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

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Eystein Jansen et al.

insolation, as was the case for the early to mid-Holocene, would exacerbate this

feature, stabilize the surface layer in the warm season, and produce a stronger

contrast between the euphotic zone proxies and those derived from thermocline

species.

For the thermal maximum this implies that the strong solar insolation

maximum of the early to mid-Holocene originating from summer season orbital

forcing did not manifest itself below the euphotic zone, and was only captured

by the temperature reconstructions derived from phytoplankton with an upper

surface ocean habitat. The difference between the alkenone- and diatom-based

SST reconstructions is most likely due to the calibration of the methodologies. The

diatom transfer functions are calibrated to surface August temperatures, whereas

alkenones are mainly produced in the spring season and calibrated to the mean

annual temperatures of the uppermost surface layer. Those foraminiferal species

(e.g. Globigerina bulloides) that normally calcify above the summer-time thermo-

cline would be expected to show the same pattern as phytoplankton-based proxies.

For foraminiferal-based transfer functions the higher degree of stratiﬁcation will

give a mixed response depending on the blend of species with surface or thermo-

cline habitat preference. In the Nordic Seas, thermocline species dominate the

100

6 7 8 9 10 11 12

Depth (m)

Annual

Winter

Spring

Summer

Temperature (°C)

(a)

(b)

100

1950 1960 1970 1980 1990

2000

1950 1960 1970 1980 1990

2000

1950 1960 1970 1980 1990 2000

Winter

Spring

Summer

Figure 5.6 (a) Seasonal and

annual water temperature

averages for the last 50 years

from Ocean Weather Ship

Mike in the upper 100 m of

the water column. The

averages are based on the

temperature records

presented in (b). (From

(Nyland et al. 2006.)

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The thermal optimum in the North Atlantic

131

foraminiferal assemblage and the resulting transfer function temperatures are

consistent with the O-isotope-based records (Andersson et al. 2003; Risebrobakken

et al. 2003). As a consequence of these ﬁndings, the wide use of N. pachyderma, as

well as foraminiferal transfer functions in paleoceanographic studies from high-

latitude areas, should be qualiﬁed with the information that the reconstruction is

primarily that of temperature in the thermocline, and thus reﬂects winter-time or

annual mean temperatures.

Dynamical responses or radiative forcing only?

So far we have seen that the seasonal character of orbital forcing explains the

thermal maximum and also the temperature reconstructions that do not display

any early to mid-Holocene optimum. The difference between the data-sets reﬂects

seasonality anomalies. It has also been inferred that the early to mid-Holocene

thermal maximum reﬂects enhanced advection of heat, either in the form of

advection of warmer waters from the North Atlantic, by stronger advection of

Atlantic waters towards the Arctic or by enhanced activity of the Westerlies (e.g. a

mechanism akin to that described by Blindheim et al. 2000).

The MD95-2011 data rule out the existence of an advection of North Atlantic

warm water anomalies to the north. The core location lies underneath the main

ﬂow path of Atlantic waters towards the Arctic. At present, the thickness of the

Atlantic water layer in this area is 400–600 m, much thicker than the suggested

depth distribution of the proxy temperature recorders from the site (Figure 5.5).

The advective time constant of the present Norwegian Sea from south to north is in

the order of several years (Skagseth 2004). Therefore, an advective temperature

anomaly would be expected to override seasonal changes and be manifested down

to several hundred meters water depth. The thermal maximum is, however, only

manifest in the upper 50 m, which implies that it is attributable to the radiative

forcing from the orbital conﬁguration. This is consistent with the model results of

Liu et al. (2003) who found a difference between the surface layer and thermocline

waters in the model’s response to orbital forcing. It remains to be tested if this is a

robust feature of model behavior in the Nordic Seas.

Ruling out advective transport of warm water anomalies as an explanation, it is

possible that oceanic heat ﬂuxes towards the Arctic, larger than later in the

Holocene, could have occurred. One possible case is that a larger heat ﬂux could

have been contained in stronger along-slope currents, similar to the positive mode

of the North Atlantic Oscillation (NOA) observed in recent decades (Blindheim

et al. 2000; Skagseth 2004). The available data cannot register such a situation with

stronger ﬂow of warm waters towards the Arctic and without any change in the

mean temperature of the Atlantic water masses. There is, at present, no reliable

kinetic ﬂow proxy data-set from which to evaluate this aspect. Nevertheless, it

seems likely that the polar ampliﬁcation of the thermal maximum is mainly due to

the polar ampliﬁcation of orbital forcing and to sea-ice albedo feedbacks due to the

summer insolation anomaly, as also noted from the PMIP2 models depicted in

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132

Eystein Jansen et al.

Cruciﬁx (this volume, Figure 4.3). This is also consistent with the shorter duration

and early timing of the maximum temperatures near the present sea-ice margin,

which implies a connection to the time of the strongest insolation anomaly along

the sea-ice margin. Further to the south the maximum temperatures occurred

later, after 8 ka (Figure 5.5), and continued at times when temperatures close to the

present sea-ice margin had started to fall (Sarnthein et al. 2003; Kim et al. 2004;

Hald et al. personal communication). Thus the polar ampliﬁcation does not

appear to be mainly due to advection of oceanic heat.

Holocene climate variability

Internal variability is an inherent part of the climate system. On sub-orbital time-

scales internal variability of the ocean and ocean–atmosphere systems has been

used to attribute the Atlantic Multi-decadal Oscillation (e.g. Sutton and Hodson

2005) to multi-century to millennial variability (e.g. Schulz et al. 2007), primarily

suggesting an origin from low-frequency variations in the Atlantic Meridional

Overturning Circulation.

Some studies have suggested that there may have existed different prevailing

atmospheric patterns in the early to mid-Holocene than in recent times, indicating

that atmospheric forcing might have inﬂuenced the ocean response and vice

versa. Rimbu et al. (2004), comparing the alkenone data-set for the Northern

Hemisphere of Kim et al. (2004) with climate model experiments, note that the

SST anomaly pattern appears to be similar to the tripole pattern of the positive

phase of the NAO, suggesting a prevailing situation with a stronger tendency

for meridional ocean and atmosphere heat transport in the eastern sector of the

high-latitude North Atlantic realm. This pattern of model response to 6 ka orbital

forcing is, however, not robust when comparing the coupled AOGCMs used in the

PMIP2 experiments. A PMIP2 model–model intercomparison shows that three of

the nine models support a positive NAO-like atmospheric circulation in the mean

state for the mid-Holocene as compared with the pre-industrial period without

signiﬁcant changes in simulated NAO variability (Gladstone et al. 2005). Some

observational evidence comes from lake studies in northern Scandinavia, indicat-

ing a generally more maritime climate in the early to mid-Holocene than in the late

Holocene (Hammarlund et al. 2002). A general feature of NAO-related responses

in northern Europe is a strong increase in winter precipitation in years of positive

NAO index, i.e. the pattern indicated by Rimbu et al. (2004). When combining

several glacier mass balance records and calculating the winter precipitation

contribution to Norwegian glaciers, Nesje (personal communication) (Figure 5.7)

ﬁnds no strong early to mid-Holocene winter precipitation maximum, thus

documenting lack of evidence for a general NAO positive state. Rather, the glacier-

based precipitation records follow the pattern of cold intervals punctuating the

overall trends and occurring at century to millennial time-scales. This is com-

parable to the pattern of warm and cold phases seen in many other records.

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The thermal optimum in the North Atlantic

133

A feature of many Holocene climate records is that variability on century to

multi-century time-scales often displays somewhat lower amplitudes between

about 8 and 6 ka, and a tendency for an increased amplitude of the variability

towards present (see Figures 5.5 and 5.7). In a number of records there is strong

variability in the early Holocene with the 8.2 ka event being probably the most

pronounced. Since much of this variability and the 8.2 ka event may plausibly be

linked with the ﬁnal demise of the large Northern Hemisphere ice sheets and thus

have a dynamical origin from Ice-Age processes, we focus here on the evolution

after the 8.2 ka event. In Figure 5.5 it is apparent that the amplitude of multi-

century to millennial scale variability is not stationary through the Holocene.

Risebrobakken et al. (2003) showed that there is an increase in the amplitude of

century to millennial scale variability after the mid-Holocene, and that there is very

little persistence in the main frequencies of variability through the Holocene. The

100

.Namredyhcap)%( )nis(

e years BP

100

120

140

160

)%( noitatipicerp retniW

0.04

0.08

0.12

0.16

12 000 10 000 8000 6000 4000 2000 0

oitar galP/ztrQ

MD99-2269

(a)

(b)

(c)

(d)

MD95-2011

Average from 4 Norwegian glaciers

Figure 5.7 (a) Mean winter

(October–April) precipitation

in percent of 1961–90 mean

(= 100%) calculated from

Jostedalsbreen, Folgefonna,

Spørteggbreen, and

Bjørnbreen (compilation by

Atle Nesje, personal

communication). (b) Relative

percent Neogloboquadrina

pachyderma (sin) in MD95-

2011 (Andersson et al. 2003;

Risebrobakken et al. 2003).

(quartz/plagioclase ratio)

from cores MD95-2011 and

(d) MD99-2269 (Moros et al.

2006).

9781405159050_4_005.qxd 6/3/08 3:55 PM Page 133

134

Eystein Jansen et al.

noticeable increased amplitude after about 4 ka in the Nordic Seas thermocline

temperature (Figure 5.5) and sea-ice proxies (Figure 5.7) indicates increased

climate variability affecting winter-time conditions. This pattern is also generally

consistent with the onset of neoglaciation in Europe (Nesje et al. 2000) and an

increase in Arctic sea-ice cover (Koç and Jansen 1994).

We do not have evidence of stronger external forcing throughout the pre-

industrial Holocene. It is therefore likely that the nonstationary aspect of the vari-

ability indicates that the stronger century to millennial scale variability is excited by

changes in boundary conditions, which amplify processes occurring at time-scales

of more than 100 years. The amplitude of millennial to sub-millennial scale

variability appears potentially linked with sea-ice extent, Northern Hemisphere

snow cover, and ocean surface temperature, and indicates that increased sea-ice

cover following the reduced summer insolation may have put in place ampliﬁca-

tion mechanisms leading to stronger ocean temperature variability. One plausible

mechanism, drawing on the general trends of orbital forcing through the

Holocene, is that the reduction in boreal summer insolation and a less pronounced

summertime surface ocean stratiﬁcation induced sea-ice/snow albedo feedbacks

which drove the overturning circulation past a threshold into a more variable

mode of operation. In the absence of a clear attribution of this variability to exter-

nal forcings (e.g. solar, volcanic; e.g. Risebrobakken et al. 2003), it appears most

likely that the century to millennial scale variability is primarily caused by the long

time-scale internal dynamics of the climate system.

Moros et al. (2006) (Figure 5.7) published records of ice-rafted debris (IRD)

in the form of quartz content of marine sediments, presumed to originate from

melting sea-ice. The data show a strong increase in the drift-ice occurrence off East

Greenland in the latest half of the Holocene. This is coherent with orbital forcing,

which led to an increased sea-ice cover in the marginal ice zone. Koç and Jansen

(1994) found a similar pattern based on sea-ice diatom records, although this

study had a much lower temporal resolution. The IRD record from the central

Nordic Seas basically follows the foraminiferal temperature data (Figure 5.7), indi-

cating that sea-ice here is related to protrusions of colder waters to the south-east

and probably related to changes in atmospheric dynamics. The IRD record from

the central Nordic Seas is, in a number of aspects, similar to the IRD data of Bond

et al. (2001), although the different methods and temporal resolution do not make

a detailed correlation possible. Overall, the data of Bond et al. as well as the IRD

data from the Nordic Seas indicate that there is not one single sea-ice response in

the region and that the general sea-ice response appears as one following the

orbital forcing, but punctuated by colder intervals at millennial to century scales

with increased presence of sea-ice and colder thermocline temperatures. As ﬁrst

noted by Risebrobakken et al. (2003), there is not a single persistent variability in

the high-latitude North Atlantic through the Holocene. The emergence of higher

amplitude millennial scale variability in many records appears to be linked to a

threshold reached after the thermal optimum, and caused by decreased summer

insolation and seasonality. The winter-time precipitation records are in line with

this evidence, indicating that the cold intervals in the ocean records are linked with

a colder, less moist winter-time climate over Scandinavia.

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The thermal optimum in the North Atlantic

135

Conclusions

The early to mid-Holocene thermal maximum is clearly documented in SST records

in the high-latitude North Atlantic–Nordic Seas, with a polar ampliﬁcation, and an

earlier and shorter lasting extension in the areas near the modern sea-ice limit in

the Nordic Seas than to the south. Further south, the maximum is most clearly

expressed between 8 and 6 ka. In the Irminger Sea the maximum SSTs were

delayed, probably due to downwind effects from the remnants of the Laurentide

Ice Sheet.

The maximum is expressed by proxy records which sample the uppermost

surface layer above the seasonal thermocline. Proxy data from recorders that

sample the sub-surface do not document the maximum temperatures. This shows

that the SST maximum is mainly forced by the summer insolation maximum and

sub-surface proxy data that reﬂect annual mean or winter-time conditions are not

inﬂuenced by the summer-time insolation forcing.

The origin of the thermal maximum is primarily from orbital forcing and not

advection of SST anomalies from the south. It is difﬁcult to attribute the thermal

maximum to long-term persistent atmospheric patterns.

Superimposed on the SST trends is century to millennial scale climate variabil-

ity. This variability does not occur as a stationary response to speciﬁc external

forcing, but appears primarily to be induced and later ampliﬁed after the thermal

maximum by changes in climate system internal dynamics.

Acknowledgments

The authors are grateful to Atle Nesje for compiling winter precipitation data and

to Øyvind Lie and Trond Dokken for fruitful discussions and comments. This

study is an outcome of the NORPAST-2 project funded by the Norwegian

Research Council, and the MOTIF and PACLIVA projects funded by the European

Union. This is publication A174 from the Bjerknes Centre.

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....

Holocene climate change

and the evidence for solar

and other forcings

Jürg Beer and Bas van Geel

Keywords

Climate change, solar forcing, orbital forcing, volcanic forcing, 850 bc climate event, socio-

economic impacts

Introduction

Future climate change may have considerable effects on the hydrologic cycle and

temperature, with signiﬁcant consequences for sea level, food production, world

economy, health, and biodiversity. How and why does the natural climate system

vary on decadal to millennial time-scales? Do we sufﬁciently understand natural

climate change, and what is the relative importance of natural processes versus

human activities in explaining the global warming of the last few decades? How

much of the recent warming is induced by natural (solar, volcanic) rather than

anthropogenic forcing?

In addition to historical documents produced by people observing environmental

change, nature itself provides a wealth of information stored in various archives

such as ice cores, sediments, tree rings, and speleothems. A growing proportion of

this stored information can be deciphered thanks to impressive developments in

analytic techniques (cf. Birks, this volume). An increasingly clear view of the past

environment is emerging. We are still far away, however, from having a complete

picture of all the physical, chemical, and biological processes involved. The main

problem is identifying all the relevant processes and quantifying the forcing fac-

tors, and the corresponding response of the numerous interlinked components of

the climate system (atmosphere, hydrosphere, cryosphere, biosphere). Therefore,

any primary change will trigger a chain of secondary reactions, some of which

amplify the primary change (positive feedback). These positive feedback mechan-

isms occur on very different time-scales, ranging from minutes (e.g. the ozone

chemistry in the stratosphere) to millennia (e.g. the formation of continental

ice sheets).

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Holocene climate change and solar and other forcings

139

In this chapter we give an overview of the mechanisms causing natural climate

change on decadal to millennial time-scales. Proxy climate records provide the

basis for both a better understanding of the processes involved, and for testing cli-

mate models. Based on selected literature from high-resolution Holocene records

of climate change, we evaluate the role of the various potential forcing factors.

Forcing mechanisms

The climate system generally can be described in terms of energy transport. The

main energy source is the Sun, which emits electromagnetic radiation with wave-

lengths covering the full spectrum and peaking in the visible part (400–750 nm).

On its way through the atmosphere to the Earth’s surface, part of this radiation is

reﬂected, scattered, or absorbed. The absorbed energy (~70 percent) is ultimately

re-emitted into space with much longer wavelengths. Since the incoming solar

radiation covers only half the globe (day side) and peaks at low latitudes, there are

permanent energy gradients on the Earth’s surface, which the climate system tries

to eliminate by transporting energy through the atmosphere and the ocean (ther-

mohaline circulation).

All the radiative processes depend strongly on the composition of the atmo-

sphere (gases, aerosols, dust, clouds), which provide another level of complexity, as

these components are strongly connected to chemical, thermal, and dynamical

changes taking place in the atmosphere on vastly differing time-scales.

Any change in these complex processes can force the climate to change. We

distinguish between external (orbital and solar) and internal (volcanic and ocean

circulation) forcings which we will address in the following sections.

Orbital forcing

The amount of solar radiation arriving at a given point on the top of the atmo-

sphere depends on the solar luminosity (the total amount of radiation emitted by

the Sun) and the relative position of Sun and Earth in space (distance, direction of

the Earth’s rotational axis relative to the ecliptic). Whereas the former depends on

processes taking place within the Sun, the latter is determined by the distortion of

the Earth’s orbit by the gravitational forces of the other planets (mainly Jupiter and

Saturn) (Laskar et al. 2004). Orbital forcing affects three parameters with typical

periodicities: the eccentricity (the deviation of the orbit from a circle, with periods

of ~400 and ~100 kyr), the obliquity (the tilt angle of the Earth’s axis with a period

of ~40 kyr), and the precession of the Earth’s axis (~20 kyr period). Whereas the

eccentricity leads to changes in the mean annual distance between Sun and Earth,

and therefore to changes in the total incoming radiation, the other two parameters

affect only the relative distribution of the solar radiation, with implications for the

energy transport within the climate system. At the time-scale of the Holocene,

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