V. A. Dergachev1, P. B. Dmitriev1, O. M. Raspopov2, and B. Van Geel3
1 Ioffe Physico-Technical Institute, Russian Academy of Science,
2 Sankt-Petersburg Branch of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Russian Academy of Science;
3 Amsterdam University, Holland
For more than a hundred years many geoscientists discuss the effect of solar activity variations on the Earth's atmosphere and climate changes. This problem became more acute during the last years in connection with the global warming during the last century. The physical causes of this warming are a subject of hot discussions. The popular view that the global warming is caused by purely anthropogenic factors is argued by the growing number of opponents whose arguments suggest the possibility of a high natural climatic change. The natural changing of the climate may be associated both with the outside effect on the climate parameters (for example, with the solar activity (SA) and with the solar radiation), and with some internal processes operating in the atmosphere-ocean-continent (A-O-C) system, and also with the interaction between the outside factors and the internal processes operating in a strictly nonlinear A-O-C system with its own frequencies and noise. The latter may intensify the outside signal several times, this naturally leading to the higher efficiency of the outside factor effects. [Drijhout at al., 1999; Lawrence and Ruzmaikin, 1998; Waple et al., 2002; White et al., 1997].
The authors of some papers [Mann and Jones, 2003; Mann et al., 2003] state that the global warming in the last century was unprecedented during the last two millennia. This does not agree with the estimates reported by other authors [Esper et al., 2002; Soon et al., 2003; and others]. The authors of the latter publications claim that the climate of the 20th century was not uniquely warm during the last millennium. Although Soon et al. [ agree that in some local areas the anthropogenic effect on the climate may be quite substantial at the present time. Some authors [Dergachev and Raspopov, 2000a, 2000b; Lockwood et al., 1999] emphasize that the global warming of the last century was favored by the long-term growth of the solar activity. As follows from the paper by Dergachev and Raspopov [2000b], the whole of the last century was at the ascending wave of the quasi two hundred-year cycle of the solar activity. As follows from Lockwood and Stamper , a change in the intensity of the solar activity in the last century might have caused a 52-percent growth of the global temperatures during the time period of 1910 to 1960 and a 31-percent growth of temperature from the end of 1970 to the present time. It should also be noted that about 120-130 thousand years ago, during the interglacial period ("Eemian'' period in the western terminology and "Mikulinean'' period in the Russian terminology) the global temperature was almost the same as the modern one, although the anthropogenic effect was obviously absent. Therefore the discussion of the effects of natural factors and, primarily, of the solar activity and the associated variation of the solar radiation and the fluxes of galactic cosmic rays (GCR), as well as of the effect of the geomagnetic dipole variations on the climatic processes, is important for understanding the physical causes of the modern climatic changes. The modern instrumental data available for the solar activity and the instrumental series of temperature data are highly restricted in time. For this reason in our analysis of the association of long-term solar activity variations, GCR intensity, and geomagnetic dipole changes with climatic changes we must rely on the use of paleodata that have preserved in the Earth's "archives'' (glaciers, tree growth rings, bottom marine and lacustrine sediments, loess, and the like). The aim of this study was to estimate the contribution of the natural factors associated with the solar and geomagnetic variability and with the galactic cosmic ray flux to the global climatic changes. This analysis was made for different time intervals, such as, the last millennium, the Holocene epoch (up to 10-12 thousand years ago) and the time interval of 10-50 thousand years ago.
where M d is a dipole magnetic moment, r0 is the radius of the Earth, and a is geomagnetic latitude. The curve plotted in Figure 2a shows variations in the cutoff rigidity as a function of the latitude for the modern geomagnetic dipole moment M0. The abscissa of this plot also shows a scale which allows one to make allowance for the effect of the changing dipole moment ( M d/M0 ). Note that the screening of cosmic rays by the geomagnetic field is the highest in the equator region. Figure 2b shows variations in the energy spectrum of GCR proton flux at the maximum and the minimum of the 11-year solar activity cycle in comparison with an interstellar spectrum [Weber and Lockwood, 1998]. The analysis of the data plotted in Figure 2a suggests that where the dipole moment reduces to the value of 0.2 M0, the rigidity R e in the equatorial region must decline from 15 to 3 GeV (Figure 2a), as a result of which the amount of galactic cosmic rays reaching the Earth atmosphere must grow several times larger (Figure 2b). Therefore, variations in the concentrations of cosmogenic isotopes provide information of changes in the geomagnetic dipole for the last 12000 years [Teanby and Gubbins, 2000], obtained by way of generalizing the archeo- and paleomagnetic data. When we examined the time interval of 10-50 thousand years, we used the data reported by Lehman et al.  for changes in the geomagnetic moment.
The common practice is to use the concentrations of stable oxygen isotope 18O ( d18O) in the cores of ice and marine deposits. The concentration of this isotope is essentially a parameter that provides information for the temperature of the environment where the precipitation was formed (clouds, surface water layers, etc.). In this study we used the data available for d18O variations in the cores of the Greenland ice from the GISP-2 Hole [Grootes and Stuiver, 1997].
When discussing the paleodata for the solar activity, it need be kept in mind that the Sun is the main source of energy that arrives at the Earth. However, solar activity may control climate changes in two ways. On the one hand, associated with the solar activity are changes in the solar radiation, including the ultraviolet rays, whose variations may be as high as 10% during the eleven-year cycle of the solar activity. However, no experimental data have been obtained to prove any long-term (more than decades of years) solar radiation variability. On the other hand, the fluxes of galactic cosmic rays, modulated by the solar activity, can affect some atmospheric processes, including the state of the cloud cover, and, hence, may control the climatic processes. For this reason, when we use the paleodata for long-term variations in the solar activity, we actually discuss the effects of variations in the galactic cosmic ray fluxes on the climate.
The cosmogenic 14C and 10Be isotopes are significant not only as the important indicators of the solar activity and galactic cosmic ray fluxes and establishing a correlation between the data available for the evidence of the natural processes obtained from oceanic and terrestrial records or from ice cores, but also provide critically important data for precipitation rates and the dynamics of the climatic system. Note, that in contrast to 14C found in tree rings (the 14C, after oxidation to 14CO 2 gas mixes between the atmosphere and ocean in the complex exchange of the carbon reservoirs), 10Be has a simpler exchange reservoir system in ice layers (after its formation 10Be grows attached to aerosol and settles in the ice layers or in the oceanic sediments). In turn, the comparison of the behaviors of these two radioisotopes in their exchange reservoirs provides data on the changes that take place in the characteristics of the carbon exchange system. As a result of the damping effect of the carbon exchange system, the cycle amplitudes in the 14C concentration, lasting hundreds of years, are suppressed 10-20 times, with the phase shift being several tens of years, whereas the amplitudes of the ten-year cycles are suppressed 50-100 times with a phase shift of a few years [Dergachev and Stupneva, 1975].
In order to exclude the meteorologic effects that may be present in the annual data, the above-mentioned data series were smoothed with the help of a linear filter [Alavi and Jenkins, 1965] with the gradual growth of its smoothing parameter: T cut-off = 3, 5, 10, and 20 years, which corresponds to the filter cut-off point at the signal half-power. The next step was to plot some selected estimates of the cross-correlation function for each pair of the initial series of data and their smoothed components [Jenkins and Watts, 1972]. The resulting values were used to estimate the data reliability (see Table 1) at the significance level of 95%, which proved that the respective pairs of the initial data and their smoothed components were not "white'' noise or, to be more exact, not correlated sequences of purely random numbers. Table 1 demonstrates that in the case of the concentration of the 14C and 18O isotopes the significant correlation exists only for the smoothed components, beginning with the smoothing parameter of 10 and more years, whereas in the case of the 10Be and 18O isotope pair concentration it exists both for the initial series themselves and for their smoothed components. Moreover, this correlation intensifies with the growing value of the linear filter smoothing parameter. Consequently, since the concentration of the 18O is the indicator of the atmosphere temperature, the meteorologic effect masking the relationships between the data series concerned can be smoothed using a linear filter with the optimal value of the smoothing parameter equal to 10 years.
Figure 3 shows the smoothed components of the initial data with the optimal smoothing parameter value of 10 years (a, b, and c) and some selected estimates of their cross-correlation values (d, e, and f), which showed the 95-percent significance of the potential existence of a correlation relationship between the concentration of the isotopes of the respective pairs. It follows from Figures 3d and 3e that the changes in the 14C and 18O concentrations, as well as in the 10Be and 18O concentrations, are in opposite phases, that is, the maximum concentrations of the former isotope correspond to the minimum concentrations of the latter, which is clearly shown in Figures 3a, 3b, and 3c. These figures also show the shifts between the compared pairs of the data (for instance, a 3- or a 4-year shift for 14C and 10Be, with the latter being ahead of the former), this being associated with the formation conditions of the respective isotopes and with their respective reservoirs. The high correlation between the 14C and 10Be series (45% compared to the threshold value of 5%, see the table) suggests the common modulation mechanism for the formation rate of these isotopes.
The analysis of the data presented in the tables and of the correlated functions of the investigated data pairs presented in Figure 3 suggests the conclusion of a close relationship between the changes in the intensity of galactic cosmic rays and in the climate during a time period of hundreds of years and allows one to trace some common regularities in their variation, namely, the increase of the galactic cosmic ray intensity correlated with the temperature decline.
The climate during the last 10 thousand years was more stable and warm compared with the previous period prior to and after the retreat of the last glaciation, which began approximately 21 thousand years ago. It should also be noted that the magnetic dipole moment of the Earth was the highest during the last 10 thousand years, compared to the previous period, which might have resulted in the much lower level of the galactic cosmic rays arriving to the Earth atmosphere during this long period of time.
The levels of galactic cosmic rays (GCR) in the Earth's atmosphere are inversely related to the intensities of helio and geomagnetic fields. The calculations reported by Dergachev  and Castagnoli and Lal  show that the 14C production rate becomes more sensitive to changes in the solar modulation during the minimum values of the dipole moment. At higher latitudes, where the values of the dipole moment of the Earth are 10-20% of the modern field value, the GCR flux may be more than twice higher than the modern GCR flux, whereas with the doubled value of the dipole moment the GCR flux is not higher than the 50-percent value of the modern flux. It is important to emphasize that in the case of very low values of the dipole moment, differences in the GCR levels between the maxima and minima, associated with the 210-year solar cycle, grow at least by an order of magnitude. Consequently, both of these factors affect, to a higher or lesser degree, ionization in the atmosphere and, hence, produce climatic changes.
In order to analyze a relationship between the geomagnetic field variations and the concentrations of cosmogenic isotopes and stable oxygen using a 10,000-year scale, we carried out a cross-correlation analysis of the long-term trend of radiocarbon data and of the smoothed curve of the Earth dipole moment variation. The smoothing scale was taken to be ~1000 years in order to exclude the contribution of the nondipole variations of the field. As long as this pair of data series is concerned, 75% of the cross correlation values exceeded the 95-percent level of significance, which proves the high correlation between these data, the correlation coefficient between these data series being - 0.6. The high-frequency portion of the radiocarbon data series, after the long-term trend had been subtracted, did not show any significant possibility for the existence of correlation with the smoothed curve of the Earth dipole moment variation. An approximately similar pattern was observed for the cross-correlation analysis of the long-term trend of the radiocarbon series and the smoothed d18O series: 82% of the values of the cross-correlation function exceeded the 95% level of significance with the correlation coefficient being 0.6. Similarly not high was the significance of the potential correlation for the high-frequency data. At the same time the cross correlation analysis of the radiocarbon data series, from which the trend corresponding to the curve of the Earth's dipole moment variation was extracted, the high-frequency component of the radiocarbon series and of the smoothed series of d18O variations showed the significant possibility for the existence of correlation (69% of the correlation function values exceeded the 95-percent level of significance), the correlation coefficient being equal to 0.6. Hence, these data prove that d18O variations reflect the main features of the cosmic ray effect.
To sum up, the cyclic changes of climate in the time scale of tens of years to millennia show that the fairly stable climate during the last 10 thousand years experienced the effect of galactic cosmic rays (GCR), modulated by the solar activity the effect of which is reduced significantly by the high magnetic field of the Earth. The period of ~2500 years expressed in the D14C and 10Be data seems to be of the solar origin. A 2400-year periodicity has been established in the Sun motion around the center of the solar system masses [Charvatova, 2000]. Periodicities and variations in the maxima and minima magnitudes often show 1400 to 1600-year cycles in the paleoclimatic data. Historic observations show good correlation between the climate at the time scales of approximately this length and the observed solar phenomena, such as the number of sunspots, as well as auroras.
It should also be noted that during the time interval discussed the modulating effect of the geomagnetic field controlled the lower GCR effect on the Earth's atmosphere, this obviously controlling the warmer climate of the Holocene. The high geomagnetic field also lowered the efficiency of the modulating effect of the solar fields on GCR, as a result of which only the long-term cycles can be traced in the concentrations of cosmogenic isotopes, in particular, the ~2500-year cycle which is followed by the ~2500-year quasiperiodic cooling. On the whole, our analysis suggests that cosmic rays prove their association with a climatic change during the Holocene, when the dipole moment of the Earth remained to be sufficiently high.
A distinctive feature of the time interval of 50 to 10 thousand years ago was a high climate variability. About 20 000 years ago the last glaciation period experienced its maximum. Moreover, characteristic of this period were 3o-5oC quasiperiodic variations in the global temperature (during a few decades of years). This time period experienced three geomagnetic digressions: Gotenburg, Mono, and Lashamp [Petrova et al., 1992], during which the northern geomagnetic pole moved into the Southern Hemisphere and back. It is known that geomagnetic excursions usually take place during the low values of the geomagnetic dipole [Cox, 1969]. Therefore the chosen time interval is favorable for analyzing cosmic ray variations ( 10Be concentration variations in the Earth records), caused by the solar activity and geomagnetic dipole variations, and climatic changes.
The climatic events of the period concerned can be estimated quantitatively and dated exactly, this removing the problem of interpreting these data which are associated with the uncertainty of the age variation with depth. All climate indications derived from the data of ice cores and marine and continental deposits point to the rapid high-amplitude mode variations between the wholly glacial and relatively mild interglacial conditions. These rapid events can be grouped into characteristic cycles of various durations, such as the Bond cycles (10-15 thousand years), Heinrich cold events (about 6-7 thousand years), and warm cyclic events (~2500 years), known as the Dansgaard-Oeschger events [Bradley, 1999]. Without discussing these cycles, it should be noted that the ~2500-year cycle (or the 1400-1600-year cycles) all began with the abrupt 5oC warming for a few decades or a shorter period of time and were modulated during the whole of this time period [Bradley, 1999].
Worthy of noting are the correlation coefficient values and the high degree of certainty of the potential existence of the mutual relationships for the respective pairs of the data: - 0.65 (70%) for the Earth's magnetic moment and the 10Be concentration, +0.71 (72%) for the magnetic moment of the Earth and the 18O concentration, and - 0.89 (53%) for the 10Be and 18O concentrations. The remarkably coincident variations of these data suggest the effect of galactic cosmic rays on the 18O concentration. The pattern of the significant cyclic millennial-year variation of both cosmogenic isotopes and d18O depicts first of all the effect of solar modulation on the galactic cosmic rays, because the dipole moment of the Earth is very low in this time interval. In the case of the unstable climate it becomes colder under the conditions of the low magnetic field and high GCR activity.
The most detailed data for obvious and high climatic oscillations are available for the last glacial-interglacial transition period (15000-11500 years ago). Let us see the pattern of climate and GCR variations in this time interval. Figure 8 shows temperature variations ( d18O) in the Greenland region and GCR variations ( 14C and 10Be) over the time interval of the glacial to interglacial transition. The changes in the 14C and 10Be concentrations demonstrate clearly the long-term changes that lasted thousands of years with the thousand-year changes overlapping them. Therefore, the presence of similar frequencies in the spectra of the data discussed and, moreover, the similarity of the details prove their interconnection. The data presented in the last two figures show that the d18O concentrations are coherent with both the 10Be concentrations and also with the D14C data. It is remarkable that these data series are coherent not only in the phase but also in the amplitude, this providing a reliable proof for a definite relationship in the cosmic rays-Sun-climate chain.
The analysis of a relationship between the climatic changes and the variations of galactic cosmic rays, modulated by the solar activity and by changes in the geomagnetic field, carried out in this study for three intervals of time (the last millennium, the Holocene Epoch, and the period of 50-10 thousand years ago), allow us to make the following conclusions: (a) galactic cosmic rays (GCR) seem to be the main factor that affect the climate; (b) the solar modulation of the galactic cosmic rays controls this effect, and (c) the significant decline of the geomagnetic dipole, compared to the present one, resulted in the higher flow of galactic cosmic rays into the atmosphere and in the cooling of the climate. It appears that the ratio: Q m/Q0 = const(M/M0)-0.5 between the change of the Earth's magnetic moment M and the production rate of cosmogenic Q isotopes in the Earth atmosphere, derived from the experimental data obtained for a short period of cosmic ray observation in the epoch of a comparatively high magnetic moment [Elsasser et al., 1956], cannot be used for very low M values (here M0 and Q0 are the values of the magnetic moment and the production rate of 14C in the present-day epoch).
In order to prove the critical importance of accounting for the GCR effect on the climate, we have to trace a correlation between the GCR intensity variations and the climate characteristics over a longer time scale. In the last years significant attention has been given to collecting detailed data not only for ice cores, but also for marine sediments, which allow one to extend the time scale for hundreds and millions years ago. Frank et al.  analyzed 19 cores from the Atlantic and Pacific ocean floor deposits and obtained changes in the 10Be formation rate for the last 200 thousand years. Combining the data of magnetic measurements in oceanic drill-holes from the 17 localities, Guyodo and Valet  obtained the relative changes of paleointensity of the geomagnetic field for the last 200,000 years.
Being unable to make a detailed analysis of their data here, we can note that the effect of changes in the intensity of galactic cosmic rays and in the geomagnetic field on the climate was much more complex. One of the problems is the variation of the 10Be concentrations in the deep-sea sediments, associated with the uncertainty of Be accumulation rate and tranfer in oceanic deposits during the glacial cycle. High 10Be concentrations might have accumulated in oceanic deposits during the time of warm interglacial periods as result of glacier melting. This might have caused more sediment accumulation and, hence, the higher 10Be concentration in the cores corresponding to the given period of time, whereas ice cores contain 10Be which precipitated from the atmosphere. For lack of reliable data for GCR variations during large time intervals, we analyzed a relationship between the variations in the Earth's dipole magnetic moment obtained from 33 series of the data available for deep-sea sediments [Guyodo and Valet, 1999] and 18O concentrations in oceanic deposits [Shackleton et al., 1990] for the last 800 thousand years. The peaks of the field intensity corresponded to the temperature lows. The correlation coefficient was - 0.6. Note that the long series of these data did not show any stable periodicity, whereas the d18O variation series showed peaks at 23, 41, and 100 thousand years, which are usually associated with terrestrial orbital parameters [Milankovich, 1939].
In the overwhelming majority of studies the large-scale climatic changes with periods of tens and hundreds of thousand years are described in terms of the Milankovich theory. At the same time the analysis of the numerous climatic data available shows that many aspects of explaining the climate in terms of the astronomic theory remain unsolved. The longest climatic cycles seem to result from GCR modulation by the Earth magnetic field and from the effect of some orbital parameters on the galactic cosmic rays. This follows from the fact of finding a relationship between the Earth position and the glacial-interglacial chronology. The mechanism responsible for this relationship may be caused by the solar-latitudinal GCR variations as a result of the latitudinal variations of the solar magnetic fields or as a result of changes in the Earth orbit inclination, the plane of this orbit precessing with a period of 70 thousand years, which must induce changes in the gravitational effect of the Sun and planets on the geomagnetic field which, in its turn, modulates the GCR penetration into the Earth atmosphere. Generally, in terms of large time scales the efficiency of the heliosphere for the GCR penetration into the Earth's atmosphere can vary greatly. This problem calls for a particular discussion.
Let us consider the potential mechanisms of the effects of the solar variability and cosmic rays on the atmospheric processes. It is known that the most reliable data for climatic changes in many regions of the world embrace the time period slightly longer than a century. Using short time scales and based on the direct measurements, the physical processes operating on the sun and affecting the Earth's atmosphere, thus leading to changes of the climate, are changes in the solar radiance [Lean and Foukal, 1988], changes in the solar activity which produce large changes in ultraviolet radiation [Lean et al., 1998], this causing changes in the stratospheric ozone content [Van Geel et al., 1999], and the effect of GCR on clouds and atmospheric circulation, this leading to changes in the ion production rate in the troposphere [Pudovkin and Raspopov, 1992; Pudovkin and Veretenenko, 1995; Svensmark, 1998; Svensmark and Friis-Christensen, 1997]. Each of these potential solar mechanisms has its own advantages and disadvantages, the latter being justly criticized. Nevertheless, each of them rests on the solid physical basis, and at the present time there are reasons to establish a causal relationship between these mechanisms and climatic changes, which will eventually allow us to determine which one (or ones) of them can be potentially important for the problem concerned.
In terms of the potential climatic effect the most vigorous is the total solar irradiation. The discovery of variations in the total solar irradiation (approximately 0.1% for the 11-year cycle), coherent with changes in the spot formation activity of the Sun [Fröhlich and Lean, 1998], suggests the possibility of the direct mechanism of the Sun effect on the climate. Unfortunately, the recorded small variation in the solar energy flow and the limited interval of measurements, where no potential long-term trend in the variation of the Sun energy flow can be recorded, leaves a high freedom of activity for both the proponents and opponents of this mechanism.
The physical mechanism, which does not require large variations in the total solar irradiation, can be associated with the high variations of the complete solar radiation can be associated with high variations in the solar ultraviolet radiation during the 11-year solar cycle (up to 10%). This mechanism is responsible for the formation and destruction of ozone in the stratosphere, which must cause changes in the chemical composition of stratospheric gases and, hence, in the troposphere. However, the problem of stratospheric effects on the troposphere and climate remains to be controversial and partially contradictory. This mechanism of the indirect effect of the solar activity on the lower atmosphere is associated with a necessity to account for complex dynamic processes in the atmosphere. Neither is there any convincing evidence that it may be one of the main mechanisms responsible for climatic changes.
In contrast to the above mechanisms, the physical mechanism associated with the GCR flow attaining the Earth's lower atmosphere can cause changes in the climate by way of affecting the processes of cloud condensation and atmospheric circulation. It is important to note that this mechanism removes the problem of incompatibility between the energy flows from the solar electromagnetic radiation (~1010 erg m-2s-1 ) and from cosmic rays (~102 erg m-2s-1 ).
It has been established that the GCR flux arriving into the Earth atmosphere is modulated by the processes operating in the heliosphere, associated with the variations of the magnetic field, convected from the Sun by solar wind, and varies in connection with the 11-year cycle of the solar activity. The GCR flows have an inverse correlation with the cycle of the solar spots: they have maximum values at the cycle minimum, and minimum values at the cycle maximum. In contrast to the solar cosmic rays, the substantial galactic cosmic ray fluxes of high energy are able to reach the upper layer of the troposphere. Penetrating into the stratosphere and troposphere, the flow of the particles may produce cloud condensation nuclei and, hence, create ionization which is the main cause of the atmospheric conductivity. The GCR fluxes are responsible for the ionization of the Earth atmosphere below 35 km. Geomagnetic activity also affects the GCR intensity and, hence, the cloud condensation nuclei, causing changes in the cloudiness. The growth of GCR fluxes in the Earth's atmosphere must lead to the growth of low clouds, especially in the tropic regions, to the growth of the albedo of these clouds, and to the decline of the lower atmosphere temperature. The effect of the charged particles of cosmic origin on the cloudiness and precipitation has been proved by many geoscientists [Pudovkin and Raspopov, 1992; Pudovkin and Veretenenko, 1995; Roble, 1985; Stozhkov et al., 1996]. Svensmark and Friis-Christensen  showed, as a result of their observations, that ionization may control the global cloud cover. A change in the low clouds during one cycle is about 3-4% [Pallé Bago and Butler, 2002], whereas a change in the GCR level from the maximum to the minimum of the cycle was roughly 10-12%, a change in the ionization at the level of the troposphere turned out to be about 20% of this period.
Clouds play an important role in the Earth radiation budget, capturing the outgoing radiation and reflecting the radiation falling to the atmosphere. It is highly necessary to carry out experimental observations for establishing a correlation relationship between the GCR flux and cloudiness, which would provide a missing connection of the solar-climatic relationships. Recent satellite observations proved the existence of correlation between the GCR flux and the temperature of the upper boundary of the low clouds with a 99.8% significance level [Svensmark, 1998]. Svensmark  proved that a temperature change produced by the GCR effect on the clouds from 1975 to 1989 was 3-5 times greater than the temperature change caused by changes in the total solar irradiation. The temperature of the low clouds was found to be > 273 K, that is, they were warm and consisted of liquid water drops. The subsequent studies, for instance, [Pallé Bago and Butler, 2000], demonstrated that the mechanism for explaining a relationship between clouds and cosmic rays can be found via the role of atmospheric ionization in aerosol formation. In the case of typical atmospheric oversaturation (~1%) a liquid cloud drop is formed only in the presence of aerosol which acts as a condensation center.
Although there is some correspondence between the low cloud cover, observed from the satellites, and the GCR intensity at the high level of significance, the length of the data base is too short to eliminate a number of uncertainties in the attempt to ascertain the long-term behavior of the cloud cover. The proving of a relationship between GCR and the cloud cover will provide a new mechanism for climate changes. Moreover, in contrast to the other mechanisms considered above the study of the GCR effect on the weather and climate has a few advantages, namely:
(1) the galactic cosmic rays (GCR) are recorded in the form of 14C, 10Be, 26Al, and other isotopes of cosmic origin in annual tree rings ( 14C), in ice layers ( 10Be), and in marine deposits ( 10Be and 26Al), this allowing us to trace their detailed chronologic histories over thousands and even tens and hundreds of thousand years;
(2) the snow produced from the clouds under the GCR effect falls to ice cupolas and glaciers and accumulates in the ice layers, the stable isotope ratios in the ice cores providing information on the temperature of the cloud from which snow precipitated;
(3) the study of variations in the concentrations of cosmogenic isotopes over a large time scale, caused by temporal variations of the solar and terrestrial magnetic fields, as well as of variations in the concentration of stable 18O isotope in the ice layers, allows one to estimate the role of this mechanism in climate changes.
Analysis of the data available for cosmogenic nuclides, namely, for 14C in tree rings, for the period of more than 10,000 years and for 10Be in the ice cores for the period of almost 40,000 years provides a convincing proof for the long-term modulation of the intensity of cosmic rays in the vicinity of the Earth by the solar and geomagnetic fields. The history of the climate variations over the studied time interval can be easily traced in the data of stable isotopes from the cores of ice and oceanic sediments.
The results of this study prove the existence of a direct relationship between the long-term variations in the intensity of cosmic rays and the surface temperature of the Earth. Most clearly traced is the relationship between the long-term variations in the intensity of galactic cosmic rays, modulated by the solar and geomagnetic activity, and climate during a period of ~10 to ~40 thousand years from the data available for changes in the concentrations of stable oxygen isotopes and 10Be in the cores of Greenland ice.
Since the ratios of stable oxygen isotopes provide information on the temperature of the cloud from which snow precipitated, and the cosmic rays were the nuclei of this cloud condensation, leading to precipitation from it, the discovered relationship proves that cosmic rays were the main factor affecting the weather and climate during tens of thousand years. Therefore, there is no need to require any significant long-term changes in the solar radiation.
Nevertheless, although there is a strictly substantiated correlation between the galactic cosmic rays, modulated by the solar and geomagnetic fields, and climate over different time scales, further research need be carried out to prove a physical relationship among the galactic cosmic rays, the solar and terrestrial fields, and the orbital movements of the Earth.
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