"SECULAR AND LARGE-SCALE CHANGES IN SOLAR ACTIVITY, COSMOGENIC ISOTOPES"
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: email@example.com 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 57 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. 58 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. 59 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 60 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. 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