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.     

地磁活动对气候要素影响的研究进展

地磁活动是太阳爆发现象引起地球近地空间磁场扰动的重要空间天气过程之一.地球磁场的变化具有多种时间尺度,其中从数十年到数世纪的长时间地磁场变化主要是由地核磁场引起的,而从数秒到数年的短时间地磁变化与太阳活动有关.近年来,越来越多的统计研究表明,地磁活动与太阳活动和地球气候变化之间存在着显著的相关性.地球磁场和地球大气系统的耦合现象驱动着人们探索地磁活动对地球天气和气候系统影响的研究.本文的目的就是综述国内外地磁变化对气候影响的研究进展,介绍我们最新的研究成果,探索地磁活动对气候要素的影响特征和可能机理过程,为深入研究地磁活动对地球天气和气候的影响提供基础和依据,以期对地磁活动和气候要素关系有进一步的认识.     

Climate hypersensitivity to solar forcing?

We compare the equilibrium climate responses of a quasi-dynamical energy balance model to radiative forcing by equivalent changes in CO₂, solar total irradiance (S_tot) and solar UV (S_UV). The response is largest in the S_UV case, in which the imposed UV radiative forcing is preferentially absorbed in the layer above 250 mb, in contrast to the weak response from global-columnar radiative loading by increases in CO₂ or S_tot. The hypersensitive response of the climate system to solar UV forcing is caused by strongly coupled feedback involving vertical static stability, tropical thick cirrus ice clouds and stratospheric ozone. This mechanism offers a plausible explanation of the apparent hypersensitivity of climate to solar forcing, as suggested by analyses of recent climatic records. The model hypersensitivity strongly depends on climate parameters, especially cloud radiative properties, but is effective for arguably realistic values of these parameters. The proposed solar forcing mechanism should be further confirmed using other models (e.g., general circulation models) that may better capture radiative and dynamical couplings of the troposphere and stratosphere.     

The importance of ocean temperature to global biogeochemistry

Variations in the mean temperature of the ocean, on time scales from millennial to millions of years, in the past and projected for the future, are large enough to impact the geochemistry of the carbon, oxygen, and methane geochemical systems. In each system, the time scale of the temperature perturbation is key. On time frames of 1-100 ky, atmospheric CO₂ is controlled by the ocean. CO₂ temperature-dependent solubility and greenhouse forcing combine to create an amplifying feedback with ocean temperature; the CaCO₃ cycle increases this effect somewhat on time scales longer than ∼5-10 ky. The CO₂/T feedback can be seen in the climate record from Vostok, and a model including the temperature feedback predicts that 10% of the fossil fuel CO₂ will reside in the atmosphere for longer than 100 ky. Timing is important for oxygen, as well; the atmosphere controls the ocean on short time scales, but ocean anoxia controls atmospheric pO₂ on million-year time scales and longer. Warming the ocean to Cretaceous temperatures might eventually increase pO₂ by approximately 25%, in the absence of other perturbations. The response of methane clathrate to climate change in the coming century will probably be small, but on longer time scales of 1-10 ky, there may be a positive feedback with ocean temperature, amplifying the long-term climate impact of anthropogenic CO₂ release.     

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.     

Space- and ground-based CO2 measurements: A review

The climate warming is mainly due to the increase in concentrations of anthropogenic greenhouse gases, of which CO₂ is the most important one responsible for radiative forcing of the climate. In order to reduce the great estimation uncertainty of atmospheric CO₂ concentrations, several CO₂-related satellites have been successfully launched and many future greenhouse gas monitoring missions are planned. In this paper, we review the development of CO₂ retrieval algorithms, spatial interpolation methods and ground observations. The main findings include: 1) current CO₂ retrieval algorithms only partially account for atmospheric scattering effects; 2) the accurate estimation of the vertical profile of greenhouse gas concentrations is a long-term challenge for remote sensing techniques; 3) ground-based observations are too sparse to accurately infer CO₂ concentrations on regional scales; and 4) accuracy is the primary challenge of satellite estimation of CO₂ concentrations. These findings, taken as a whole, point to the need to develop a high accuracy method for simulation of carbon sources and sinks on the basis of the fundamental theorem of Earth’s surface modelling, which is able to efficiently fuse space- and ground-based measurements on the one hand and work with atmospheric transport models on the other hand.     

Evaluating sun–climate relationships since the Little Ice Age

From the coldest period of the Little Ice Age to the present time, the surface temperature of the Earth increased by perhaps 0.8°C. Solar variability may account for part of this warming which, during the past 350 years, generally tracks fluctuating solar activity levels. While increases in greenhouse gas concentrations are widely assumed to be the primary cause of recent climate change, surface temperatures nevertheless varied significantly during pre-industrial periods, under minimal levels of greenhouse gas variations. A climate forcing of 0.3 W m⁻² arising from a speculated 0.13% solar irradiance increase can account for the 0.3°C surface warming evident in the paleoclimate record from 1650 to 1790, assuming that climate sensitivity is 1°C W⁻¹ m⁻² (which is within the IPCC range). The empirical Sun–climate relationship defined by these pre-industrial data suggests that solar variability may have contributed 0.25°C of the 0.6°C subsequent warming from 1900 to 1990, a scenario which time dependent GCM simulations replicate when forced with reconstructed solar irradiance. Thus, while solar variability likely played a dominant role in modulating climate during the Little Ice Age prior to 1850, its influence since 1900 has become an increasingly less significant component of climate change in the industrial epoch. It is unlikely that Sun–climate relationships can account for much of the warming since 1970, not withstanding recent attempts to deduce long term solar irradiance fluctuations from the observational data base, which has notable occurrences of instrumental drifts. Empirical evidence suggests that Sun–climate relationships exist on decadal as well as centennial time scales, but present sensitivities of the climate system are insufficient to explain these short-term relationships. Still to be simulated over the time scale of the Little Ice Age to the present is the combined effect of direct radiative forcing, indirect forcing via solar-induced ozone changes in the atmosphere, and speculated charged particle mechanisms whose pathways and sensitivities are not yet specified.     

Climatic and environmental changes over the last millennium recorded in the Malan ice core from the northern Tibetan Plateau

In this paper, climatic and environmental changes were reconstructed since 1129A.D. based on the Malan ice core from Hol Xil, the northern Tibetan Plateau. The record of δ 18O in the Malan ice core indicated that the warm-season air temperature variations displayed a general increase trend, the 20th-century warming was within the range of natural climate variability, and the warmest century was the 17th century while the warmest decade was the 1610s, over the entire study period. The “Medieval Warm Epoch” and “Little Ice Age” were also reflected by the ice core record. The dust ratio in the Malan ice core is a good proxy for dust event frequency. The 870-year record of the dust ratio showed that dust events occurred much frequently in the 19th century. Comparing the variations of δ 18O and the dust ratio, it is found that there was a strong negative correlation between them on the time scales of 101―102 years. By analyses of all the climatic records of ice cores and tree rings from the northern Tibetan Plateau, it was revealed that dust events were more frequent in the cold and dry periods than in the warm and wet periods.     

Evidence for solar forcing of climate variation from δ18O of peat cellulose

There have been a number of investigations for examining the possible link between long-term climate variability and solar activity. A continuous δ¹⁸O record of peat cellulose covering the past 6 000 years and the response of climate variation inferred from the proxy record to solar forcing are reported. Results show that during the past 5 000 years the abrupt climate variations, including 17 warming and 17 cooling, and a serious of periodicities, such as 86, 101, 110, 127, 132, 140, 155, 207, 245, 311, 820 and 1 050 years, are strikingly correlative to the changes of solar irradiation and periodicity. These observations are considered as further evidence for a close relationship between solar activity and climate variations on time scales of decades to centuries.     

Influence of climatic factors on the past atmospheric content of the C-14 isotope

Reconstructions of solar activity in the past epochs based on information on the past atmospheric content of the cosmogenic ¹⁴C isotope are nowadays actively discussed. The ¹⁴C isotope is generated in the atmosphere of the Earth under the influence of cosmic rays, and its concentration in annual tree rings carries information on the past solar activity. However, the concentration of this isotope in annual tree rings may also be influenced by climatic factors. In the present work, the possible correlation between variations in the ¹⁴C atmospheric content and in the Earth’s global temperature from the late 14th century to the middle of the 19th century is studied. It is shown that variations in global temperature may produce changes in the ¹⁴C atmospheric content and consequently have to be taken into account in reconstructions of the past solar activity.     

Responses of foraminiferal isotopic variations at ODP Site 1143 in the southern South China Sea to orbital forcing

The foraminiferal δ ¹⁸O and δ ¹³C records for the past 5 Ma at ODP Site 1143 reveal the linear responses of the Plio-Pleistocene climatic changes in the southern South China Sea to orbital forcing at the obliquity and precession bands. The phase of the δ ¹⁸O variations with the orbital forcing is opposite to that of the δ ¹³C, which may be caused by the frequent El Niño events from the equatorial Pacific. The amplification of the Northern Hemisphere Ice Sheet at ñ3.3 Ma probably affected the development of the 100-ka climatic cycles. Its further spreading may spur the 100-ka climatic cycle to become the dominant cycle in the late Pleistocene. The “Mid- Pleistocene Transition” event has localized influence on the isotopic variations in the southern South China Sea. The foraminiferal δ ¹³C records for the past 5 Ma at Site 1143 are highly coherent with the orbital forcing at the long eccentricity band, and lead the δ ¹⁸O records at the shorter eccentricity band, highlighting the importance of the carbon cycle in the global climate change.     

Photosynthetic responses of a humid grassland ecosystem to future climate perturbations

Increases in atmospheric CO₂ concentration not only affects climate variables such as precipitation and air temperature, but also affects intrinsic ecosystem physiological properties such as bulk stomatal conductance and intercellular CO₂ concentration. De-convolving these two effects remains uncertain in biosphere–atmosphere water and carbon cycling. Using a simplified analytical net ecosystem CO₂ exchange (NEE) model, tested with recently collected flux measurements in a humid grassland ecosystem in Ireland, we assess how much projected climate shifts affect net canopy photosynthesis (A) without physiological adjustments and contrast those findings with published field data on physiological adjustments for several grassland ecosystems. Our analysis suggests that the intrinsic grassland ecosystem physiological adjustment of A is about 45 times more important than the resulting climatic forcing shifts from the IS92a scenario (and a doubling of atmospheric CO₂ concentration). Also, our analysis shows that increase in precipitation results in concomitant decrease in the two climate variables—net radiation and vapor pressure deficit, and these decreases have opposite (and almost canceling) effects on A. Implications to afforestation policy and future experimental efforts to quantify the carbon sink from humid grassland ecosystems are also discussed.     

LONG-TERM VARIATIONS IN INSOLATION AND THEIR EFFECTS ON CLIMATE, THE LLN EXPERIMENTS

Used to test the Milankovitch theory over the last glacial-interglacial cycles, the Louvain-la-Neuve two-dimension Northern Hemisphere climate model shows that orbital and CO2 variations induce, in the climate system, feedbacks sufficient to generate the low frequency part of the climatic variations over the last 200 kyr. Initiation and termination of glacial cycles cannot indeed be explained without invoking both the fast feedbacks associated with atmospheric processes (water vapor, cloud, snow and sea ice) and the slower feedbacks associated with coupling to other parts of the climate system, in particular the land ice-sheet buildup and disintegration. This model shows that long-term changes in the Earth's orbital parameters lead to variations in the amount of solar radiation received at the top of the atmosphere, which in turn act as a pacemaker for climatic variations at the astronomical frequencies, through induced albedo-temperature and greenhouse gases-temperature feedbacks. Spectral analysis of the Northern Hemisphere global ice volume variations simulated under both insolation and CO2 forcings reproduces correctly the relative intensity of the peaks at the orbital frequencies as seen in SPECMAP data. Except for variations with time scales shorter than 5 kyr, the simulated long-term variations of total ice volume are comparable to that reconstructed from deep-sea cores. For example, the model simulates glacial maxima of similar amplitudes at 134 kyr BP and 15 kyr BP, followed by abrupt deglaciations.     

Modelling the Northern Hemisphere temperature for solar cycles 24 and 25

It is uncertain whether the solar cycle 24 will have a high or a low sunspot maximum number. In its last revision the Solar Cycle 24 Prediction Panel indicates that the low prediction is the most likely. Also, solar cycle 25 is considered to present an equal or lower activity than cycle 24. In order to assess the possible effect of the solar activity on temperature, in the present work we attempt to model the tendency of the Northern Hemisphere temperature for the years 2009–2029, corresponding to solar cycles 24 and 25, using a thermodynamic climate model. We include as forcings the atmospheric carbon dioxide (CO₂) and the solar activity by means of the total solar irradiance, considering that the latter has not only a direct effect on climate, but also an indirect one through the modulation of the low cloud cover. We use two IPCC-2007 CO₂ scenarios, one with a high fossil consumption and other with a low use of fossil sources. Also we consider higher and lower solar activity conditions. We found that in all the performed experiments the inclusion of the solar activity produces a noticeable reduction in warming respect to the IPCC-2007 CO₂ scenarios. Such reduction goes between ～14% and ～44%. In order to evaluate the efficiency of the TCM, we use the root mean square (RMS) between the observed and model temperatures for the period 1980–2003. We find that the RMS for the experiment using the CO₂ as the only forcing is 0.06 °C,while for the experiment that includes also the solar activity it is higher, 0.13 °C.     

Secular variation in aerosol transparency of the atmosphere as the possible link between long-term variations in solar activity and climate

Information about variations in solar activity and climate on the time intervals from 130 years to four–five last centuries, including results of instrumental measurements (Wolf numbers, actinometry, thermometry) and indirect indicators (ice core acidity, NO ₃ ^? ion concentration in polar ice, temperature tree-ring reconstructions), has been analyzed for the Northern Hemisphere and its high-latitude part. It has been obtained that the observed relation between secular variations in solar activity and near-Earth temperature resulted from the effect of the corresponding variation in aerosol transparency of the stratosphere on terrestrial climate. It has been also indicated that long-term variations in the aerosol content of the stratosphere can, in turn, be related to secular cycles in atmospheric ionization caused by variations in fluxes of ionizing cosmic particles.     

The Earth's Climate in the Next Hundred Thousand years (100 kyr)

One of the most striking features of the Quaternary paleoclimate records remains the so-called 100-kyr cycle which is undoubtedly linked to the future of our climate. Such a 100-kyr cycle is indeed characterised by long glacial periods followed by a short-interglacial (10–15 kyr long). As we are now in an interglacial, the Holocene, the previous one (the Eemian, which corresponds quite well to Marine Isotope Stage 5e, peaking at 125 kyr before present, BP) was assumed to be a good analogue for our present-day climate. In addition, as the Holocene is 10 kyr long, paleoclimatologists were naturally inclined to predict that we are quite close to the next ice age. Simulations using the 2-D climate model of Louvain-la-Neuve show, however, that the current interglacial will most probably last much longer than any previous ones. It is suggested here that this is related to the shape of the Earth's orbit around the Sun, which will be almost circular over the next tens of thousands of years. As this is primarily related to the 400-kyr cycle of eccentricity, the best and closest analogue for such a forcing is definitely Marine Isotopic Stage 11 (MIS-11), some 400 kyr ago, not MIS-5e. Because the CO₂ concentration in the atmosphere also plays an important role in shaping long-term climatic variations – especially its phase with respect to insolation – a detailed reconstruction of this previous interglacial from deep sea and ice records is urgently needed. Such a study is particularly important in the context of the already exceptional present-day CO₂ concentrations (unprecedented over the past million years) and, even more so, because of even larger values predicted to occur during the 21st century due to human activities.     

Simulated effect of sunshade solar geoengineering on the global carbon cycle

Solar geoengineering has been proposed as a potential mechanism to counteract global warming. Here we use the University of Victoria Earth System Model (UVic) to simulate the effect of idealized sunshade geoengineering on the global carbon cycle. We conduct two simulations. The first is the A2 simulation, where the model is driven by prescribed emission scenario based on the SRES A2 CO₂ emission pathway. The second is the solar geoengineering simulation in which the model is driven by the A2 CO₂ emission scenario combined with sunshade solar geoengineering. In the model, solar geoengineering is represented by a spatially uniform reduction in solar insolation that is implemented at year 2020 to offset CO₂-induced global mean surface temperature change. Our results show that solar geoengineering increases global carbon uptake relative to A2, in particular CO₂ uptake by the terrestrial biosphere. The increase in land carbon uptake is mainly associated with increased net primary production (NPP) in the tropics in the geoengineering simulation, which prevents excess warming in tropics. By year 2100, solar geoengineering decreases A2-simulated atmospheric CO₂ by 110 ppm (12%) and causes a 60% (251 Pg C) increase in land carbon accumulation compared to A2. Solar geoengineering also prevents the reduction in ocean oxygen concentration caused by increased ocean temperatures and decreased ocean ventilation, but reduces global ocean NPP. Our results suggest that to fully access the climate effect of solar geoengineering, the response of the global carbon cycle should be taken into account.     

Greenhouse effects of aircraft emissions as calculated by a radiative transfer model

With a radiative transfer model, assessments are made of the radiative forcing in northern mid-latitudes due to aircraft emissions up to 1990. Considered are the direct climate effects from the major combustion products carbon dioxide, nitrogen dioxide, water vapor and sulphur dioxide, as well as the indirect effect of ozone production from NO_x emissions. Our study indicates a local radiative forcing at the tropopause which should be negative in summer (-0.5 to 0.0 W/m²) and either negative or positive in winter (-0.3 to 0.2 W/m²). To these values the indirect effect of contrails has to be added, which for the North Atlantic Flight Corridor covers the range -0.2 to 0.3 W/m² in summer and 0.0 to 0.3 W/m² in winter. Apart from optically dense non-aged contrails during summer, negative forcings are due to solar screening by sulphate aerosols. The major positive contributions come from contrails, stratospheric water vapor in winter and ozone in summer. The direct effect of NO₂ is negligible and the contribution of CO₂ is relatively small.     

Climate sensitivities of two versions of FGOALS model to idealized radiative forcing

Projections of future climate change by climate system models depend on the sensitivities of models to specified greenhouse gases.To reveal and understand the different climate sensitivities of two versions of LASG/IAP climate system model FGOALS-g2 and FGOALS-s2,we investigate the global mean surface air temperature responses to idealized CO2 forcing by using the output of abruptly quadrupling CO2 experiments.The Gregory-style regression method is used to estimate the"radiative forcing"of quadrupled CO2 and equilibrium sensitivity.The model response is separated into a fast-response stage associated with the CO2 forcing during the first 20 years,and a slow-response stage post the first 20 years.The results show that the radiative forcing of CO2 is overestimated due to the positive water-vapor feedback and underestimated due to the fast cloud processes.The rapid response of water vapor in FGOALS-s2 is responsible for the stronger radiative forcing of CO2.The climate sensitivity,defined as the equilibrium temperature change under doubled CO2 forcing,is about 3.7 K in FGOALS-g2 and4.5 K in FGOALS-s2.The larger sensitivity of FGOALS-s2 is due mainly to the weaker negative longwave clear-sky feedback and stronger positive shortwave clear-sky feedback at the fast-response stage,because of the more rapid response of water vapor increase and sea-ice decrease in FGOALS-s2 than in FGOALS-g2.At the slow-response stage,similar to the fast-response stage,net negative clear-sky feedback is weaker in FGOALS-s2.Nevertheless,the total negative feedback is larger in FGOALS-s2 due to a larger negative shortwave cloud feedback that involves a larger response of total cloud fraction and condensed water path increase.The uncertainties of estimated forcing and net feedback mainly come from the shortwave cloud processes.     

Carbon cycle and climate change during the Cretaceous inferred from a biogeochemical carbon cycle model

The carbon cycle and climate change during the Cretaceous are reconstructed by using a carbon cycle model, and discussed. The model takes into account the effects of the enhanced magma eruption and organic carbon burial rates, both of which characterize the carbon cycle during the Cretaceous. The result for the CO₂ variation is roughly consistent with the pattern of paleoclimate change inferred from the geological record. The CO₂ level during the mid-Cretaceous is estimated to be 4–5 times the present atmospheric level, corresponding to a surface temperature of 20–21°C. The warm, equable Cretaceous resulted from the effects of tectonic forcing such as enhanced CO₂ degassing, although the enhanced organic carbon burial has a tendency to decrease the CO₂ level. The organic carbon burial rate during the Cretaceous is generally larger than those for the Cenozoic, and is characterized by three major peaks (~ 1.5–1.8 times the present-day value) corresponding to the major oceanic anoxic events. In the case for the extensive mantle plume degassing, although the CO₂ levels are only 10% higher than those for the standard case during 120–100 Ma, the causes for the enhanced CO₂ levels would be quite different. If the globally averaged surface temperature had increased due to paleogeographic forcing effects, the greenhouse effect of CO₂ (and thus the CO₂ level) should be lower than the values estimated for the standard case. If the CO₂ levels are similar to, but the surface temperature is higher than, those for the standard case, either the parameter β (an influence of the Himalayas–Tibetan Plateau on the global weathering today) may be unreasonably large or the dependence of the silicate weathering rate on the CO₂ partial pressure and the surface temperature should be much weaker than those previously proposed.