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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 329 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 330 V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339 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 331 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 332 V. Courtillot et al. / Earth and Planetary Science Letters 253 (2007) 328–339 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 334 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. 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