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

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.     

The Dicke cycle: A ∼27-day solar oscillation

Measurements of solar EUV irradiance show, besides the ～11-year Schwabe cycle, a series of oscillations with a ～27-day period. They are generally explained by the passage of active regions across the solar surface resulting from the Sun's rotation, but the calculated amplitude underestimates the observed long-term variation in irradiance (Lean 1991). The variant of this model proposed here is modulation of EUV emission from the corona by rotation of the Sun's radiative zone. The response would be immediate, raising the prospect of short-term forecasting of EUV effects on space weather and on the Earth's atmospheric circulation.     

Trends in the F2 ionospheric layer due to long-term variations in the Earth's magnetic field

The Earth's magnetic field presents long-term variations with changes in strength and orientation. Particularly, changes in the dip angle (I) and, consequently, in the sin(I)cos(I) factor, affect the thermospheric neutral winds that move the conducting plasma of the ionosphere. In this way, a lowering or lifting of the F2-peak (hmF2) is induced together with changes in foF2, depending on season, time and location. A simple theoretical approximation, developed in a previous work, is extended to a worldwide latitude–longitude grid to assess hmF2 and foF2 trends due to Earth's magnetic field secular variations. Compared to the greenhouse gases effects over the ionosphere, the Earth's magnetic field may be able to produce stronger trends which vary with season, time and location. However, to elucidate the origin of F2-region trends, long-term variations in the three possible known mechanisms should be considered altogether—greenhouse gases, geomagnetic activity and Earth's magnetic field.     

Variability of the solar shape (before space dedicated missions)

Shrinking or expansion of the solar shape and irradiance variations are ultimately related to solar activity. We give here a review on existing ground-based or space solar radius measurements, extending the concept to shape changes. We show how helioseismology results allow us to look at the variations below the surface, where changes are not uniform, putting in evidence a new shallow layer, the leptocline, which is the seat of solar asphericities, radius variations with the 11-yr cycle and the cradle of complex physical processes: partial ionization of the light elements, opacities changes, superadiabaticity, strong gradient of rotation and pressure. Based on such physical grounds, we show why it is important to get accurate measurements from scheduled dedicated space missions: PICARD, SDO, DynaMICCS, ASTROMETRIA, SPHERIS. Such measurements will provide us a unique opportunity to study in detail the relationship between global solar properties and changes in the Sun's interior.     

Shielded by the wind: the influence of the interstellar medium on the environment of Earth

We report on the recent studies on the long-term influence of cosmic rays on the Earth's environment. While on short time-scales solar activity is the driver for atmospheric changes suspected to be due to cosmic rays, for long time-scales the heliosphere, i.e. the circumsolar region occupied by the expanding part of the Sun's atmosphere, has to be considered. The heliosphere is identified as an important shield against interstellar influences and hazards. It has been demonstrated by quantitative modelling that a change of the interstellar medium surrounding the heliosphere as a result of the Sun's quasi-Keplerian motion around the galactic center triggers significant changes of planetary environments caused by enhanced fluxes of neutral atoms as well as by the increased cosmic ray fluxes. We give a compilation of recent space science results of interest to the atmospheric science community.     

Regional millennial trend in the cosmic ray induced ionization of the troposphere

Long-term trends in the tropospheric cosmic ray induced ionization on the multi-millennial time scale are studied using the newly released paleomagnetic reconstruction models. Spatial and temporal variations of the tropospheric ionization has been computed using the CRAC:CRII model and applying the paleomagnetic CALS7k.2 reconstruction. It has been shown that long-term variations of the tropospheric ionization are not spatially homogeneous, and they are defined not only by solar (i.e., covariant with solar irradiance) changes but also by the geomagnetic field. The dominance of the two effects is geographically separated, which makes it possible to distinguish between direct and indirect solar–terrestrial climate effects. Possible climate applications are considered.     

Signatures of solar activity variability in meteorological parameters

Solar radiation (both total and in various wavelengths) varies at different time scales—from seconds to decades or centuries—as a consequence of solar activity. The energy received from the Sun is one of the natural driving forces of the Earth's atmosphere and since this energy is not constant, it has been argued that there must be some non-zero climate response to it. This response must be fully specified in order to improve our understanding of the climate system and the impact of anthropogenic activities on it. However, despite all the efforts, if and how subtle variations of solar radiation affect climate and weather still remains an unsolved puzzle. One key element that is very often taken as evidence of a response, is the similarity of periodicities between several solar activity indices and different meteorological parameters. The literature contains a long history of positive or negative correlations between weather and climate parameters like temperature, rainfall, droughts, etc. and solar activity cycles like the 27-day cycle, the prominent 11-year sunspot cycle, the 22-year Hale cycle and the Gleissberg cycle of 80–90 years. A review of these different cycles is provided as well as some of the correlative analyses between them and several stratospheric parameters (like stratospheric geopotential heights, temperature and ozone concentration) and tropospheric parameters (like temperature, rainfall, water level in lakes and river flooding, clouds) that point to a relationship of some kind. However, the suspicion on these relationships will remain as long as an indisputable physical mechanism, which might act to produce these correlations, is not available.     

The role of the stratosphere in climate change

A variety of climate perturbations have the potential to alter the thermodynamic and dynamical characteristics of the middle atmosphere, which may then affect tropospheric climate. Increased thermal emission from rising stratospheric CO₂ levels and scattering of solar radiation from stratospheric volcanic aerosols have a direct impact on surface temperatures, while variations in stratospheric water vapor and ozone can affect tropospheric temperatures. Observations and modeling experiments suggest that these perturbations, as well as solar irradiance variations operating through the stratosphere, may affect tropospheric dynamics, such as planetary wave amplitudes and Hadley cell intensity. In addition, climate changes will probably alter tropospheric/stratospheric exchange, with the potential for modifying trace gas distributions and climate forcing. These issues are reviewed in the light of the incorporation of middle atmosphere studies into IGBP.     

Total solar irradiance variations

Total solar irradiance has been monitored from space for nearly two decades. These space-borne observations have established conclusively that total solar irradiance changes over a wide range of periodicities—from minutes to the 11-year solar cycle. Since the total energy flux of the Sun is the principal driver for all Earths atmospheric phenomena, the accurate knowledge of the solar radiation received by the Earth and its variations is an extremely important issue. In this paper we review the long-term variations of total solar irradiance during solar cycles 21 and 22. We conclude that, within the current accuracy and precision of the measurements, the minimum level of total solar irradiance is about the same for both solar cycles 21 and 22.     

A 14.7 Ka record of earth surface processes from the arid‐monsoon transitional zone of China

The stability of Earth's critical zone is intimately linked with erosion, weathering and vegetation type and density. Therefore, it affects global biogeochemical processes which in turn affect the global climate by absorbing and reflecting solar radiation, and by altering fluxes of heat, water vapour, carbon dioxide and other trace gases through various feedback mechanisms. However, there is a lack of knowledge about how Earth's critical zone processes have changed over time and their link with past monsoon variability, especially in Asia. The study of lake sediments, which contain a suite of inorganic elemental and isotopic proxies, may facilitate the understanding of the Earth's critical zone processes on millennial timescales. Here we reconstruct the history of erosion–weathering–vegetation interactions since ~14.7 ka using geochemical records from a radiocarbon‐dated sediment core from Lake Gonghai in the monsoon‐arid transitional zone of north China. Detrital (Al, Ti, K, Rb) and authigenic (Ca, Sr) elemental records reveal distinct, millennial‐scale, late deglacial‐Holocene erosion and weathering patterns and transitions with the former (latter) elements showing higher (lower) values in warm intervals and vice versa. Chemical Index of Alteration (CIA) molar, a humidity proxy, suggests low humidity during the late deglacial ~11.5–14.7 ka, high humidity during the early‐mid Holocene ~11.5–3.2 ka, and intermediate humidity during the late Holocene interval since ~3.2 ka. The results of cross‐spectral analysis and comparison of our records with other climate reconstructions also suggest a pattern of orbitally‐phased humidity changes in north China. Overall, our results provide evidence for the solar‐forcing of Earth's surface processes in mid‐latitude China under natural climatic conditions. Copyright © 2017 John Wiley & Sons, Ltd.     

Paleointensity variations of Earth's magnetic field and their relationship with polarity reversals

Power-spectral analyses of the intensity of Earth's magnetic field inferred from ocean sediment cores and archeomagnetic data from time scales of 100 yr to 10 Myr have been carried out. The power spectrum is proportional to 1/f where f is the frequency. These analyses compliment previous work which has established a 1/f² spectrum for variations at time scales less than 100 yr. We test the hypothesis that reversals are the result of variations in field intensity with a 1/f spectrum which occasionally are large enough to cross the zero intensity value. Synthetic binormal time series with a 1/f power spectrum representing variations in Earth's dipole moment are constructed. Synthetic reversals from these time series exhibit statistics in good agreement with the reversal record, suggesting that polarity reversals may be the end result of autocyclic intensity variations with a 1/f power spectrum.     

Possible link between multi-decadal climate cycles and periodic reversals of solar magnetic field polarity

The linkage between multi-decadal climate variability and activity of the sun has been long debated based upon observational evidence from a large number of instrumental and proxy records. It is difficult to evaluate the exact role of each of solar parameters on climate change since instrumentally measured solar related parameters such as Total Solar irradiance (TSI), Ultra Violet (UV), solar wind and Galactic Cosmic Rays (GCRs) fluxes are more or less synchronized and only extend back for several decades. Here we report tree-ring carbon-14 based record of 11-year/22-year solar cycles during the Maunder Minimum (17th century) and the early Medieval Maximum Period (9–10th century) to reconstruct the state of the sun and the flux of incoming GCRs. The result strongly indicates that the influence of solar cycles on climate is persistent beyond the period after instrumental observations were initiated. We find that the actual lengths of solar cycles vary depending on the status of long-term solar activity, and that periodicity of the surface air temperatures are also changing synchronously. Temperature variations over the 22-year cycles seem, in general, to be more significant than those associated with the 11-year cycles and in particular around the grand solar minima such as the Maunder Minimum (1645–1715 AD). The polarity dependence of cooling events found in this study suggests that the GCRs can not be excluded from the possible drivers of decadal to multi-decadal climate change.     

A Tikhonov regularization method to estimate Earth's oblateness variations from global GPS observations

Earth's oblateness is varying due to the redistribution of Earth's fluid mass and the interaction of various components in the Earth system. Nowadays, continuous Global Positioning System (GPS) observations can estimate Earth's oblateness (J₂) variations with the least squares method, but are subject to ill-conditioned equations with limited GPS observations and aliasing errors from truncated degrees. In this paper, a Tikhonov regularization method is used to estimate J₂ variations from global continuous GPS observations. Results show that the J₂ has been better estimated from GPS observations based on a Tikhonov regularization method than the usual least squares method when compared to SLR solutions. Furthermore, the amplitudes and phases of the annual and semi-annual J₂ variations are closer to the SLR results with truncated degrees from 2 to 5. Higher truncated degrees will degrade the J₂ estimate. Annual J₂ variations are best estimated from GPS observations with truncated degree 4 and semi-annual J₂ variations are best estimated with truncated degree 2.     

Solar variability and its implications for the human environment

Solar variability can affect human activities in a variety of ways, from changing our climate to disrupting power distribution facilities and shortening the orbital lifetime of satellites. This tutorial paper will be concerned only with effects on the surface environment that can have a direct impact on our everyday life, such as variations in the stratospheric ozone layer that shields us from harmful ultraviolet radiation, and changes in global climate that can hinder or delay the detection of climate changes that might result from our own technological activities. The emphasis is on potential mechanisms, rather than on reported correlations between solar and terrestrial parameters, but reference to certain observations will be made. Realization of a potential impact of solar variability on our local environment has progressed a long way in the last few decades, from denial to partial acceptance, but a complete assessment of its reality and magnitude remains a distant goal.     

Impact of the geomagnetic field and solar radiation on climate change

Recent studies have shown that, in addition to the role of solar variability, past climate changes may have been connected with variations in the Earth??s magnetic field elements at various timescales. An analysis of variations in geomagnetic field elements, such as field intensity, reversals, and excursions, allowed us to establish a link between climate changes at various timescales over the last millennia. Of particular interest are sharp changes in the geomagnetic field intensity and short reversals of the magnetic poles (excursions). The beginning and termination of the examined geomagnetic excursions can be attributed to periods of climate change. In this study, we analyzed the possible link between short-term geomagnetic variability (jerks) and climate change, as well as the accelerated drift of the north magnetic pole and surface temperature variations. The results do not rule out the possibility that geomagnetic field variations which modulate the cosmic ray flux could have played a major role in climate change in addition to previously induced by solar radiation.     

Review of electromagnetic coupling between the Earth's atmosphere and the space environment

We review our understanding of the electrical properties of the lower and upper atmosphere along with various possible sources of the electromagnetic energy near and far above the Earth's surface. The transport of electromagnetic energy from the atmosphere to the ionosphere and then to the magnetosphere and back to the Earth's surface via ionosphere and lower atmosphere is discussed. The electromagnetic coupling of various regions is also discussed.     

Local turbulence in the Earth's core

Abstract

It is shown that in the Earth's core, where the geodynamo is at work (and is supplied with energy by the prevailing unstable density stratification), a buoyancy instability of a local character exists which is highly supercritical. This instability results in fully developed turbulence dominated by small scale vortices. The influence of the Earth's rotation and of the magnetic field produced by the geodynamo makes this small scale turbulence highly anisotropic. A qualitative picture of this local anisotropic turbulence is devised and the main parameters characterizing it are estimated. Expressions for the turbulent diffusivity are developed and discussed.     

Growth in the atmospheric aerosol concentration as a climate forcing agent

The level of scattered radiation is analyzed using data of diurnal solar coronograph observations at the Mountain Astronomic Station in the period 1957?C2010 around coronal spectral lines at 5303 ? and 6374 ?. The observations were performed near the solar limb and were normalized by the intensity of the solar disk center. The measurements revealed variations on different timescales: seasonal variations, local maxima on timescales of a few years, and long-term trends. The local changes in the level of scattered radiation were found to be probably due to volcanic eruptions. An analysis revealed a tendency towards an increased in scattered radiation by approximately 40% during the last 50 years. The variations in the level of scattered radiation are compared with the concentration of atmospheric aerosols. The long-term growth in scattered radiation compares well with changes in the Earth??s near-surface temperature and is possibly associated with global climate change.