Battarbee R.W., Binney H.A. (Eds.) Natural Climate Variability and Global Warming: A Holocene Perspective

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Jürg Beer and Bas van Geel

30 40 50 60 70

12 14

0 2 4 6

PCI in GISP2

(normalized units)

Residual Δ

Mid-European

lake-level chan

Ice rafted debris

in the North Atlantic

Solar activity

High level Low level

Time (calendar years BP)

01020 –20

–0.500.5

a. b. c. d. e.

Figure 6.8 Comparison of various climate parameters (redrawn after Magny 2004) with the detrended Δ

C and the solar modulation function Φ.

The records show the following parameters: (a) the Polar Circulation Index (PCI) at GISP2 (Mayewski et al. 1997); (b) the detrended deviation of the

atmospheric

C ratio from 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 et al. 2001); (e) the solar modulation

function of Figure 6.2.

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(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. Lake-

sediment records have also yielded strong evidence for relationships between solar

forced changes to cooler, wetter climatic conditions and the economy of prehis-

toric 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 condi-

tions. 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

C 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 Φ 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 oxy-

gen 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 Δ

C record, suggesting that changes of monsoon intensity are driven

by solar activity ﬂuctuations. δ

O in stalagmites from the Dongge cave in south-

ern 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) recon-

structed 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

C, 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 re-

gional 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).

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Jürg Beer and Bas van Geel

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).

The 850 BC

event: an example of climate change with

socio-economic impacts

As pointed out by Oldﬁeld (this volume) climate change can have important

impacts on human society. The two fundamental characteristics of a change are its

magnitude and rate. Although the former is obvious, the latter is often not taken

seriously enough. Although every change causes problems of adaptation it is much

easier to deal with if it occurs slowly.

The climate shift around 850 cal. bc (Sub-boreal–Sub-atlantic transition), one

of the most important climate shifts during the Holocene, with evidence for cli-

matic tele-connections is such an example. It had strong socio-economic impacts

in areas that were marginal from a hydrologic point of view. In lowland regions in

The Netherlands, the climate shift caused a sudden, considerable rise of the

groundwater table so that arable land was transformed into wetland, where peat

growth started. Farming communities living in such areas were forced to migrate

because they could no longer produce sufﬁcient food (van Geel et al. 1996). In

Figure 6.9, the landscape development in the northern Netherlands is shown for

three successive phases. At the start of the second phase, coincident with an abrupt

decline of solar activity, the atmospheric circulation changed, leading to cooler and

wetter climate conditions. The rise of the water table forced the farmers to migrate

to well-drained areas in the northern Netherlands where salt marshes offered them

new fertile land. Magny (2004) showed that over a period of several millennia the

presence of lakeside villages in south-eastern France and adjacent Switzerland was

strongly linked with lake levels and solar activity. No lakeside villages occurred

after 850 cal. bc.

In north-west and central Europe, the climatic shift around 850 cal. bc preceded

a considerable rise in palynological indicators of human impact on the landscape.

Van Geel and Berglund (2000) suggested a causal link between the climate shift

around 850 bc and the evidence for a subsequent increase in human population

density. They postulated that the climatic crisis in the ﬁrst instance caused an

environmental and social crisis. A collapse of societies resulted in a weakening

of the position of dominating groups, which brought about a change in the social

structure of farming communities. This facilitated the introduction of a new tech-

nological complex, which again created further social change combined with a

leap forward in production, food consumption, and population density.

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Holocene climate change and solar and other forcings

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In south-central Siberia, archaeological evidence suggests an acceleration of

cultural development and a sudden increase in density and geographic distribution

of the nomadic Scythian population after 850 bc. Van Geel et al. (2004a) hypo-

thesized a relationship with an abrupt climatic shift towards increased humidity

(equatorward relocation of mid-latitude storm tracks). The hypothesis is sup-

ported by pollen-analytic evidence. Areas that initially may have been hostile semi-

deserts changed into attractive steppe landscapes with a high biomass production

and carrying capacity. Newly available steppe areas could be utilized by herbivores,

making them attractive for nomadic tribes. The Central Asian horse-riding

Scythian culture expanded, and an increased population density was a stimulus for

westward migration towards south-eastern Europe.

There is strong evidence for climate change in the Central African rain forest belt

around 850 bc (van Geel et al. 1998). Palynological studies point to a drastic

Figure 6.9 The effects of climate change during the period

ca. 950–750 bc in the northern Netherlands. From phase A

to phase B, solar activity abruptly declined (the Sub-

boreal–Sub-atlantic transition). Atmospheric circulation

changes occurred and the Westerlies became more intense.

Precipitation in north-west Europe suddenly increased,

while temperatures declined. As a consequence, a sudden

rise in the water table occurred. Fenlands extended, often at

the expense of arable land. Species composition in raised

bogs changed as a consequence of cooler, wetter climatic

conditions (compare Figure 6.3c). Farmers in marginal

areas could no longer produce enough food and they had to

migrate to well-drained areas. The thermal contraction of

ocean water caused a temporary pause or slowing of the rise

in sea level that had been occurring since the end of the last

Ice Age. Salt marshes occurred for the ﬁrst time along the

coast of the northern Netherlands. These fertile areas were

ideal for farmers who had lost their habitat as a

consequence of rising groundwater tables. Artiﬁcial earth

mounds were constructed for protection.

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Jürg Beer and Bas van Geel

change in the vegetation cover (from predominantly rain forest to a more open

savannah landscape) as a consequence of an aridity crisis. The change of climate

and vegetation was also important for prehistoric humans. A population of farm-

ers migrated from the south into the area. The contrast between this change to dry-

ness in central west Africa and the contemporary increase of precipitation in the

temperate zones ﬁts well with the hypothesis that, after a decline of solar activity,

there was a decrease in the latitudinal extent of the Hadley Cell circulation and

consequently the monsoon decreased in intensity, while the mid-latitude storm

tracks in the temperate zones were enhanced and moved in the direction of the

Equator (van Geel and Renssen 1998). A dryness crisis caused by a weak monsoon

intensity in north-west India after 850 bc also supports this hypothesis (van Geel

et al. 2004b). Furthermore, massive glacier advance in the south-central Andes of

Chile, probably resulting from an equatorward relocation of mid-latitude storm

tracks (like in the Northern Hemisphere), forms part of a wealth of evidence for

worldwide climate change around 850 bc (van Geel et al. 2000).

Evidence from paleodata indicates that the climate shift around 850 bc occurred

suddenly, probably within a decade. The end of the event, however, seems to have

been gradual (a time-transgressive passing of thresholds), so that, given present

knowledge, it is not yet possible to pinpoint an end of the event. Changing climatic

conditions at 850 bc may have been similar to climatic cooling shifts during the

Little Ice Age (Mauquoy et al. 2002).

Discussion

The attribution of climate change to potential causes is a key issue in climate

research. It is very important to be able to separate climate effects due to human

activities from the response due to natural climate factors and internal climate

variability. Climate feedbacks dominate the hydrologic cycle due to the large

amounts of water on our planet and its strong impact on the energy balance (van

Dorland 2006). Climate change caused by freshwater ﬂux may have been restricted

to the early Holocene (Clarke et al. 2003; Wiersma and Renssen 2006).

Since the atmospheric greenhouse gas concentration did not change much

before the start of the fossil fuel combustion by people, greenhouse gas forcing was

not an important forcing on climate during the Holocene, with the exception of

the past few decades. In spite of the fact that single volcanic eruptions are of short

duration the combination of many eruptions can have signiﬁcant climatic effects

on annual to decadal time-scales (Crowley 2000b; Robock 2000; Ammann et al.

2003).

On longer time-scales there is an increasing amount of evidence for solar forcing

of climate change (van Geel et al. 1999; Renssen et al. 2000; Bond et al. 2001;

Mauquoy et al. 2002; Hu et al. 2003; van der Plicht et al. 2004; van Geel et al.

2004a,b; Maasch et al. 2005; Versteegh 2005; Xiao et al. 2006). Climate proxy data

indicate that variations in solar activity are an important driving force of variations

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Holocene climate change and solar and other forcings

155

in climate. The apparently large sensitivity of the climate system to small changes

in solar activity indicates that ampliﬁcation by feedback processes in the climate

system has to be taken into account. Two mechanisms have been proposed for this:

cosmic ray-cloud correlations and UV-ozone atmospheric circulation changes.

Variation in solar UV radiation controls stratospheric ozone production and

may trigger climate change. Haigh (1994, 1999) performed simulations with

climate models to study the relation between the 11-year solar activity cycle, ozone

production, and climate change. A chemical-atmospheric model showed that a

1 percent increase in UV radiation at the maximum of a solar cycle generated

1–2 percent more ozone in the stratosphere. This increased amount of ozone

used as input in a climate model resulted in the warming of the lower stratosphere

by the absorption of more UV radiation. In addition, stratospheric winds were

strengthened, and the tropospheric westerly jet streams were displaced poleward.

The position of these jets determines the latitudinal extent of the Hadley Cell

circulation and, therefore, the poleward shift of the jets resulted in a similar dis-

placement of the descending parts of the Hadley Cells. The change in circulation

ultimately caused a poleward relocation of mid-latitude storm tracks. The opposite

effect to that described by Haigh may have played a role during the climate shift of

850 bc. Reduced solar activity, as indicated by the observed strong increase of

atmospheric

C and

Be, may have resulted in a decrease in stratospheric ozone.

A decrease in latitudinal extent of the Hadley Cell circulation with equatorward

relocation of mid-latitude storm tracks would consequently follow (Haigh 1994,

1999; van Geel et al. 1998).

The second, and more controversial, potential ampliﬁcation factor involves

changes in cosmic ray ﬂux (as a consequence of a ﬂuctuating solar wind) and

its relationship with global cloud cover. According to Pudovkin and Raspopov

(1992), and Raspopov et al. (1997), ionization by cosmic rays positively affects

aerosol formation and cloud nucleation. Svensmark and Friis-Christensen (1997)

found indications for the possible importance of this process. They recorded a

correlation between the variation in cosmic ray ﬂux and observed global cloud

cover over the solar cycle from 1983 to 1995. An increase in global cloud cover was

thought to cause global cooling. The increase in cloudiness and accompanying

cooling corresponds with reconstructed wetter and cooler conditions at mid-

latitude in Europe at around 850 bc, when there was an abrupt and steep increase

of cosmogenic radiocarbon in the atmosphere, but according to van Geel et al.

(2001) the UV-ozone ampliﬁcation mechanism is more probable than the

cosmic–ray–cloud hypothesis. Wagner et al. (2001) also did not ﬁnd evidence for a

cosmic ray climate connection.

Conclusions

The past few decades were the warmest during the past millennium (IPCC 2001;

Osborn and Briffa 2006, and references therein) owing to greenhouse gas-induced

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Jürg Beer and Bas van Geel

warming. The temperature rise during the ﬁrst half of the last century, however, is

clearly linked to increased solar activity (Lockwood et al. 1999). Reconstruction of

the solar variability reveals that, at present, the Sun is in a very active phase, but

shows no clear indication of a trend since 1980 when the satellite-based measure-

ments of the total solar irradiance (TSI) began. Based on paleorecords, and consid-

ering the fact that ampliﬁcation mechanisms for changing solar activity are not

well understood – and therefore cannot yet be sufﬁciently quantiﬁed in climate

models – we conclude that the solar forcing of climate change may be more import-

ant than has been suggested by the Intergovernmental Panel on Climate Change

(IPCC 2001). If the Little Ice Age and the subsequent warming were mainly driven

by changes in solar activity this component of natural forcing plays an important

role in estimating the future climate warming induced by greenhouse gases due to

human activities. A future major decline of solar activity as predicted by de Jager

(2005) may bring further progress in our understanding of present and future

climate change.

Acknowledgments

The authors thank Jan van Arkel (IBED-UvA) for help with the illustrations and

Rick Battarbee and two anonymous reviewers for valuable comments. Investiga-

tions by BvG and colleagues were supported by INTAS and the Research Council

for Earth and Life Sciences (ALW) with ﬁnancial aid from The Netherlands

Organization for Scientiﬁc Research (NWO). The work of JB is supported by the

Swiss National Science Foundation. Dan Yeloff kindly corrected the English.

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