud” mechanisms.The clear air mechanism relies on the production ofultrafine sulfate aerosols from ions, which act as cloudcondensation nuclei. Atmospheric measurements sup-port such a mechanism [46,45]. The near cloudmechanism relies on the production of ice nuclei inthe vicinity of clouds (even if these are not thunderstormclouds) induced by the perturbation in the globalatmospheric electrical circuit due to the ionization ofthe atmosphere by the GCR flux [47,48,45]. Theamplitudes of these effects are still uncertain and atthe frontier of cloud physics research. Carslaw et al. [36]

conclude “It will be difficult to separate solar andcosmic ray effects (…) Geomagnetic field variationscould in principle untangle this ambiguity because theyaffect cosmic rays but not solar irradiance, but thesevariations occur on much longer time scales than thesolar variations”. We will see below that this may notalways be the case when one turns to the record of thepast millennia, over which the internal geomagneticfield has varied significantly on time scales even shorterthan some solar time-scales…

Also, we now have more robust direct and indirectevidence of a GCR–climate relation. Very recently,Vieira and da Silva [49] argue that in the southernPacific Ocean variations in cloud cover are related to thepresence of the Southern Hemisphere magnetic anom-aly, and that the causal mechanism involves strongercosmic ray/cloud interaction in the lower field region.Even more recently, Svensmark et al. [50] report on alaboratory experiment where a gas mixture attemptingto represent the chemical composition of the loweratmosphere was subjected to UV light and cosmic rays.These authors find that released electrons promote fastand efficient formation of the building blocks for cloudcondensation nuclei. Moreover, GCR are well correlatedwith continuous satellite data measuring low cloudcover over much of this period [45]. Using a globalnumerical model of ion production, Usoskin et al. [42]calculated the expected distribution of correlationbetween cosmic ray ionization and low-cloud amountfor the years 1984–2000. A significant correlation,higher than 90%, was simulated, mainly over oceans (inparticular above the North Atlantic). Conversely,correlation is zero at low latitudes, as expected.

In order to check the GCR-climate hypothesis, com-parisons have been made over a longer time interval. Forinstance, Wagner et al. [51] have compared the 10Be and36Cl records (GCR proxies) with a climate proxy record(δ18O) between 20 and 50 kyr BP. But because pro-duction of cosmonuclides is in part controlled by thestrength of the geomagnetic field (e.g. [52]) and becausethe geomagnetic field can vary quite rapidly and in an asyet insufficiently constrained way [53], it may not bepossible to use these cosmonuclides in a robust way asproxies of climate on a time scales from 103 to 105 years(Bard and Frank, [20]: “…with the currently availablereconstructions of field intensity and cosmogenic nuclideproduction over the past 200 kyr, it is not possible toextract a solar component with the precision required todraw meaningful conclusions.”). However, based on avery careful analysis of correlations between solaractivity, cosmic rays and Earth's temperature over thelast millennium, Usoskin et al. [14] find that “periods of

Fig. 4. Geomagnetic field intensity variations in Western Europeduring the past 1300 yr determined from archeomagnetic analyses.Vertical and horizontal error bars correspond to standard deviations ofintensity means and age brackets of dated sites, respectively. Thegeomagnetic field intensity variations deduced from geomagnetic fieldmodels from 1850 onwards are indicated by small crosses. Climaticvariations during the past millennium are deduced from retreats andadvances of Alpine glaciers. Cooling periods are indicated by shadedbands. Figure from Gallet et al. [61].

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higher solar activity and lower cosmic ray flux tend to beassociated with warmer climate, and vice-versa”. Theyalso show that cosmic ray flux correlates with temper-ature better than with sunspot numbers and that a positivecorrelation between geomagnetic dipole moment andtemperature further supports the role of cosmic rays.

Important changes have come from evolution ofmagnetospheric configuration: as geomagnetic activity(of external origin) increases, the auroral oval expandsequatorward and the subauroral region moves to lowergeomagnetic latitudes. Feynman and Ruzmaikin [54]have shown that, from 1890 to 1985, the area of theauroral oval has evolved in parallel with the aa index andEarth's global temperature. The trend follows the “overallmagnetic trend” which Le Mouël et al. [29] haveidentified in all magnetic observatories with long enoughrecords, and again the correlation fails only after ∼1985.

So far, observed correlations between Earth's climateand geomagnetism have involved mostly that part ofmagnetic changes which is controlled by external(solar), not internal (core) variations. Yet, moving onto longer time scales may have uncovered links betweenthe internal field and climate.

5. Moving on to the archeomagnetic time scale(103–104 yr)…

Based on newly acquired archeo-intensity data fromarcheological and historically dated material recoveredfrom western Europe and the Middle-East, Genevey andGallet [55] and Gallet et al. [56] observed rather sharpmaxima in time variations of the intensity of the ancientfield, associated with sharp curvature changes indirection. These features, which are stronger and fasterthan previously realized [57,58,53] have been called“archeomagnetic jerks” (Fig. 4). The name may be a bitmisleading, as these are rapid, ∼100-yr long increasesin field intensity by 15–30%. But the idea behind theterm was to note previously unrecognized sharp featuresat a time-scale intermediate between geomagnetic jerks(1–2 yr in a centennial time series) and excursions orreversals (103–104 yr in 105–106 yr time series). Notethat these archeomagnetic jerks are being observed overa widening area, first centered in Europe and thenextending to the Middle East. Similar observations arementioned by Snowball and Sandgren [59] in NorthernEurope and Stoner et al. [60] in Northern Canada. Theglobal extent and time coincidence of some of thesefeatures 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

correspond to previously unrecognized abrupt geomag-netic features of internal origin with rather low sphericalharmonic degree.

Archeomagnetic jerks are found around 1400, 800,200AD and 800 BC. Subsequent work [61] has added anevent around 1600, with other less robust possibilities at1800, 600 AD and 350 BC. This implies 4 rather clearevents in the past two millennia (i.e. a “repeat time” onthe order of 500 yr, a time-constant characteristic ofsecular variation of the equatorial dipole field — e.g.[57]) if only the more robust events are included. Thenumber goes to 7 if all suggested events are included,implying a time constant of 200–300 yr, not verydifferent from the duration of the events themselves andmore characteristic of non-dipole secular variation(though see [53]). In this paper, we restrict ourselves tothe better-identified events, which remain short andrather rare on the time scales considered.

Gallet et al. [61] have compared the occurrences ofarcheomagnetic jerks with paleo-climate indicators,

Fig. 5. Geomagnetic field intensity variations in Mesopotamia duringthe four millennia BC determined from archeomagnetic analyses,compared to climate change determined in the North Atlantic fromBond et al. [65]. On top are indicated the main societal changes in theMiddle East. Figure from Gallet et al. [64].

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such as advances and retreats of glaciers in the SwissAlps [62,63], which are good indicators of climatechange in Europe, at least during the past few millennia,when there are data. There is a good correspondencebetween jerk times (particularly their rising period) andadvances in glaciers. The correlation between coldevents 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 BCin 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 coincidein time with cooling periods detected in the NorthAtlantic from ice-rafted debris [65].

6. …and to the paleomagnetic time scale (excursionsand reversals)?

As one attempts to go back further in the past, databecome scarce and interpretations more speculative.Fuller [66] argues that the strongest minima in paleoin-tensitywithin the Brunhes chron (i.e. the past∼800 ka; cf.the paleointensity stack of Guyodo and Valet [67])correlate with obliquity minima, and that reversalspreferentially occur when the average amplitude of the

obliquity signal is low. This interpretation is at odds withthe analysis of Kent and Carlut [68], who find nodiscernible obliquity modulation (41 ka) of the geomag-netic field in sedimentary paleointensity records (neitherthose of reversals in the past 5.5 Ma nor those ofexcursions in the past 800 ka). Other authors claim that anumber of paleomagnetic field excursions occurring inthe past 800 ka correlate with minima in precession andwarmer interglacial episodes, hence with climate changeas seen in the series of δ18O measurements. Carcailletet al. [69,70] find that over the past ∼1 Ma geomagneticdipole moment variations exert the main control on 10Beproduction variations. They also find a correlation (lesssignificant but intriguing) between these records and δ18Ovariations measured on benthic foraminifera, whichwould indicate that geomagnetic excursions occurpreferentially during interglacial/glacial transitions ordeglaciation events. A similar claim has recently beenput forward by Acton [71]. Some authors also find aspectral peak at∼100 ka period in both the 10Be/9Be ratioand paleointensity and paleoinclination variations[70,72,73]. A possible connection between Earth'smagnetic field and Milankovic orbital cycles is thereforepresently hotly debated. In this debate, a critical aspectconcerns the fact that part of the observed correlationsbetween magnetic intensity and climate could be due tochanging carbonate content of the sediments, itselfcontrolled by climate fluctuations. As a consequence,current opinions range from meaningful correlation toartefact and lack of significance (e.g. [74,67,75]).

7. Are there possible mechanisms?

We acknowledge the fact that the correlations we haveoutlined between Earth's mean temperature and varia-tions in the geomagnetic field are tentative, and becomebetter grounded in observations as one moves from thedistant past to the present. And of course, a correlationdoes not in itself suffice to demonstrate a causalconnection. Nevertheless, we feel the community shouldbe 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 isconcerned, there can be no doubt on the sense of thecausal relationship. Orbital changes are the main causeof temperature and CO2 variations in the atmosphere.Could they also generate the observedmagnetic changes,through direct destabilization of convection due to thevery tiny and regular changes in the distribution of forcesacting on the fluid core? This is reminiscent of forcingof the geodynamo at precessional frequencies [76].

Fig. 6. Schematic representation of sources of forcing of climate change.The roles of the Sun's and Earth's magnetic fields in modulatingincoming cosmic ray flux is emphasized. Adapted from Veizer [84].

336 V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339

Precession was abandoned for decades as a potentialmechanism to drive a dynamo, but Tilgner [77] hasrecently found that precession-driven dynamos can existat magnetic Reynolds numbers characteristic of theEarth's core. Another possibility invokes an indirectmechanism [78], in which magnetic instabilities wouldbe driven by variations in polar ice caps, leading(through loading and unloading) to variations in theEarth's rotation speed and then to changes in core flow.This mechanism is highly speculative and not yet borneout by modeling or dynamo theory.

Then comes the correlation of archeomagnetic jerkswith cold episodes (at least in western Europe). Intensitychanges may either reflect changes in axial dipolestrength, changes in dipole tilt, or else changes in loworder components of the non-dipole field. New dataseem to confirm the existence of several “archeomag-netic jerks” [59,60]. There is ongoing debate as to thegeographical extent of the events, for which the mostreliable data come from a large but not global area:Gallet et al. [56,61,64] argue for a minimum extentranging from the Eastern Atlantic to Central Asia,implying low degree and order features, whereasGomez-Paccard et al. [79] do not find significantgeomagnetic features in the (far lower quality) datafrom Japan and the South-western USA. The resolutionin 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 ofcosmic rays and ensuing modifications in low-cloudcover), one would expect paleointensity maxima tocorrelate with maxima in shielding, hence minima incosmic rays reaching low clouds, hence lesser cloudcover and albedo and higher temperatures, the oppositeof what is apparently observed at millennial andmultimillenial scales. Biblical accounts of exceptionalluminosity events in the first millennium BC have beeninterpreted by Siscoe et al. [80] as showing that coronalauroras can occur at low latitudes under the hypothesis ofa much reinforced geomagnetic dipole intensity. On thecontrary, Raspopov et al. [81] and Dergachev et al. [82]suggested that these luminosity events could be linkedwith a “Sterno-Etrussia” excursion, implying very lowdipole field intensity (which is not observed in archeo-magnetic data of Genevey et al. [83]). But Gallet et al.[56,61,64] have noted that if archeomagnetic jerkscorrespond to extrema in dipole tilt, this could dragthe auroral oval and the subauroral regions to lower(geographical) latitudes, where cosmic rays couldinteract with a more humid troposphere, causing moreintense cloud condensation and therefore cooling: this is

how the internal geomagnetic field could in part controlgalactic cosmic rays impinging on the troposphere, in thecloud-effective energy range [54]. If archeomagneticjerks involve low degree non-dipole components of thefield, the geometry of the cosmic-ray/troposphere inter-action can be more complex but the effects could besimilar to those due to a strongly inclined dipole. Re-solving this would require far better global coverage ofarcheointensity data than are presently available.

The third observation set is the correlation of decadelong magnetic trends with recent cooling and warmingepisodes over much of the past century: Solanki [12]and Le Mouël et al. [29] claim a good correlationbetween magnetic field changes, solar irradiance andglobal temperature from the late 19th century to the mid-1980s. Causality in that case of course places theprimary agent in the Sun. Solar variability at these time-scales affects in parallel the part of total solar irradiancewhich interacts with the atmosphere, causing tempera-ture changes, and the part of TSI which (in addition tothe solar wind) interacts with the magnetosphere andionosphere, generating the observed geomagnetic varia-tions. Over this period, the “overall magnetic trend” istherefore essentially at the same time the “ solar evo-lution (irradiance) trend” and the “global temperature

337V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339

trend”, but it is far easier to extract because of theenhanced sensitivity discussed above.

Fig. 6 shows a simplified model of the relationsbetween the various forcing functions and envelopes inwhich climate develops. Solar irradiance S(t) sum-marizes the time-varying input from the Sun over a widerange of frequencies, and intimately implies the co-variant solar magnetic field SMF(t). Cosmic-ray fluxCRF(t) is another time-varying forcing function ofextra-solar and solar origins. These are modulated byboth the Solar and Terrestrial magnetic fields SMF(t)and EMF(t).

The resulting time-varying forcing function, relatedto changes in solar photon and particle fluxes but also tothe variations of the magnetic fields of Sun and Earth,acts on the Earth's fluid envelopes, being involved (withvarying degrees of certainty and of understanding ofcausative mechanisms) in cloud nucleation, 14C and10Be generation, and changes in the water cycle (themain greenhouse gas). This complex series of processesfinally result (among many other variables) in time-varying temperature T(t) at lower tropospheric levels,considered as one aspect of “climate”. Of course, theamount of CO2 in the atmosphere is also a time-functionwith feedbacks on the climate system itself. Theproblem is to determine how much of the climatechange signal stems from pCO2 changes.

The observed correlation between temperature andmagnetism fails after the mid-1980s, when solarirradiance and magnetic activity drop, whereas temper-ature continues an accelerated rise [12]. This is whenanthropogenically-induced global warming might firstbecome apparent. Having lost the “Sun–Magnetism–Climate connection”, which seems to have prevailedover geological until very recent times, may be aworrying loss…

8. Conclusion

In conclusion, correlations between magnetic varia-tions and climate may be more significant thanpreviously realized. We see that no forcing factor, be itchanges in CO2 concentration in the atmosphere orchanges in cosmic ray flux modulated by solar activityand geomagnetism, or possibly other factors, can atpresent be neglected or shown to be the overwhelmingsingle driver of climate change in the past century. Mostof the time, the prime, joint forcing factor is in solarvariations (at the decadal time scale) or orbital forcing(at the Milankovic scale). The Sun is clearly a signif-icant driver of changes not only in climate but in theoverall behavior of the ionosphere and magnetosphere,

and external geomagnetic field; this modulates incom-ing fluxes of cosmic rays which are increasingly re-cognized as potential drivers of changes in cloud coverand albedo. The work of Le Mouël et al. [29], based onvery sensitive yet robust magnetic indices, shows thatthis situation may have prevailed until the mid-1980s.At longer time scales, we have seen that changes in theinternal geomagnetic field itself might somewhatunexpectedly trigger significant climate change: arche-omagnetic jerks may be the only evidence that changesin the internal magnetic field itself can at times have asignificant influence on climate, possibly through thecosmic-ray/low-cloud connection at times of extremaltilt of the dipole. Although still in need of confirmation,their detection is therefore particularly exciting: Galletet al. [64] have recently underlined a potential con-nection between these geomagnetic events and somemajor societal changes in the Middle East throughclimatically driven environmental fluctuations (Fig. 5).A correlation at the longer time scales of Milankoviccycles remains very speculative at this time.

Acknowledgements

We thank five anonymous reviewers for helping us toimprove our manuscript. IPGP Contribution NS 2175.

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