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

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140

Jürg Beer and Bas van Geel

orbital forcing is slow. Nevertheless, the change in insolation over the past 12 000 years

is considerable and cannot be neglected (cf. Jansen et al., this volume). Figure 6.1a

shows the change in insolation in June for the Holocene period as a function of lat-

itude. At high latitude the change in insolation amounts to 54 W m

−2

. In Figure

6.1b the change in insolation is shown for December. In this case, the insolation

increases from the beginning of the Holocene and reaches a maximum between

4000 and 2000 years BP. The maximum change amounts to 37 W m

−2

. Orbital

forcing is the only forcing that is fully understood, and can be calculated not only

for the past but also for the future several million years (Berger and Loutre 2004).

Solar activity and solar forcing

Since the Sun drives the climate system, solar variability is obviously a serious

candidate for forcing climate change. This is especially so in view of the fact that the

–90

–60

–30

–3

–90

–60

–30

–3

–30

–20

–10

Age [kyr BP]

Latitude

Change in December insolation

–20

Latitude

Change in June insolation

Age [kyr BP]

Insolation (W m

–2

)

Insolation (W m

–2

)

(a)

(b)

Figure 6.1 Changes in

orbital forcing during the

Holocene (12 000 years BP to

3000 years in the future) for

the months June (a) and

December (b). The ﬁgures

show clear trends depending

on latitude. In June there is a

strong decreasing trend in the

north (a). In December, the

insolation on the Southern

Hemisphere ﬁrst increases

and then reaches its

maximum between 2000 and

4000 cal. years BP before

decreasing again (b).

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Holocene climate change and solar and other forcings

141

Sun is a variable star showing considerable cyclic changes in its magnetic activity,

as expressed, for example, in the sunspot number.

In the past, many attempts have been made to ascertain whether the solar radi-

ation arriving at the top of the atmosphere at a distance of 1 astronomical unit

from the Sun is constant (denoted by the solar constant or total solar irradiance,

TSI). All the early attempts, however, failed due to the above-mentioned radiative

atmospheric processes. Only after it became possible to mount radiometers on

satellites outside the atmosphere was it discovered that the solar constant ﬂuctuates

in phase with the magnetic activity of the Sun. It is possible to divide the measured

total solar irradiance (TSI) into three components: a background component, a

darkening component controlled by the sunspots, and a brightening component

related to the faculae, which overcompensate the negative effect of the sunspots

and lead to a positive correlation between TSI and solar activity (Fröhlich and

Lean 2004).

The instrumental data from almost the past three decades show that the change

of the TSI over an 11-year Schwabe cycle is about 0.1 percent, corresponding to

0.25 W m

−2

(the global mean value at the Earth’s surface), an estimate that is quite

small compared with the 3.7 W m

−2

estimated for a doubling of CO

. The variabil-

ity of the solar radiation, however, is strongly wavelength dependent and reaches

values of more than 100 percent in the UV part of the spectrum. Such large changes

in the spectral solar irradiance (SSI) strongly inﬂuence the photochemistry in the

upper atmosphere, and in particular the ozone concentration. Model calcula-

tions show that through dynamical coupling SSI changes can cause shifts in the

tropospheric circulation systems and therefore change the climate (Haigh and

Blackburn 2006).

From a climate perspective, changes in forcings on decadal and shorter time-

scales are less important, because many processes within the climate system

occur on much longer time-scales (e.g. the thermohaline circulation, build up of

ice-sheets). Therefore, the crucial questions are whether changes of TSI and SSI

occur on centennial and millennial time-scales, and how large these changes are.

These questions are still being debated. They can be answered in two steps: (i) how

variable is the Sun’s magnetic activity? (ii) how is this magnetic activity related to

TSI and SSI? As we will show, the magnetic variability is indeed larger on longer

time-scales. From the solar physics perspective, however, it is not yet clear if this is

also the case for the TSI and the SSI. On the other hand, paleoclimate reconstruc-

tions provide growing evidence for larger changes in solar forcing than has been

experienced during the past 30 years (Bond et al. 2001; Neff et al. 2001; Wang et al.

2005; Haltia-Hovi et al. 2007).

The longest historical record of solar activity is the sunspot record, which goes

back to 1610 when the telescope was invented. It shows the well-known 11-year

Schwabe cycle superimposed on a generally increasing trend from 1610 to the

present, which is interrupted by distinct periods of low solar activity (the Maunder

minimum of 1645–1715, and the Dalton minimum of 1795–1820).

To extend this record of solar activity beyond the era of direct observations we

have to rely on indirect proxy data. Such data can be derived from measurements

of the cosmogenic radionuclides

Be and

C in natural archives such as ice cores

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Jürg Beer and Bas van Geel

and tree rings (Beer et al. 1990; Stuiver et al. 1991; Muscheler et al. 2004).

Cosmogenic radionuclides are produced continuously in the atmosphere as a

result of the interaction of galactic cosmic rays with nitrogen and oxygen (Masarik

and Beer 1999). The higher the cosmic ray intensity, the larger the production rate,

and vice versa. The cosmic ray intensity is modulated by two magnetic effects: the

solar activity and the geomagnetic ﬁeld. Depending on the magnetic activity, the

Sun emits plasma (solar wind) carrying magnetic ﬁelds into the heliosphere, which

acts as a shield and reduces the cosmic ray intensity. The second shielding effect is

due to the geomagnetic ﬁeld, which prevents cosmic rays with too low an energy

from penetrating the atmosphere and producing cosmogenic radionuclides. After

production, the fate of the cosmogenic radionuclides depends on their geochem-

ical properties.

Be becomes attached to aerosols and is removed from the atmo-

sphere mainly by wet deposition within 1–2 years. Ice cores are excellent archives

to measure

Be and to reconstruct its production rate in the past. On the other

hand,

C forms

and exchanges between atmosphere, biosphere, and ocean.

As a consequence of the large size of these reservoirs and the long residence times

(ocean: 1–2 kyr), the amplitude of an observed

C change in the atmosphere is

considerably smaller than the corresponding change in the production rate and the

C change is delayed. The ideal archive for the reconstruction of past

C changes

are tree rings.

Although the physics of the production processes in the atmosphere are well

understood, the transport from the atmosphere into the archives is rather complex

and has not yet been fully elucidated (Beer et al. 2002). Comparisons between

and

C records show that during the Holocene the production signal was domin-

ant and system effects generally can be neglected in a ﬁrst-order approach. This

means that both

Be and

C provide an independent record of the cosmic ray

intensity of the past. To extract the solar component from this signal the geo-

magnetic effect has to be removed. This can be achieved by taking into account

the changes in the geomagnetic ﬁeld intensity derived from archeomagnetic and

paleomagnetic measurements. The result is a record of the solar modulation func-

tion Φ (Figure 6.2) (Vonmoos et al. 2006). Φ=0 means a completely quiet Sun, a

condition that has probably never occurred. Φ=1000 MeV corresponds to an

active Sun as typical for a solar cycle maximum during recent times. The Φ-record

is characterized by a long-term trend superimposed on which are short-term

ﬂuctuations. These short-term ﬂuctuations can be divided into cyclic and episodic

features. The cyclic features show periodicities around 11 years (Schwabe cycle),

80 years (Gleissberg cycle), 205 years (DeVries or Suess cycle), and 2200 years

(Halstatt cycle) (Stuiver and Braziunas 1993).

Most episodic features are strong negative spikes corresponding to so-called

grand solar minima, periods when the Sun was very quiet as during the Maunder

minimum. It should be noted that the record does not cover the past 300 years BP.

The mean Φ value of the past ﬁve decades is about 700 MeV. This means that

we are presently in a period of high solar activity, and that in the past there were

periods with considerably lower solar activity. Similar conclusions were obtained

by Solanki et al. (2004). Whether these large changes of activity are also reﬂected in

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Holocene climate change and solar and other forcings

143

the TSI and SSI is not known. Therefore we prefer not to speculate about the cor-

responding forcing in W m

−2

. There is clear evidence for larger changes from

paleoclimatic records, as we will show below. Most authors have used the Δ

record as an indicator of past solar activity. However, Δ

C reﬂects the deviation of

the atmospheric

C content relative to a standard value, so it also contains a long-

term geomagnetic and a system component which perturbs the solar signal to

some extent. Consequently, the

Be ﬂux and the

C production rate are better

proxies, although they still contain a geomagnetic component.

In Figure 6.3, the x-axis is based on tree ring counting. At 850 bc in Figure 6.3a, a

rapid decrease of the radiocarbon age occurs followed by a plateau. The decrease

corresponds to the Δ

C rise in Figure 6.3b (red data points). The blue curve in

Figure 6.3b depicts the changes in the radiocarbon production that are needed

to cause the observed Δ

C ﬂuctuations. Note the difference in the amplitudes

(ca. 40 percent in production, ca. 2 percent in Δ

C) and the lag of the Δ

C peak by

about 20 years relative to the production. These system effects are caused by the large

C reservoirs (ocean, atmosphere, biosphere) and the exchange between them.

Volcanic forcing and ice cores

Volcanic eruptions are the main cause of strong short-term (annual) climate forc-

ing. The injection of large amounts of gases (SO

, CO

, H

O, N

) and dust into the

atmosphere perturbs the radiative balance via enhanced absorption and scattering

of solar radiation leading to a warming in the upper atmosphere and a cooling in

the lower atmosphere (Figure 6.4). Due to the short tropospheric lifetime (1–3

weeks) the injection of gases must reach the stratosphere (lifetime: 1–3 years) to

become globally active. The most important substance is SO

, which oxidizes

quickly to H

. Beside volcanic eruptions, there are other comparatively stable

200

400

600

800

1000

02000400060008000

10 000

low-pass 100 low-pass 500

[MeV]

e (cal yr BP)

Figure 6.2 Reconstruction of

the solar modulation function

Φ covering the period

9300–340 cal. years BP.

depends on the intensity of

the solar open magnetic ﬁeld,

which reduces the cosmic ray

ﬂux and therefore the

production rate of

cosmogenic radionuclides.

This record is based on

data from the GRIP ice core

and shows short-term as well

as long-term ﬂuctuations. The

average solar activity of the

past few decades corresponds

to a value of 700 MeV. The

data have been low-pass

ﬁltered with cut-off

frequencies of 1/(100 years)

and 1/(500 years).

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Jürg Beer and Bas van Geel

–10

0.6

0.8

1.2

1000 900 800 700 600 500

2400

2500

2600

2700

2800

2900

Dendro a

e (BC)

Volume percentage

C (per mil)

Radiocarbon age (BP)

C production (rel. units)

Sphagnum

imbricatum

Sphagnum

papillosum

Sphagnum

cuspidatum

Sphagnum

sect. acutifoli

(a)

(b)

(c)

Figure 6.3 (a) The

C calibration curve for the period

1000–500 bc. The horizontal axis shows the calendar age.

The vertical axis shows the

C age in BP (BP: before 1950).

The calibration curve is based on mainly decadal

measurements of exactly dated (dendrochronology) wood.

Fluctuations of the curve are caused by changes of the

cosmic ray intensity due to ﬂuctuations in solar activity. (b)

C is the deviation of the atmospheric

C ratio from

a standard value in ‰ as calculated from the data of the

calibration curve. The blue curve depicts the changes in the

radiocarbon production that are needed to cause the

observed

C data. The Δ

C curve provides information

about the

C production rate and the carbon system in the

past. The interval ca. 850–750 bc represents the phase of

extreme climate at the start of the Sub-atlantic. The “

C-

clock” accelerates during that period (300

C “years” in

about 100 calendar years). The Δ

C curve shows a sharp

rise of atmospheric radiocarbon, related to a temporary

decline in solar activity. (c) Changes in species composition

of Sphagnum during the Sub-boreal–Sub-atlantic transition

in Engbertsdijksveen, a raised bog (after van Geel et al.

1996). Sphagnum species of the section acutifolia prefer

relatively dry and warm climatic conditions. Sphagnum

cuspidatum and S. papillosum prefer a relatively high water

table. Sphagnum imbricatum prefers high air humidity and

cool conditions. A major change started ca. 850 bc, when

solar activity showed a temporary decline (a fast increase of

C).

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Holocene climate change and solar and other forcings

145

sources of SO

such as the marine biota producing dimethylsulﬁde, and nonerup-

tive volcanic emissions. Figure 6.4 is a schematic overview of the main processes

involved in volcanic forcing (Fischer 2003, 2005).

Ice cores are an important source of information on volcanic eruptions. Figure 6.5

shows a record of volcanic sulfate derived from the GISP2 ice core (Zielinski and

Mershon 1997). Using additional information about the volcanoes responsible for

the emissions, Crowley (2000a) estimated the corresponding forcing for the past

1000 years (inset in Figure 6.5). The very sharp peaks indicate that the sulfate is

rapidly removed from the atmosphere. Bay et al. (2004) linked volcanic ash layers

in Siple Dome (Antarctica) with the onset of millennium-scale cooling recorded in

a Greenland ice core and interpreted the results as evidence for a causal connection

between volcanism and millennial climate change. The high volcanic activity

during the main deglaciation of the early Holocene suggests that ice-sheet unload-

ing and/or sea-level rise was responsible for increased volcanism during that

period. One of the difﬁculties in quantifying the volcanic forcing is that with-

out additional information it is impossible to distinguish between regional

tropospheric and global stratospheric eruptions. Whereas one single eruption can

Figure 6.4 Schematic overview of the main atmospheric processes related to volcanic forcing (Fischer 2003, 2005). (From

Robock (2000), plate 1.)

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Jürg Beer and Bas van Geel

change weather patterns signiﬁcantly over a 1–2 year period, a series of eruptions

may have a long-term climatic effect. Castellano et al. (2004) compared millennial

scale volcanic frequencies against δD in the EPICA-Dome-C ice core; they found

no clear evidence for a close relationship between climatic change and volcanism.

Oscillations in the ocean system

An abrupt cold period occurred around 8200 calendar years BP in the North

Atlantic area. It lasted for ca. 300 years and in Greenland ice-core records it is

characterized by a reduction in temperature greater than 1°C, a decrease in ice

accumulation rate, increasing wind speeds, and a drop in atmospheric methane

levels (Wiersma and Renssen 2006, and references therein). A slowing down of

the thermohaline circulation as a result of a freshwater perturbation has been

proposed as the cause of the event. The slowdown resulted in a decrease of the

northward heat transport in the North Atlantic Ocean, leading to pronounced

cooling. The proglacial Laurentide Lakes in front of the Laurentide ice sheet were

most probably the source of the freshwater pulse (Clarke et al. 2003). Model–data

comparisons by Goosse et al. (2002) and Wiersma and Renssen (2006) conﬁrm the

catastrophic drainage of Laurentide Lakes as a forcing mechanism.

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

200

400

600

800

1000

0200040006000800010 00012 000

–12

–10

–8

–6

–4

–2

1000 1200 1400 1600 1800 2000

Forcing [W m

–2

]

Year [AD]

e [yr BP]

Volcanic sulfate [ppb]

Figure 6.5 A record of

volcanic sulfate

concentrations in ppb derived

from H

measurements on

the GISP2 ice core (Zielinski

and Mershon 1997) covering

the past 12 000 calendar years.

The inset shows an estimate of

the sulfate-induced forcing in

−2

by Crowley (2000a).

Note that the forcing can be

very strong, but does not last

long (1–2 years).

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Holocene climate change and solar and other forcings

147

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 radiocar-

bon 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)

C dates from peat

deposits, however, display wiggles that can be ﬁtted to the wiggles in the radiocar-

bon calibration curve. The practice of dating peat samples using

C “wiggle-match

dating” has greatly improved the precision of radiocarbon chronologies since its

application by van Geel and Mook (1989). By wiggle-matching

C 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

C during the early Holocene, the

Sub-boreal–Sub-atlantic transition (Figure 6.3b: a sharp increase of

C production

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Jürg Beer and Bas van Geel

and evidence for wetter conditions in Figure 6.3c), and the Little Ice Age (Wolf,

Spörer, 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 6.6 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.)

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Holocene climate change and solar and other forcings

149

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

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 Δ

C and Φ),

whereas high lake-stands occurred when solar activity was low. Holzhauser et al.

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 sub-

dividing the Holocene.

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