3. Long-Term Solar Activity Variations in the Holocene and Their Connection With Abrupt Climate Change

Figure 3
[7]  Data with a high time resolution of the order of 10-20 years are available for the Holocene for both climate change and solar activity variations. This allows a more precise comparison of the development of climatic processes and solar activity variations than for the Pleistocene. Information on solar activity variations in the Holocene is obtained from measurements of the concentration of radiocarbon 14C ( Δ14C) in dated tree rings. At present a series of 14C data has been built for the last 11,400 years [Stuiver et al., 1998]. This series is shown in Figure 3. Variations in the 14C concentration in tree rings reflect slow variations in the geomagnetic field magnitude and also, as indicated above, faster solar activity variations. It can be seen from Figure 3 that the periodicity of occurrence of deep solar minima (maxima in the 14C density) is 2300-2400 years. The first of these minima corresponding to the Maunder minimum was accompanied by a sharp cooling in Europe and the start of development of the so-called Little Ice Age [Eddy, 1976; Shindell et al., 2001; Soon and Yaskell, 2003]. During the next, Homeric, minimum around 2700-2800 years BP, cooling and increase in humidity in Europe and climate change on other continents were also observed [Dergachev et al., 2005; Raspopov and Dergachev, 2003; Raspopov et al., 1998, 2000, 2005; van Geel et al., 1998]. The solar minimum around 5400-5200 years BP was also accompanied by abrupt climate change [Thompson et al., 2006]. At present data on climate change during the next deep solar minimum ~7000 years BP are available: advance of glaciers occurred in Alaska and Canada [Koch and Clague, 2006]. However, it should be borne in mind that the development of this solar minimum took place on the background of the climatic optimum of the Holocene, and cooling could be not so noticeable. The next two deep solar minima also fall on the years of abrupt climate changes, i.e., near the boundary of the Holocene and Younger Dryas. Thus, almost all deep solar minima that occurred with a periodicity of 2300-2400 years were accompanied by large-scale climate changes. Along with this, analysis of abrupt climate changes in the Holocene has shown that not only deep solar minima. but also other processes caused coolings and abrupt climate changes.
Figure 4
This is evident from the data presented in Figure 4. The upper panel of Figure 4 shows 200-year averaged data on the content of potassium (K) and sodium (Na) aerosols in Greenland ice for 11,400 years [Mayewsky et al., 2004]. These data contain information on the character of atmospheric circulation in the North Atlantic region. A 2300-2400-year periodicity in the atmospheric circulation is clearly seen in the figure. The lower panel shows results of wavelet filtering (Morle basis) of variations in the 14C concentration shown in Figure 3 for the range of periods 2000-3000 years. It can be seen that the solar minima repeating with a periodicity of 2300-2400 years coincide with increases in the contents of aerosols in Greenland ice, which points to intensification of the atmospheric circulation during these time intervals. Figure 4 also shows time intervals of glacier advance in Central Asia, the Southern Hemisphere, North America, and Scandinavia [Denton and Karlén, 1973; Haug et al., 2001] and also the time intervals of glacier retreat in Switzerland [Hormes et al., 2001]. It is evident from Figure 4 that the 2300-2400-year solar activity variations in the years of solar minima are accompanied by glacier advance. However, there are also the time intervals of cooling (glacier advance) that occurred at a high solar activity level. These intervals (around 1200-1400, 4000 years BP) are marked by arrows in Figure 4. Coolings during these time intervals are evidenced by the data on displacement of the Northern timberline in Northern Scandinavia and Canada [Grudd, 2006; Helamma et al., 2004]. The timberline is a sensitive indicator of climatic conditions and ecological situation on the whole. It corresponds to a mean July isotherm of +11.5oC. A number of authors reported on multiple variations in the latitudinal and altitudinal timberline in Scandinavia and North European part of Russia derived from palynological and dendrochronological data [Bjune et al., 2004; Kultti et al., 2006; MacDonald et al., 2000]. The maximum northward extent of forest was observed in the interval 4300-4000 years BP. Beginning from 4000 years BP, a southward retreat of the timberline associated with cooling has been taking place everywhere.

Figure 5
[8]  Figures 5a,c show variations in the altitudinal timberline in Northern Finland and Northern Sweden, respectively, for the Holocene. Figure 5d presents time distribution of the number of subfossil logs used for plotting Figure 5a. In essence, this distribution also gives information on the timberline displacement in Northern Finland. The curves demonstrate that the timberline displaced around 5400-5200, 4200-3800, 2700-2200, 1500-1400, and 800-600 years BP. The first, third, and fifth time intervals correspond to deep solar minima, and the second and forth intervals correspond to advance of glaciers shown in Figure 4. Figure 5e presents variations in the high-latitude timberline in Canada, and Figure 5f shows reconstructed temperature variations in this region. Like in Scandinavia, coolings around 5000, 3800, 2500 and 800 years BP are observed. Note that the curves of average annual temperature in Scandinavia plotted on the basis of palynological data by Seppä and Poska [2004] and Heikkila and Seppä [2003] demonstrate temperature variations in the time intervals 5400-5100, 4300-3800, and 2800-2200 years BP. During these periods, a slight temperature increase was at first observed, then the temperature sharply fell, and after this an abrupt warming occurred.

[9]  Analysis of palaeoclimatic data has shown that climate changes around 4200-3800 and 1500-1300 years BP had a global character [Mayewsky et al., 2004; Ristvet, 2003]. While in Northern regions, such as Scandinavia and Canada, they were accompanied by coolings, in southern regions, such as, for example, Mesopotamia and Mexico, they were accompanied by the droughts that led to collapse of civilizations, such as the Akkadian Empire and Maya civilization, respectively [Gill, 2000; deMenocal et al., 2000; Hodell et al., 1991]. It is likely that the reason for such abrupt climate changes, which are not associated with deep solar minima of the Maunder type, is the development of internal processes in the atmosphere-ocean system.

[10]  Analysis of bottom sediments of the North Atlantic has shown that ice-rafting events occurred in the North Atlantic during both the Holocene and Pleistocene. A vivid example of ice rafting and a massive freshwater outburst is the outbreak of Lake Agassiz from the North American continent around 8200 years BP, which is well fixed in different palaeodata [Renssen et al., 2001; Rohling and Palike, 2005]. This outbreak resulted in cooling on the global scale and was recorded in bottom sediments in the North Atlantic by an increase in the density of haematite grains and Iceland spar [Bond et al., 2001].

Figure 6
[11]  Comparison of the IRE development and solar activity variations (Figure 6) indicates that the IRE onsets coincide with or follow the time intervals of a high solar activity. This is logical because, as simulation has shown, the IRE development results in an increase in the ocean surface temperature [Shaffer et al., 2004], which must indeed occur in the case of a higher solar activity and, hence, a higher level of solar irradiance. It becomes evident from comparison of the times of glacier advance (Figure 4b) and time intervals of IRE (Figure 6) that the time intervals 1500-1400 and 4200-3800 years BP correspond to development of IRE (no. 1 and 3 in Figure 6) and, hence, the IRE could be stimulaters of abrupt global climate changes during these time intervals. Thus, abrupt climate changes in the past could be caused by not only a low but also a high level of solar activity.


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