Reactions of the climate system to forcings

It is well known that the climate system shows variability on vastly different time-scales. It is not yet clear as to what extent this variability is caused by internal processes within the climate system, and which role the different forcing factors play. The results of climate model experiments indicate significant internal variability, as well as a significant sensitivity to solar, volcanic, and greenhouse gas forcing (see Goosse et al., this volume; Claussen, this volume). During the Holocene, greenhouse gases did not vary greatly. Volcanic aerosols remain in the atmosphere for only a few years. Therefore, as the focus here is on climate change on decadal to millennial time-scales, we are principally concerned with solar forcing, although we need to recognize that multiple volcanic eruptions can cause climatic effects also on decadal time-scales (Crowley 2000b). There is a rapidly growing number of examples pointing to the Sun as a major forcing factor during the Holocene (Beer et al. 2000, 2006). Here we discuss evidence for solar-induced climate change based on records from various natural archives.

Peat deposits and climate change

Holocene peat deposits, especially the rainwater-fed raised bogs such as those in north-west Europe, are natural archives of climate change (cf. also Verschuren and Charman, this volume). Climate-related changes in precipitation and temperature are reflected in the changing species composition of the peat-forming vegetation (Figure 6.3c). Plant remains can be identified and, by using ecological information about peat-forming species, changes in species composition of sequences of peat samples can be interpreted as evidence for changing local hydrologic conditions, often linked to climate change. The degree of decomposition of the peat-forming plants is also related to former climatic conditions (peat decomposes more under drier conditions; wetter climatic conditions allow plant remains to be preserved better; Figure 6.6). Blytt and Sernander sub-divided the Holocene in alternating periods, which were supposed to represent differing climatic conditions (Blytt, 1882; Sernander 1910) (Figure 6.7; Birks, this volume). Later researchers found the Blytt-Sernander sub-division to be too simple, but the so-called Sub-boreal-Sub-atlantic transition of the Blytt-Sernander scheme, which occurred around 850 bc, is a consistently observed abrupt and intense climate shift. In north-west Europe, there is strong peat-stratigraphic and archaeological evidence that the climate changed from relatively dry and warm to cooler, wetter conditions (Figures 6.3 and 6.6; van Geel et al. 1998). The radiocarbon method is generally used for dating of Holocene climate-induced shifts in peat deposits. Calibration of a single radiocarbon date usually yields an irregular probability distribution of calendar ages, quite often over a long time-interval. This is problematic in paleoclimatological studies, especially when a precise temporal comparison between different climate proxies is required. Closely spaced sequences of (uncalibrated) 14C dates from peat deposits, however, display wiggles that can be fitted to the wiggles in the radiocarbon calibration curve. The practice of dating peat samples using 14C "wiggle-match dating" has greatly improved the precision of radiocarbon chronologies since its application by van Geel and Mook (1989). By wiggle-matching 14C measurements, high precision calendar age chronologies for peat sequences can be generated (Blaauw et al. 2003), which show that mire surface wetness increased together with rapid increases of atmospheric production of 14C during the early Holocene, the Sub-boreal-Sub-atlantic transition (Figure 6.3b: a sharp increase of 14C production

Figure 6.6 Sampling a profile in the former raised bog area of Bargerveen (Drenthe, The Netherlands). The Sub-boreal-Sub-atlantic transition is indicated by an arrow. (Photograph by Bas van Geel. Left, D.G. van Smeerdijk; right, W.A. Casparie.)

Sub Atlantic Sub Boreal Peat

and evidence for wetter conditions in Figure 6.3c), and the Little Ice Age (Wolf, Spôrer, Maunder, and Dalton minima of solar activity). Peat records show that this phenomenon occurred in The Netherlands (Kilian et al. 1995; van Geel et al. 1998; van der Plicht et al. 2004), the Czech Republic (Speranza et al. 2003), and the UK and Denmark (Mauquoy et al. 2002). The production of radiocarbon is regulated by solar activity, and therefore periods of increased mire surface wetness have been interpreted as evidence for solar forcing of climate change (the effects of sudden declines in solar activity).

Lake sediments, glacier variations, and solar activity

Denton and Karlén (1973) linked the radiocarbon record with geologic data such as the extension of glaciers and made important conclusions about the solar

Figure 6.7 Map of southern Norway (after Blytt 1882) showing areas characterized by specific vegetation types related to differences in temperature and precipitation. This pattern was the modern reference for Axel Blytt in his work subdividing the Holocene.

Figure 6.7 Map of southern Norway (after Blytt 1882) showing areas characterized by specific vegetation types related to differences in temperature and precipitation. This pattern was the modern reference for Axel Blytt in his work subdividing the Holocene.

forcing of climate change. Magny (2004, 2007) published a long record of Holocene climate-related water table changes in lakes in south-eastern France and adjacent Switzerland. The lake-level fluctuations closely correspond to the atmospheric 14C fluctuations and therefore also to the history of solar activity (Figure 6.8). Lake levels were low during periods of high solar activity (low values of A14C and O), whereas high lake-stands occurred when solar activity was low. Holzhauser et al.

un o

0 -20 Residual A14C

14 12 10

30 40 50 60 70 80 Solar activity

Figure 6.8 Comparison of various climate parameters ( redrawn after Magny 2004 ) with the detrended A14C and the solar modulation function The records show the following parameters: (a) the Polar Circulation Index (PCI) at GISP2 (Mayewski etal. 1997); (b) the detrended deviation of the atmospheric 14C/12C ratio trom a standard value as an indicator of solar activity; ( c ) lake-level changes after Magny (2007); (d) the amount of ice-rafted debris found in sediment cores as a measure of the southward drift of icebergs in the North Atlantic ( after Bond etal. 2001 ); (e) the solar modulation function of Figure 6.2.

ta oa

PCI in GISP2 (normalized units)

0 -20 Residual A14C

Mid-European lake-level changes

14 12 10

Ice rafted debris in the North Atlantic

30 40 50 60 70 80 Solar activity

Figure 6.8 Comparison of various climate parameters ( redrawn after Magny 2004 ) with the detrended A14C and the solar modulation function The records show the following parameters: (a) the Polar Circulation Index (PCI) at GISP2 (Mayewski etal. 1997); (b) the detrended deviation of the atmospheric 14C/12C ratio trom a standard value as an indicator of solar activity; ( c ) lake-level changes after Magny (2007); (d) the amount of ice-rafted debris found in sediment cores as a measure of the southward drift of icebergs in the North Atlantic ( after Bond etal. 2001 ); (e) the solar modulation function of Figure 6.2.

(2005) compared glacier and lake-level fluctuations in west-central Europe over the past 3500 years and demonstrated synchroneity between glacier advances, periods of higher lake levels, and maxima of atmospheric radiocarbon. Lakesediment records have also yielded strong evidence for relationships between solar forced changes to cooler, wetter climatic conditions and the economy of prehistoric people. Based on changes in proportions of wild to domesticated animals and archaeobotanical data in excavated lake-side settlements Schibler et al. (1997) and Arbogast et al. (2006) concluded that people shifted their caloric intake from domesticated cereals to wild sources of food in response to adverse climatic conditions. There are numerous other examples of sediment and lake-level data that point to a solar link (Verschuren et al. 2000; Patterson et al. 2004; Baker et al. 2005; Stager et al. 2005; Morrill et al. 2006; Wu et al. 2006).

Marine sediments and stalagmites

Precise radiocarbon dating of marine sediments is problematic, because the 14C/12C ratio in the ocean differs from the one in the atmosphere (reservoir effect). Nevertheless, Bond et al. (2001) showed that more ice-rafted debris in the North Atlantic Ocean was transported to the south during periods of relatively low solar activity (low O values). The agreement between cosmogenic isotope fluctuations and ice-rafted debris points to a dominant influence of solar activity changes on the North Atlantic climate (Figure 6.8).

Neff et al. (2001) studied the climate archive of stalagmites in Oman. The oxygen isotope record was interpreted as a proxy for fluctuations in monsoon rainfall. After some adjustments of the U-Th chronology, the oxygen isotope record was linked to the A14C record, suggesting that changes of monsoon intensity are driven by solar activity fluctuations. 818O in stalagmites from the Dongge cave in southern China also show clear evidence for a solar signal in the monsoon variability on decadal to centennial time-scales (Wang et al. 2005). Mangini et al. (2005) reconstructed temperature changes during the past 2000 years based on a stalagmite from a cave in the Central Alps. Based on a high correlation of that record with A14C, the conclusion was made that solar variability was a major driver of climate in central Europe during the past two millennia.

Tree rings and climate

Localized site tree-ring studies mainly reflect the complex ecological processes that operate on small scales in forest ecosystems. Tree-ring density and tree-ring width data enhance our understanding of past temperatures and other climate changes. Extensive sets of tree-ring data can be used for the reconstruction of regional and even hemispheric-scale temperature changes (Esper et al. 2002; Briffa et al. 2004; D'Arrigo et al. 2006). Tree rings are excellent proxies to study short-term effects such as cooling induced by large volcanic eruptions (Briffa et al. 1998).

The reconstruction of long-term climate changes is, however, more ambiguous because of the necessity to remove tree-age-related (biological) trends that do not reflect climate signals. It is particularly difficult to retain long-term climate information on time-scales longer than the individual length of the tree-ring series combined in a mean chronology (Cook 1995). Methods to overcome this limitation and to reconstruct the full spectrum of climate variability include "Regional Curve Standardization" (Esper et al. 2003) and "Age Banding" (Briffa et al. 2001).

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