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

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H. John B. Birks

Christian Vaupell (1821–1862; Figure 2.3) continued investigations on pine

megafossils in Denmark. In addition to providing an ecologic explanation for the

causes of long-term forest succession, he examined the width of the tree-rings of

the buried pines. He concluded (1857) that the pines had not grown in cold condi-

tions, as Steenstrup had proposed in his forest history, but that the pines had

grown under warm but dry conditions. Vaupell suggested therefore that there had

been changes not only in moisture (to explain the changes in peat stratigraphy)

but also in temperature (to explain the changes in tree composition and growth)

during the Holocene. The Swedish paleobotanist A.G. Nathorst (1850–1921;

Figure 2.4) investigated plant macrofossils in the clays underlying the peat in

southern Sweden (Nathorst, 1870) and subsequently elsewhere in Europe

(Nathorst, 1892). He was the ﬁrst to ﬁnd remains of arctic plants such as Dryas

octopetala, Salix polaris, and S. reticulata, plants that today are conﬁned to high

(a)

Figure 2.1 (a) Fossil pine stumps on Rannoch Moor,

western Scotland. These stumps are about 4000 years old

(Birks 1975). (b) Fossil pine stump at Cooran Lane,

Galloway, south-west Scotland. This stump is about

6000 years old (Birks 1975). (c) The “Grenzhorizont” or

(b)

(c)

major recurrence surface at Chat Moss, Lancashire, north-

west England. There is a conspicuous change from dark,

humiﬁed peat to pale, less decomposed peat. This transition

is radiocarbon-dated to about 2600 years ago.

(Photographs: John Birks.)

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latitudes and/or high altitudes. With these discoveries Nathorst transformed

peat-stratigraphic and macrofossil research and early ideas about climate change

(Holmboe 1921). Temperature change immediately became recognized as the

major factor inﬂuencing the history of plants and animals since the last Ice Age as a

result of Nathorst’s macrofossil studies. The ﬁnal acceptance of the Ice Age theory

Figure 2.2 Japetus

Steenstrup (1813–1897),

a Danish scientist who

pioneered the study of plant

and animal remains preserved

in peat as a means of

reconstructing past

environmental change and

who provided one of the ﬁrst

explanations for how pine

trees could once grow on

Danish peat bogs. (With

permission from the Royal

Library, Denmark.)

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H. John B. Birks

by geologists at the same time also contributed greatly to climate change occupying

a key role in Quaternary research (Iversen 1973).

The idea that there had also been changes in temperature during the Holocene

was investigated in detail in Sweden in the early 20th century. Sweden is ideal for

such studies as there are marked south–north gradients in summer warmth today

and hence well-deﬁned northern geographic limits of many thermophilous plants

and animals. Gunnar Andersson (1865–1928) discovered plant macrofossils

(seeds, fruits, nuts, leaves, etc.) preserved in peat bogs. Finds of fossil nuts of

Corylus avellana (hazel) well north of the present range of hazel suggested that the

climate had once been warmer than today (Figure 2.5). The present-day northern

limit of hazel coincides closely with the mean July temperature isotherm of 12°C

today, whereas fossil nuts occurred as far north as the present-day July isotherm of

Figure 2.3 Christian Vaupell (1821–1862), a Danish scientist

who pioneered the use of tree-rings in fossil pine stumps as a

means of inferring past conditions of temperature and

moisture. (Picture from Iversen 1973. Copyright: Geological

Survey of Denmark and Greenland.)

Figure 2.4 Alfred Gabriel

Nathorst (1850–1921), a Swedish

paleobotanist who studied plant

macrofossils in late-glacial and

Holocene deposits in Scandinavia

and elsewhere in Europe. His

discovery of the remains of

arctic–alpine plants in the

clays below Holocene peats

highlighted the large changes

in temperature that occurred at

the onset of the Holocene.

(Photographer: Thor G. Halle,

1916. The Swedish Museum of

Natural History.)

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9.5°C, suggesting a change in July temperature of 2–2.5°C. On the basis of these

and related investigations, Andersson (1902, 1909) emphasized changes in sum-

mer temperature and presented the idea of one long early- to mid-Holocene

period with a “higher-than-today” temperature comparable with the modern

Figure 2.5 The present-day

distribution (red cross-

hatching) of Corylus avellana

(hazel) in Denmark and

Sweden in relation to the

mean July temperature

isotherm of 12°C and the

distribution of fossil hazel

nuts (red dots) of early- or

mid-Holocene age. (From

Wright 1936. Reproduced

with permission of Palgrave

Macmillan.)

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H. John B. Birks

concept of the Holocene thermal maximum. Support for this idea came from

discoveries of macrofossils of Cladium mariscus (saw-sedge), Carex pseudocyperus

(cyperus sedge), Trapa natans (water-chestnut), Emys orbicularis (European pond

tortoise), and other warmth-demanding plants and animals well north of their

present northern limits in Europe.

In Norway, Axel Blytt (1843–1898; Figure 2.6), perhaps inﬂuenced by the work

of Dau (1829), Steenstrup (1841), and Vaupell (1857), interpreted tree layers in

peat bogs and changes from dark, humiﬁed to pale, fresh peat (Figure 2.1c) as

evidence for alterations between dry (continental = Boreal) and wet (oceanic =

Atlantic) periods (Blytt, 1876). He proposed an elaborate theory for the immigra-

tion of the Norwegian ﬂora and its various ﬂoristic elements during these oceanic

and continental periods. Blytt assumed that the ﬂoristic elements of the present-

day ﬂora of Norway (e.g. boreal, atlantic) had immigrated during successive

climatic periods, with the arctic element ﬁrst and the sub-atlantic element last

(Mangerud et al. 1974). In light of the present-day distributions of these elements,

Blytt proposed that the boreal and sub-boreal elements had immigrated during

periods of continental climate, whereas the atlantic and sub-atlantic elements had

immigrated during periods of oceanic climate. In this early work Blytt used the

terms boreal and atlantic to refer to ﬂoristic elements and not to phases within the

Holocene. He widened their use in 1893 after Sernander’s early publications.

The Swedish botanist Rutger Sernander (1866–1944; Figure 2.7), who had strong

interests in both climate history and plant geography (Fries 1950), combined the

Swedish ideas of summer-temperature changes with Blytt’s (1881) moisture

changes to propose the famous Blytt–Sernander four periods of post-glacial

time (Sernander, 1893, 1894, 1908, 1909, 1910 – see Table 2.1). Sernander (1889)

initially used the concepts of Blytt’s “atlantiska period” and “sub-boreala period”

Figure 2.6 Axel Blytt (1843–1898), a Norwegian botanist

who proposed a series of alternating wet and dry periods based

on peat-stratigraphic changes and used these periods to

explain the history of the Norwegian ﬂora. (Photograph by

Carl Størmer with permission from the Norwegian Museum

of Science and Technology.)

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to link Blytt’s ﬂoristic terms atlantic, sub-boreal, etc. to Blytt’s descriptions of peat

stratigraphy in Norway and Blytt’s associated climatic interpretations (Mangerud

et al. 1974). Later Sernander (1890) used all Blytt’s ﬂoristic terms for successive

time periods, characterized by a different climate based on Blytt’s peat investiga-

tions. Blytt (1893) then applied his own ﬂoristic terms to his peat-stratigraphic

layers, thereby introducing the sub-atlantic peat-bed, the sub-boreal stump layer,

the atlantic peat-bed, etc. (Mangerud et al. 1974).

The Blytt–Sernander paradigm of Holocene climate change, in the form that

later became widespread, was probably fully established by Sernander (1894) in his

doctoral thesis at the University of Uppsala (Mangerud 1982). In this remarkable

monograph, Sernander ﬁrst discussed the basis for dating and correlation of geologic

sequences. He regarded the stratigraphic sequences based on plant macrofossils

Figure 2.7 Rutger Sernander (1866–1944), a Swedish

botanist and Quaternary geologist who expanded Blytt’s ideas

into the Blytt–Sernander scheme for Holocene climate history

(see Table 2.1) that became the major paradigm for Holocene

climate until about 1960. (From unknown Internet source

that is no longer on line.)

Table 2.1 The Blytt–Sernander division of the Holocene as proposed by Sernander

(1890, 1894). The Pre-Boreal was added by Fægri (1940) for the earliest Holocene with a

cool–sub-arctic climate

Period Inferred climate Approximate age (years BP)

Sub-Atlantic Cool, wet 0–2500

Sub-Boreal Warm, dry 2500–5000

Atlantic Warmest, wet 5000–8000

Boreal Warm, dry 8000–10 000

Pre-Boreal Cool, sub-arctic >10 000

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H. John B. Birks

and megafossils described by Steenstrup (1841) as the basis for Holocene climate

change in Scandinavia. However, in contrast to other researchers at that time,

Sernander doubted that vegetational changes in, for example, Denmark could be

synchronous with changes in, for example, north-central Sweden because plant

spread over several hundred kilometers must have required a long time. Sernander

thus sought other methods to try to determine the age of these changes. In terms of

climate history, Sernander accepted Blytt’s observations and interpretations and

wrote (Sernander, 1894, p. 71 – translated from Swedish)

“We introduce therefore to the peat bog-sequences in Gotland the terms of Blytt, from

the top downwards: sub-atlantic, sub-boreal, atlantic, and boreal beds, and we will

also use these names for the time periods corresponding to the beds.”

An approximate chronology for these periods was proposed based on correlations

with archaeology, sea-level changes (Littorina and Ancylus transgressions), and

the Swedish varve chronology of Gerard de Geer (1858–1943) by Sernander (1894)

(see Table 2.2). The change from the Sub-Boreal to the Sub-Atlantic was thought

by Sernander to be an abrupt climate change, even a catastrophe, the Fimbulwinter

of the Sagas (Iversen 1973).

The 1910 International Geologic Congress in Stockholm was, according to Knut

Fægri (personal communication 1995), a major event in the dissemination and

Table 2.2 Sernander’s (1894) correlation scheme for the Holocene (modiﬁed from

Mangerud 1982)

Sea-level

phase

Littorina

Ancylus

Polar Sea

Peat bogs in southern

Scandinavia

(Steenstrup,

Nathorst, et al.)

Spruce zone

Oak zone

Pine zone

Aspen zone

Dryas zone

Gotland vegetation

history

Spruce

Oak

Pine

Dryas

with

Salix polaris

Period

Sub-Atlantic

Sub-Boreal

Atlantic

Boreal

Sub-Arctic

Arctic

Correlations

Littorina

transgression

maximum

Ancylus Lake

transgression

maximum

Cladium

Carex

pseudocyperus

Iris pseudacorus

Northern

salix

Betula nana

Dryas

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acceptance of the Blytt–Sernander classiﬁcation scheme. Interestingly there was

close co-operation at this time between Sernander and Lennart von Post (the

founder of pollen analysis – see below) and they organized a major excursion to

Swedish peat bogs (von Post and Sernander 1910). After 1910, the Blytt–Sernander

scheme was widely used in Scandinavia for many decades without any major

change in its meaning or interpretation (Mangerud et al. 1974; Mangerud 1982).

Fægri (1940) introduced the term Pre-Boreal as a unit between the Boreal and the

Younger Dryas of the late-glacial (Table 2.1).

A major conﬂict and acrimonious debate ensued for about 20 years between the

“grand old men” (Lundqvist 1965) of Swedish post-glacial climate research,

namely Gunnar Andersson and Rutger Sernander. A rapid polarization of ideas

developed between the Blytt–Sernander scheme with its alternating dry and wet

periods and rapid climate change and Andersson’s more uniform, gradually rising

temperature curve, a thermal maximum, and subsequent decrease. The debate was

partly one of scientiﬁc personalities (Fries 1950; Danielsen et al. 2000) and partly

one about research techniques (Iversen 1973). Andersson’s approach was based

exclusively on macrofossils whereas the Blytt–Sernander scheme was based on peat

stratigraphy and megafossils. The conﬂict between the two Swedish schools was

not satisfactorily resolved until another Swede, Lennart von Post (1946), proposed

that post-glacial climate history involved both broad-scale temperature changes

(Andersson) and ﬁner-scale precipitation changes (Blytt–Sernander). This new

paradigm did not develop until the development of pollen analysis, which will be

discussed in the next section.

As part of the Andersson–Sernander debate, Samuelsson (1916) analyzed the

northern limit of hazel in Sweden in considerable detail and showed that summer

temperature was far from uniform along the limit today. He showed that a lower

summer temperature could be compensated for by a longer growing season, and

he modeled the climatic demands of hazel in terms of both summer temperature

and the length of the growing season. He proposed that both summer and winter

temperatures, and hence the length of the growing season, may have changed

during the Holocene. This idea was followed up in detail by Iversen (1944) using

pollen analysis and by Hintikka (1963) in plant geography.

During the early part of the 20th century, the Blytt–Sernander scheme became

the dominant paradigm for Holocene climate history and many peat and mega-

fossil stratigraphies in Scotland, central Europe, and the Alps were interpreted in

terms of the Blytt and Sernander model (e.g. Samuelsson 1910; Gams and

Nordhagen 1923). As peat stratigraphies were examined in more detail, several

recurrence surfaces (changes from dark, humiﬁed to fresh, unhumiﬁed peat –

Figure 2.1c) were identiﬁed by Granlund (1932), suggesting several shifts in mois-

ture during the late Holocene. Granlund proposed at least two “Sub-Atlantic”

climate phases during Sernander’s Sub-Boreal period and after Sernander’s

Fimbulwinter the climate changed twice over to the Sub-Boreal type and then

again to the Sub-Atlantic. Von Post (1946) summarized Granlund’s (1932)

modiﬁcation of the Blytt–Sernander paradigm as follows:

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H. John B. Birks

“Granlund’s work showed that the post-glacial warm period did not end abruptly

with a climatic catastrophe about 500 bc, as supposed by Sernander; but that instead

of this event we must suppose a gradually advancing climatic deterioration spread

over at least 4000 years. This process was, however, not continuous. Just in the same

way as, during autumn, summer gradually changes to winter during repeated alterna-

tions between days of summer warmth and wintry cold, so the climate falls and rises

from the warm period to the present time, through the bronze age, iron age, and our

so-called historic time, swaying between ﬁmbul winters and phases which were echoes

of the glorious climate of the stone age.”

In other words, the accepted paradigm by the 1930s based on peat stratigraphy

and megafossils was of alternating wet and dry phases but perhaps with increased

variability and several short-lived climate shifts in the late Holocene and with a

mid-Holocene warm period.

Independently of the Scandinavian botanists and peat stratigraphers, the

paradigm of a Holocene climate optimum or thermal maximum had already been

presented by Thomas F. Jamieson (1829–1913; Figure 2.8) in his study of the fossil

molluscan fauna of mid-Holocene estuarine clays in central Scotland (Wright

1936). Jamieson found species that do not occur as far north as Scotland today. The

Irish naturalist Robert Lloyd Praeger (1865–1963) reached similar conclusions in

his studies of estuarine clays and raised beaches in Northern Ireland, as did W.C.

Brøgger (1900/01) in the Oslo Fjord (Wright 1936). Mitchell (1976) discusses

whether W.B. Wright’s (1936) dedication in his second edition of The Quaternary

Ice Ages to “R. Lloyd Praeger, Discoverer of the Climatic Optimum” is appropriate,

given Jamieson’s pioneer work in 1865 that Praeger cites frequently in his Irish

studies of 1887 and 1892. Perhaps credit for the discovery of the climatic optimum

should be given to T.F. Jamieson in 1865, and not to Praeger, Brøgger, or Andersson.

In retrospect, the advances made by Dau, Steenstrup, Vaupell, Andersson,

Jamieson, Praeger, Blytt, Sernander, Brøgger, and Granlund in reconstructing

Figure 2.8 Thomas F. Jamieson (1829–1913), a Scottish

geologist who discovered the mid-Holocene climatic

optimum or thermal maximum in his investigations of

Mollusca preserved in estuarine clays in central Scotland.

(Picture from www.fettes.com/(Artist anknown.))

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Holocene climate change were very remarkable, given the very limited range of

techniques then available. A ﬂavor of the excitement of these early investigations

based around peat stratigraphy and macrofossils can be obtained from Godwin

(1978, 1981) and Mitchell’s (1990) autobiographies. It was not until the develop-

ment of pollen analysis that the different views of Andersson and Sernander could

be satisfactorily reconciled and the Holocene climate history reconstructed for

other parts of the world where peat stratigraphy is not always possible. I will return

to modern detailed studies of peat stratigraphy later when I discuss paleolimno-

logy, paleohydrology, and evidence for hydrologic changes during the Holocene.

The study of peat stratigraphy in Scandinavia was largely ignored after the devel-

opment of pollen analysis as a tool for reconstructing Holocene climate change.

It was not until the mid-1970s that peat stratigraphy was revived in Scandinavia

using new techniques for estimating peat humiﬁcation, detecting changes in bog

moisture using testate amoebae, and establishing detailed chronologies by radio-

carbon dating (e.g. Aaby and Tauber 1975; Aaby 1976). These new studies sug-

gested consistent shifts from dry to moist conditions with a periodicity of 260 years

over the past 5500 years.

The study of plant macrofossils and megafossils in north-west Europe was

similarly ignored for about 70 years. Hilary Birks (1975) re-examined buried

pine stumps (Figure 2.1) in Scottish blanket bogs using pollen analysis, peat

stratigraphy, plant macrofossils, and radiocarbon dating and showed that the

temporal patterns of pine stumps were considerably more complex than pre-

dicted from the application of the Blytt–Sernander scheme to Scottish peats

by Samuelsson (1910). In recent decades Kullman (e.g. 1998a,b 2000, 2002) has

studied tree megafossils and macrofossils in several areas of the Swedish moun-

tains and has made spectacular discoveries of Picea and Pinus remains in the early

Holocene and the late-glacial, of thermophilous trees in the early Holocene, and of

Larix sibirica in the Swedish Mountains in the early Holocene. Larix sibirica today

is found 1000 km to the east in Russia. The ﬁnds of its cones and wood (Kullman

1998a) suggest major changes in its distribution since the early Holocene and its

extinction from Fennoscandia. These studies and others (see Birks 2005; Birks et al.

2005) emphasize the importance of integrated studies involving detailed plant

macrofossil and megafossil analyses, pollen analysis, lithostratigraphic investiga-

tions, and radiocarbon dating if the complex patterns of tree spreading and climate

change in the early Holocene are to be unraveled.

Pollen analysis

The Swedish geologist Lennart von Post (1884–1951; Figure 2.9) presented in 1916

the technique of pollen analysis at the 16th Scandinavian meeting of natural

scientists in Christiania (now Oslo) and demonstrated its potential as a technique

for relative dating and for reconstructing past vegetation and past climate.

Although the original idea of pollen analysis can be attributed to the Swedish

botanist Gustaf Lagerheim (1860–1926), it was von Post who had the vision of

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