[1] Correlations between geomagnetic‐field and climate parameters have been suggested repeatedly, but possible links are controversially discussed. Here we test if weak (Earth‐strength) magnetic fields can affect climatically relevant properties of seawater. We found the solubility of air in seawater to be by 15% lower under reduced magnetic‐field (20 μ T) compared to normal field conditions (50 μ T). The magnetic‐field effect on CO 2 solubility is twice as large, from which we surmise that geomagnetic field variations modulate the carbon exchange between atmosphere and ocean. A 1% reduction in magnetic dipole moment may release up to ten times more CO 2 from the surface ocean than is emitted by subaerial volcanism. This figure is dwarfed in front of anthropogenic CO 2 emissions.

2. Experiments 2.1. Magnetic Field Effects on Viscosity [8] Viscosities of seawater and of a 0.62 M (3.5%) NaCl solution were determined from fluorescence polarization [Marangoni, 1992], for which purpose the solutions were dyed with Rhodamine‐B (see Figures S1–S3 in the auxiliary material for setup and calibration). An electronically controlled pipette was periodically pumped up and down to induce a vertical flow in the solution (0–5 mm/s). The controlled application of a magnetic field (MF) was through Helmholtz coils, housed inside a permalloy box screening out the ambient field. When the Helmholtz coils were switched off, the residual field in the tank was about 1 μT. The effect of a MF on viscosity was investigated at T = 5 ± 0.1°C. After switching on a MF of 2 mT, the viscosity continuously increased. The effect is fully reversible when the MF is reduced to 1 μT again, with a decay time of less than 10 s (Figure S4). When the samples are agitated, the viscosity initially increases at much higher rates and reaches overall higher levels than without agitation. The increase of viscosity was 8–10% stronger for the 3.5% NaCl solution than for seawater. The viscosity enhancement due to a 2 mT field (relative to 1 μT) is equivalent to a temperature reduction of 0.7°C (placid) or 1.5°C (agitated). However, we were not able to quantify the viscosity enhancement due to an earth strength field (0.05 mT). Although the sign of the MF effect is the same, the signal‐to‐noise level here is too low to obtain decent estimates. [9] The MF effect on viscosity increases with decreasing temperature. Importantly, the viscosity effect vanished after degassing the salt solutions for 60 s under vacuum, and was re‐established by gassing. The MF effects on the viscosity of the solution when O 2 or N 2 were used instead of air were similar, but slightly stronger with CO 2 and virtually absent with argon, in distilled water, and in a glucose solution (3.5 weight %). The fact that the MF effect is similar among the molecular gases suggests that effects seen with air are not due to the paramagnetic susceptibility of O 2 , which is too small to explain the observed magnetic‐field effects in terms of magnetization effects. [10] The observed MF effect on viscosity obviously needs a combination of two components; namely, dissociated salts (electrolytes) and the presence of internal interfaces in the solution (due to air bubbles or particulate matter). Electrostatically, gas bubbles in water are similar to colloidal solutes. Both are coated by shells of ions and counterions, referred to as inner and outer Helmholtz layer, which together form a dielectric boundary layer (DBL). The potential that arises on removal of the outer Helmholtz layer, is called the zeta potential, a measure of the electric polarization. The zeta potential depends on pH and salt concentration [Creux et al., 2007], but also on viscosity according to the Smoluchowski‐Einstein equation. The observed changes in viscosity therefore are likely to represent a magnetic‐field effect on the zeta potential. 2.2. Magnetic Field Effects on Gas Exchange Between Air and Seawater [11] To measure gas concentration in sea‐water, we used an elastic light‐scattering technique [Vallée et al., 2005] with a monochromatic (400 nm) light source. Elastic light scattering is sensitive to the presence of gas bubbles, which have a different refractive index than the solution and thus act as scatterers. The intensity of the scattered light depends not only on the concentration of the gas phase in the solution, but also on bubble size. To minimize the obfuscating effects of a broad bubble‐size distribution on the scattering intensity, the solution was not agitated in this experiment. Under placid conditions, the bubble‐size distribution is narrow (bubble sizes well below 1 μm) and the bubbles are separated, whilst under turbulent conditions, bubbles are prone to interact and coalesce, which gives rise to a broad bubble‐size distribution. Thus, for the placid solution, the scattering intensity can be considered to reflect the concentration of roughly monodisperse gas bubbles in the solution. [12] The seawater was degassed before the experiment. Then the surface of the water container was allowed to exchange gas with the environment (ambient pressure) and the time course of the scatter intensity was recorded in 1 μT, 20 μT, and 50 μT, respectively. The solution was kept at 4°C. Each regassing experiment follows a similar time course, characterized by an initially linear increase in scatter intensity (gas concentration) up to 45 min, followed by an approach‐to‐saturation behaviour, with saturation achieved after about two hours (Figure 2a). The curves are normalized by the saturation value for the curve taken at 50 μT. Importantly, re‐gassing rate and saturation values increase with field strength. Additional experiments were conducted to determine the MF effect on CO 2 solubility. For ∼30% (v/v) CO 2 enriched air, a reduction in field strength from 20 μT to 1 μT has an even more pronounced effect on gas solubility compared to normal air with 380 ppm CO 2 (Figure 2b). For pure CO 2 , the extrapolated MF effect is twice as strong than for normal air, such that a 1% field change yields a 0.5% change in CO 2 solubility. Importantly, the magnetic‐field effect is absent in purified water. Figure 2 Open in figure viewer PowerPoint (a) Time course of air saturation of seawater at 1 μT, 20 μT, and 50 μT, respectively. Each curve presents the mean over five experiments each and their ±1σ standard deviation (error bars). All curves are normalized to the saturation level obtained at 50 μT. (b) As in Figure 2a, but now for air with 30% CO 2 . Averages of three experiments each, 100% is defined for normal air in 50 μT (see Figure 2a). [13] The time course of the gas‐solubility experiment represents the diffusion‐limited, molecular transfer of gas across the air‐solution interface. According to the classic models of air‐sea gas exchange [ Broecker and Peng, 1974 Liss and Slater, 1974 F of gas into the salt solution is driven by the concentration gradient Δc/d of gas across the stagnant film (thickness d), D is molecular diffusivity of gas in sea water, k = D/d is the transfer velocity (“piston velocity”), c a and c s are the gas concentrations in air and in seawater (below the stagnant film), respectively. For O 2 , N 2 , and CO 2 , D (at 4°C) is about 1 × 10−5 cm2/s [ Himmelblau, 1964 Broecker and Peng [1974] d values of 3 × 10−2 cm, resulting in k values of about 3 × 10−4 cm/s. The time course of the gas‐solubility experiment represents the diffusion‐limited, molecular transfer of gas across the air‐solution interface. According to the classic models of air‐sea gas exchange [], the main resistance to gas transport is due to the stagnant boundary layer, across which the exchanging gases transfer by molecular processes (diffusion) from the well‐mixed atmosphere to well‐mixed surface ocean and vice versa. The net fluxof gas into the salt solution is driven by the concentration gradient Δof gas across the stagnant film (thickness),whereis molecular diffusivity of gas in sea water,is the transfer velocity (“piston velocity”),andare the gas concentrations in air and in seawater (below the stagnant film), respectively. For O, N, and CO(at 4°C) is about 1 × 10cm/s []. Under low agitation,obtainedvalues of 3 × 10cm, resulting invalues of about 3 × 10cm/s. [14] With c a in t is time and L = V/A is the characteristic length, with V and A denoting the volume (1 cm3) and the surface area (1 cm2), respectively, of the liquid container. Fitting the measured time course to L/k ∼ 30 min, that is, k values of about 5 × 10−4 cm/s, in good agreement with literature data. Within equation (1) kept constant during the experiment, the integration of equation (1) yieldswhereis time andis the characteristic length, withanddenoting the volume (1 cm) and the surface area (1 cm), respectively, of the liquid container. Fitting the measured time course to equation (2) , we obtain experimental time constants of∼ 30 min, that is,values of about 5 × 10cm/s, in good agreement with literature data.

3. Discussion [15] The MF effect on viscosity and gas solubility requires the presence of dielectric boundary layers (DBL), because MF effects are absent when destilled water (or a glucose solution) is used instead of seawater. The MF effects therefore are expected to occur in the ocean as well, given the abundance of DBL in the surface ocean, be it due to submicrometre particles (108–109 particles per ml) [Wells and Goldberg, 1991], be it due to sub‐100 nm colloidal particles (108–109 particles per ml) [Isao et al., 1990]). A conservative estimation of all DBL‐generating matter yields a total surface of tens of m2 per m3 seawater. [16] The MF effect on gas‐solubility in seawater does not seem to be directly related to the MF effect on viscosity, since the latter is much less pronounced. This is also suggested by the fact that the time scale of gas uptake in our experiments shows no discernible difference between normal and reduced MF. The single‐most important difference between gas uptake under normal and reduced MF is the magnitude of the flux and the saturation level. This implies a lower gas solubility in seawater in weaker magnetic fields and we may conclude that MF strength affects the equilibrium concentration of dissolved gas in the liquid phase. It is therefore conceivable that geomagnetic field strength may act as an additional factor that modulates the air‐sea exchange of gas. [17] An important unknown is the magnetic‐field effect on the reaction rates in the dissolved inorganic carbon system. CO 2 as a reactive gas may become hydrated already in the stagnant film, whereupon concentration gradients of other inorganic carbon species (HCO 3 −, CO 3 2−) would be set up immediately (given the rapid dissociation of carbonic acid, compared to the slow hydration reaction under typical seawater pH conditions), which leads to well‐known CO 2 exchange enhancement [Hoover and Berkshire, 1969; Wanninkhof and Knox, 1996]. The fact that the MF effect on solubility is more pronounced for a CO 2 enriched atmosphere than for the standard atmosphere might also result from MF effects on the equilibrium constants in the marine carbonate system, but more experiments are needed for clarification. [18] The theoretical framework describing effects of a weak static MF on electrolyte solutions, gas solubility, and equilibrium constants is at a level that does not even allow us to make qualitative predictions. Obviously, the MF effect is related to the presence of ionic solutes. Salinity reduces solubility of hydrophobic gases, and it is possible that the magnetic field (by exerting a Lorentz force on ionic solutes at DBL) partly undoes the adverse effect of salinity on gas solubility. We speculate that under higher fields, the relative influence of thermal fluctuations on ion movement becomes weaker (particularly at DBL, where gyration of ions may be stabilized), which would lead to lower effective temperatures, hence higher gas solubility.

4. Conclusions [19] We have measured viscosity and gas solubility of seawater at different magnetic field (MF) strengths. While the MF effect on seawater viscosity is probably too small to be relevant under naturally occurring field variations, the MF effect on gas solubility may present a physical link between geomagnetic field and climate. Our small‐scale laboratory experiments indicate lower solubility of CO 2 in seawater under reduced MF intensity. The extra amount of CO 2 not dissolved due to reduced CO 2 solubility in a weaker MF would add to the greenhouse effect. This mechanism therefore has the right sign to qualitatively explain the observed anti‐correlation between archeomagnetic field strength and global temperature estimates. [20] However, the magnitude of the mechanism is small compared to the magnitude of the preponderant mechanisms driving the exchange of carbon between ocean and atmosphere, such as water temperature, biological pumping, overturning circulation. The effect of geomagnetic‐field variations therefore is to modulate the air‐sea exchange of carbon. [21] The magnitude of the MF effect is such that CO 2 solubility reduces by a maximum of 0.5% per each % decrease in MF strength. To provide a quantitative assessment of the MF effect on the global carbon dioxide budget, we assume that our experimental results can be extrapolated to the global scale and that a reduction in CO 2 solubility by 0.5% is equivalent to an emission of 0.5% of the total amount of dissolved inorganic carbon (DIC) in the surface ocean (700 Pg C) [see Sigman and Boyle, 2000, Figure 2]. A 1% decrease in magnetic dipole moment occurring over a decade would then add 1 ppm CO 2 per decade to the atmosphere, that is, 0.35 Pg C/yr. This figure is a magnitude larger than the CO 2 discharge from subaerial volcanism (0.03 Pg C/yr) [Kerrick, 2001], from which we surmise that the MF effect may have played a role in pre‐industrial times. [22] Given the high anthropogenic emission rate of CO 2 (7 Pg C/yr), it would be preposterous to make the weakening Earth's magnetic field responsible for global warming. Yet, by lowering the capacity of the surface ocean to absorb excess CO 2 from the atmosphere, the diminishing field acts in the same direction as the increase in sea‐surface temperature and acidity, thereby exacerbating the effects of global warming.

Acknowledgments [23] The authors would like to thank Professor H. Scheer and two anonymous referees for helpful comments. M. W. acknowledges funding from the DFG (Wi 1828/4‐1).

Supporting Information Auxiliary material for this article contains additional information about viscosity measurements and the magnetic‐field effect on viscosity. Auxiliary material files may require downloading to a local drive depending on platform, browser, configuration, and size. To open auxiliary materials in a browser, click on the label. To download, Right‐click and select “Save Target As…” (PC) or CTRL‐click and select “Download Link to Disk” (Mac). See Plugins for a list of applications and supported file formats. Additional file information is provided in the readme.txt. Filename Description grl24714-sup-0001-readme.txtplain text document, 2.9 KB readme.txt grl24714-sup-0002-fs01.tifTIFF image, 306.5 KB Figure S1. The experimental setup in the fluorescence spectralphotometer. grl24714-sup-0003-fs02.tifTIFF image, 179.3 KB Figure S2. Calibration of fluorescence polarization A F , based on the temperature dependence of viscosity. grl24714-sup-0004-fs03.tifTIFF image, 182 KB Figure S3. Effect of salinity changes on fluorescence polarization, for seawater and NaCl solution. grl24714-sup-0005-fs04.tifTIFF image, 259.5 KB Figure S4. Magnetic‐field induced changes in viscosity of seawater and of a 3.5% NaCl solution after applying a MF of 2 mT at t = 10 sec. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.