SECULAR AND LARGE-SCALE CHANGES IN SOLAR ACTIVITY, COSMOGENIC ISOTOPES

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					 Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)

      SECULAR AND LARGE-SCALE CHANGES IN SOLAR ACTIVITY,
          COSMOGENIC ISOTOPES AND CLIMATE CHANGES

                                                 V.A.Dergachev
          Ioffe Physico-Technical Institute of RAS, St.-Petersburg, 194021, Russia, e-mail:
                                     v.dergachev@mail.ioffe.ru


         Abstract. There is a grows body of evidence from a multi-proxy palaeoclimate records that
         hundred years and millennial years periodic climatic events have persisted during the last 10000
         years, as for instance the well-known “Little Ice Age” and cold episode about 2800 cal yr BP
         separated by about 2400-year time interval. Ice rafted debris in marine cores of the north Atlantic,
         which are attributed to changes in the north Atlantic deep water formation and probably forced by
         changes in solar activity, demonstrated about 1500-year cycles, which rather appear to reflect
         atmospheric circulation variations, ice sheet fluctuations and oceanographic changes. It is well
         established that the production of cosmogenic isotopes, such as 14C and 10Be, is modulated by solar
         activity and may thus serve as a proxy for solar activity changes. The 14C and 10Be signals from
         well-dated samples show similar trends during the last 10000 years. Removing the effects of the
         Earth’s magnetic field from the measured 14C concentration in tree-ring yields the residual
         radiocarbon signal, which potentially reflects changes in solar activity. As demonstrated by
         spectral analysis of sunspot numbers and reflected in the 14C proxies, solar activity displays a
         cyclic behavior with short-time, secular and large-scale periodicities. Hence, if solar activity is the
         driving force behind climate changes, these cyclicities should be observable in climate records.
         Evidence of warm and cold periods and of cyclic climate variability connected with secular and
         large-scale changes in solar activity are demonstrated by this work. Large-scale climate changes
         recognized as global events suggest periodicities of about 2400 years. The observed 210-year
         climate periodicity corresponds to secular changes in solar activity, such as the Maunder or
         Spoerer minimum. Direct solar forcing may account for a significant amount of the climate
         variations observed during the Holocene.


Introduction

The climate of the last millennium has been the subject of much debate in recent years, both in the scientific
literature and in the popular media. The most debated issue in contemporary science is the cause or causes of
global warming – the increase of approximately 0.8±0.1 0C in the average global temperature near the
Earth’s surface since 1900 year. The IPCC report (Climate Change 2007) concludes that the observed
warming is due to the increase in anthropogenic greenhouse gas concentration in the atmosphere. As to the
natural causes of global warming it is reported that the contribution of solar variability is negligible, to a
certainty of 95%.
      Presently, there is a grows body of evidence from a multi-proxy palaeoclimate records that the earth’s
climate experienced rapid cyclical climate change - medium-lived (hundred years) and long-lived (millennial
years) periodic climatic events - during the last 10000 years. Proxy data document mid-Holocene warming
of the Arctic as well as the Antarctic (Mayewski et al. 2004). This Holocene warming appears to be strongly
linked to solar variability and not to the greenhouse gas forcing.
      In order to understand the Holocene climate history and the forcing for natural climate variability at
decadal to millennial timescales during this epoch, records of climate variability to have the finest possible
temporal resolution and greater chronological control. Documenting the extent and persistence of centennial-
and millennial-scale variability requires global coverage. Proxy climate indicators include information
obtained from documentary and cultural sources, ice cores, glaciers, boreholes, speleothems, tree-growth
limits, lake fossils, mammalian fauna, coral and tree-ring growth, peat cellulose, pollen, phonological data,
and seafloor sediments. On the assumption that the Sun and cosmic ray intensity are the major driver of
climate changes (van Geel et al. 1999), the 14C concentration record has been used as a measure of changes
in cosmic ray flux, and solar activity in the past.
      Climate proxy records of high resolution are analyzed to demonstrate major changes in these variables
over the Holocene. A comparison between these changes in climate and changes in cosmogenic isotopes is
carried out to establish a relationship between solar variability and climate changes. The main archives in

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 Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)

this study are hydrological and atmospheric circulation changes. Lake basins present highly sensitive
archives because lake-level record can document past changes in the water budget in relation to climatic
changes. In addition, hydrology is more important for people in some cases than temperature variability.

EVIDENCE FOR REGULAR CLIMATIC CHANGES IN THE PAST

During the Holocene large fluctuations in hydrology and atmospheric circulation, as revealed by a number of
archives and proxies, took place on the continents with distinct amplitudes both in the Northern Hemisphere
and in the tropics and subtropics. The main attention in continental palaeohydrology is devoted to the
analysis of information from groundwater, mountain glaciers and permafrost, lake, wetland, soil and river
systems.
     Lake levels are influenced by climatic parameters affecting both evaporation and precipitation. To
reconstruct a Holocene mid-European lake-level record Magny (2004) used a data set of 180 radiocarbon,
tree-ring and archaeological dates obtained from sediment sequences of 26 lakes in the Jura, the northern
French Pre-Alps and the Swiss Plateau. The dates were separated into two groups, i.e. lower lake-level
versus higher lake-level episodes. The phases of high lake-level are characterized by a deposition of more
mineralized sediments, whereas the phases of low lake-level are characterized by an extension of peat or
organic detritus accumulation in the nearshore areas. According to a quantitative reconstruction of climate
variables, phases of higher lake-level coincide with an increase in annual precipitation, a decrease in summer
temperature and a shortening of the growing season. Fig. 1 shows that the dates form clusters suggesting an
alternation higher lake-level phases that point to a rather cold Holocene climate. There are clearly ca. 2000-
year quasi-periodicity in cold climate change. Thus, the mid-European lake-level record testifies to a
significant instability of the Holocene climate.




    Fig. 1. Distribution of the dates of higher lake-level events reconstructed in the Jura mountains, the
northern French Pre-Alps and the Swiss Plateau over the Holocene period (Magny, 2004). The vertical
scales represent the number of dates for successive 50 years intervals between 12,250 and 0 cal yr BP.

     The Holocene wetting of the northern desert belt of Africa was studied by Gasse (2005). Lake, pollen
and speleothem records registered weakening of the summer Indian and African monsoons and dry spans
interrupted the Holocene wet period (Fig. 2). As can be seen from Fig.2, major Holocene droughts are
repeated every 2000-2500 years.
     On the basis of the results of palynological research on two cores from the Song Hong (Red River) delta
in the sub-tropical zone of Asia, centennial- to millennial-scale climate changes and human impacts during
the Holocene were clarified by Li et al. (2006). Three cycles of cooling and warming were identified during
the last 5000 year: a cool and wet climate during 4530–3340 cal yr BP, 2100–1540 cal yr BP, and 620–130
cal yr BP, a warm and dry climate during 3340–2100 cal yr BP, 1540–620 cal yr BP and the present warm
climate. The first and last cooling events correspond to global Holocene cooling events, the Neoglacial
Period and the Little Ice Age, respectively. Each persisted for 500–1000 yr, and they occurred at intervals of
1500–2000 years.


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 Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)




    Fig.2. Comparison of lake-level fluctuations in the Ziway-Shala Lake in the Sahara–Sahel (Hoelzmann
et al. 1998) and Ethiopian Abhe Lake (Gasse 2000), reflected Indian and African monsoons, with dry spells
due to major Holocene droughts (Gasse 2005).

     The storm chronology was inferred by Noren et al. (2002) from terrigenous sedimentations in-wash
layers, which reflect rainfall events of exceptional intensity/duration in the 13 lake drainage basins in the
northeastern United States. The frequency of storm-related floods in the northeastern United States has
varied in regular cycles during the past 13,000 years, with a characteristic millennial periodicity. Maxima of
terrigenous influx coincide with high storminess and flooding episodes in other records from the North
Atlantic area, and with cool periods in Greenland and Europe as recorded in glaciers by Hormes et al.
(2001).
     Presently, there is a growing body of evidence that short-lived periodic events have persisted into the
Holocene epoch as for instance the 8200 cal BP and 2800 cal BP cold periods (e.g., Dergachev et al 2004;
Veski et al 2004) together with the perhaps more well-known ‘Little Ice Age’ (Matthews and Briffa 2005).
     Neff et al. (2001) presented a high-resolution study of variation in the Indian Ocean monsoon during the
time span from 9600 to 6100 BP derived from oxygen isotope variation (the stable isotopes are used to
provide information concerning climate changes) in a Th/U dated speleothem from Oman. The speleothem
δ18O values serve as a proxy for estimating variation in monsoon intensity by measuring past changes in δ18O
of monsoon rainfall as recorded in speleothem calcite δ18O. In the time span from 8500 to 8000 cal year BP
there are strong peaks at 8400, 8200 and 8000 cal yr BP with the 200-year periodicity (Neff et al. 2001) in
δ18O data similar to the pattern of climate change during the Little Ice Age in the past millennium. As was
shown by Fleitmann et al. (2003) from δ18O monsoon record in a stalagmite of Qunf Cave in Southern Oman
(17°10’ N, 54°18’ E; 650 m above sea level), between 10,300 and 8000 BP decadal to centennial variations
in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores. Taking
into account both the stalagmite and GRIP records, decadal scale intervals of reduced monsoon precipitation
(more positive δ18O values) correlate with cooling events in Greenland and vice versa, as best expressed at
9100 and 8200 BP.
         Olsen (2007) discussed the climate variability based on the Blinden Lake (Denmark) record in
relation to regional and northern hemisphere climate by combining the sedimentological and geochemical
evidence. An estimate of the paleolake water isotope composition (δ18Ow) and changes of the lake water
level (ΔW) and thereby also an effective humidity were derived. Figure 3 presents the wavelet power
spectrum of the inferred δ18Ow and ΔW values.




    Fig. 3. The absolute values of the wavelet coefficient using a morlet wavelet on the δ18Оw (lake water
isotope composition) and on ΔW (lake water level) from Blinden Lake (Denmark) sediment.
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 Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)

Both wavelet power spectra reveal a similar pattern, verifying that most of the observed variance is likely to
originate from changes in the isotope composition of the lake water and probably reflects changes in the
evaporation to inflow balance. The wavelet spectra suggest periodicities of mainly 210 years.

CLIMATE CHANGES, SOLAR ACTIVITY AND COSMOGENIC ISOTOPES

Understanding the mechanisms and history of natural climate variability is important for improving climate
predictability and properly attributing ongoing climate changes to human and natural forcings. The general
state of the Earth's climate is controlled by the balance of energy on the Earth received from the Sun and the
amount of energy released back to space. The Sun provides more than 99% of the energy to the Earth’s
climate. Causes of climate change involve any process that can alter this global energy balance. Energy from
the Sun drives the Earth’s weather and climate. A number of studies have sought to find correlations between
the changes in solar activity and the temperature of the Earth’s atmosphere. Good correlations have been
found for the past millenium on a time scale of decades to centuries (e.g., Solanki et al. 2004). It should be
particularly emphasized that solar activity during the Little Ice Age is extremely weak, and during the
Medieval Warm Period is high.
     Cooling and glacier advances during the Little Ice Age are widespread at high northern latitudes. For the
low altitudes new high resolution lacustrine records (Verschuren et al. 2000) show that equatorial East Africa
experienced humid condition. In equatorial Africa, lake levels can be used as an indicator of climate
changes. It is interest to consider past levels in Lake Victoria (Stager et al. 2005) and Lake Naivasha
(Verschuren et al. 2000). Lake Victoria, located on the equator between the two main branches of the East
African Rift Valley system is well situated to record large-scale climate events that affected not only tropical
Africa but also the polar regions. About 90% of the lake's water arrives and exits through the atmosphere,
making it extremely sensitive to changes in rainfall. The balance of this lake is regulated by evaporation
processes as a result of solar variability
     Fig. 4 shows the comparison between each of the lakes and the proxy of solar activity – radiocarbon
concentration (∆14C) measured in tree rings. As can see from this figure the Victoria and Naivasha basins
were unusually arid during Europe's Medieval Warm Period and unusually wet during cool phases of the
globally distributed Little Ice Age.




    Fig. 4. Comparison of proxy records for changes in the hydrology with the proxy for solar activity based
on the ∆14C record. SWD is the shallow water depth. The minima of solar activity are W –Wolf, S –Spoerer,
M – Maunder, D – Dalton.

    Comparison between atmospheric radiocarbon and hydrological data from tropical Africa demonstrates
the relationship between a variable solar activity and climate. Maasch et al. (2005) compared eight well-
dated high-resolution records, reflecting the range and rate of change of atmospheric circulation and
hydrology, obtained at latitudes extending from the Arctic to the Antarctic with the ∆14C record over the 2
millennia and showed that such relationship is seen on a global scale.
    Exact measurements of the 14С concentration in year-by-year tree rings allow to trace continuous long-
term changes in level of solar activity during more than the last 10 thousand years (Stuiver et al. 1998).
Vasiliev and Dergachev (2002) analyzed the primary properties of the decadal data of ∆14C record during the
Holocene using power spectrum, time-spectrum and bispectrum analysis. They established that the
amplitudes of the radiocarbon content vary periodically in time, the changes of amplitudes are synchronous
in the wide frequency band. A bispectrum analysis of data demonstrates the existence of amplitude
modulation with period of ~2400 years. In addition, a bispectrum analysis allows to classify three primary
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 Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)

lines of the power spectrum: 710, 420 and 210 years, and was shown that the line component corresponding
to 210 years has first harmonics. The long period of ~2400 years is the characteristic property of major
climatic changes in Fig. 5.




     Fig. 5. Comparison of the Polar Circulation Index from GISP2 (Mayewski et al. 1997) with the mid-
European lake-level fluctuations (Magny 2004), with the ice-rafting debris events in the North Atlantic
Ocean (Bond et al. 2001), and with the atmospheric residual 14C contcentration (Stuiver et al. 1998) during
of the Holocene.

     The mechanism for the connection between solar variability and atmospheric circulation may be due to
solar ultraviolet radiation or cosmic ray flux modulated by solar activity. Changes in ultraviolet radiation
from the Sun may lead to a change in ozone production in the lower stratosphere accompanied by the change
in tropospheric dynamics, whereas cosmic ray flux changes may directly lead to a change in global cloud
cover, as demonstrated by the correlation between the variation in cosmic ray flux and the observed global
cloud cover. An increase in the low cloud cover due to cosmic ray flux may lead to wetter and cooler
conditions at different latitudes (Svensmark et al. 2007).

CONCLUSION

Thus, the analysis of the numerous varieties of proxy climatic records is indicative of high variability of the
Holocene climate. Furthermore, palaeoclimate records reveal the presence of fairly regular quasi-periodic
patterns of major large-scale global climate changes. A correlation of historical records of solar activity and
climate change and also cosmogenic isotopes, proxies for solar activity, and millennial scale variability in
palaeoclimate records demonstrates the connection between solar variability and climate change. As
mentioned above, cosmogenic isotope records can be used as a measure of changes in solar activity and in
cosmic ray flux in the past. More significant changes in the Holocene climate are characterized by a quasi-
2400-year periodicity in cold conditions possibly caused by changes in solar activity. The Sun-climate
relation is most clearly seen during the Little Ice Age. Additional study is needed to investigate the rate and
change of atmospheric circulation in the past. The change in the processes of atmospheric circulation may
alter the distribution of precipitation both high and low latitudes that may lead to the large fluctuations in
lake levels, monsoon activity and redistribution of moisture and heat on the Earth’s surface.

Acknowledgements

This work was supported by the Russian Foundation for Basic Research (projects 06-02-16268, 06-04-
48792, 06-05-64200, 07-02-00379), Presidium of RAS (program «Environmental and Climatic Changes»),
and Presidium of the St.-Petersburg Scientific Centre of RAS (Regional Programs).

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