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The effects of land use and climate change on the carboncycle of Europe over the past 500 yearsJ ED O . KAPLAN* , KR I STEN M . KRUMHARDT * and NIKLAUS E. ZIMMERMANN†

*Soil-Vegetation-Atmosphere Research Group, Institute of Environmental Engineering, Ecole Polytechnique Federale de Lausanne,

Station 2, 1015 Lausanne, Switzerland, †Landscape Dynamics Unit, Swiss Federal Research Institute WSL, Zuercherstrasse 111,

8903 Birmensdorf, Switzerland

Abstract

The long residence time of carbon in forests and soils means that both the current state and future behavior of the

terrestrial biosphere are influenced by past variability in climate and anthropogenic land use. Over the last half-mil-

lennium, European terrestrial ecosystems were affected by the cool temperatures of the Little Ice Age, rising CO2

concentrations, and human induced deforestation and land abandonment. To quantify the importance of these pro-

cesses, we performed a series of simulations with the LPJ dynamic vegetation model driven by reconstructed

climate, land use, and CO2 concentrations. Although land use change was the major control on the carbon inventory

of Europe over the last 500 years, the current state of the terrestrial biosphere is largely controlled by land use

change during the past century. Between 1500 and 2000, climate variability led to temporary sequestration events of

up to 3 Pg, whereas increasing atmospheric CO2 concentrations during the 20th century led to an increase in carbon

storage of up to 15 Pg. Anthropogenic land use caused between 25 Pg of carbon emissions and 5 Pg of uptake over

the same time period, depending on the historical and spatial pattern of past land use and the timing of the reversal

from deforestation to afforestation during the last two centuries. None of the currently existing anthropogenic land

use change datasets adequately capture the timing of the forest transition in most European countries as recorded

in historical observations. Despite considerable uncertainty, our scenarios indicate that with limited management,

extant European forests have the potential to absorb between 5 and 12 Pg of carbon at the present day.

Keywords: carbon cycle, carbon sequestration, climate change, dynamic global vegetation model, European history, forest tran-

sition, land use

Received 13 July 2011 and accepted 26 September 2011

Introduction

The state of terrestrial ecosystems at the present day is

a product of past climate and anthropogenic land cover

change (ALCC; Pongratz et al., 2009; Strassmann et al.,

2008). Many processes in the terrestrial biosphere oper-

ate on centennial to millennial timescales including the

growth and development of forests and the dynamics

of soil organic matter decomposition (Trumbore, 2000;

Canadell et al., 2007). Although there is evidence that

even old trees continue to accumulate carbon at a

steady rate (Luyssaert et al., 2008), forests tend to be

most productive during their early stages of growth; as

a forest ecosystem matures, the rate at which it seques-

ters carbon tends to decrease (Albani et al., 2006) and

net ecosystem production approaches zero (Canadell

et al., 2007). Thus, the potential for carbon to be stored

in terrestrial ecosystems in the future depends strongly

on past ecosystem history and the trajectory of ecosys-

tems at present (Magnani et al., 2007).

In Europe, the five centuries following 1500 CE are

distinguished by climate variability, including the Lit-

tle Ice Age (LIA), and intense human impact on ter-

restrial ecosystems, mainly through the conversion of

forests to cropland and pasture, and the exploitation

of forests for fuel and construction materials. Follow-

ing the Industrial Revolution, most European coun-

tries experienced a forest transition (Mather et al.,

1998b; Mather, 1999), the afforestation that typically

accompanies societies reaching a certain level of eco-

nomic development, urbanization and technological

advancements in agriculture. Forest transitions com-

monly occurred in central, west, and Nordic European

countries from the beginning of the 19th century

(Mather, 1999; Rudel et al., 2005; Bradshaw, 2008) and

later during the 20th century in Eastern Europe

(Mather, 1992; Kauppi et al., 2006; Kozak et al., 2007).

The shift of European ecosystems from source to sink

and subsequent rates of carbon sequestration with

forest regrowth depend on the timing of the initial

forest transition and external environmental factors

including changes in climate and atmospheric CO2

concentrations.Correspondence: Jed O. Kaplan, tel. + 41 21 693 8058,

fax + 41 21 693 3913, e-mail: [email protected]

902 © 2011 Blackwell Publishing Ltd

Global Change Biology (2012) 18, 902–914, doi: 10.1111/j.1365-2486.2011.02580.x

It is generally thought that ecosystems in the tem-

perate latitudes of the northern hemisphere represent

the main natural terrestrial sink for CO2 over recent

decades as a result of land use change, and CO2 and

N fertilization (McGuire et al., 2001; Gurney et al.,

2004; Friedlingstein et al., 2006; Denman et al., 2007).

In Europe, evidence from both forest inventory data

and modeling studies suggests that forests represented

a substantial carbon sink in the late 20th century. In a

synthesis of forest inventory data, Ciais et al. (2008)

observed large increases in forest biomass and net pri-

mary production (NPP) during the period from 1950

to 2000. These authors concluded that environmental

factors, such as increasing atmospheric CO2, were the

main reason for the increase in NPP, whereas

increases in biomass were attributed to changes in for-

est management. In contrast, the model intercompari-

son study of Churkina et al. (2010) attributed a large

portion of the net carbon uptake in European ecosys-

tems to CO2 fertilization, and suggested that climate

and land cover changes over the second half of the

20th century were either neutral or sources of atmo-

spheric CO2.

Despite the fact that carbon dynamics in terrestrial

ecosystems evolve over much longer timescales than

just a few decades, few studies have tried to take the

multicentennial to millennial view of changes in terres-

trial carbon stocks and vegetation productivity on con-

tinental scales. Reliable land use and forest inventory

data exists only from the mid-20th century (Rama-

nkutty & Foley, 1999; Klein Goldewijk, 2001; Klein

Goldewijk et al., 2011), so most attempts to quantify

changes in the terrestrial biosphere and the carbon bal-

ance of ecosystems as a result of ALCC over longer

time periods have been limited to bookkeeping meth-

ods (Houghton, 1999) and/or simple extrapolation of

present day land use patterns to the distant past (Jain &

Yang, 2005; Olofsson & Hickler, 2008).

Recently, a study of ALCC in Europe offered an alter-

native view of land cover change for this highly

impacted area of the world, depicting much higher lev-

els of preindustrial anthropogenic land use than previ-

ous extrapolation-based methods (Fig. 1; Kaplan et al.,

2009). Expanding their method of reconstructing ALCC

to global scale and employing a dynamic global vegeta-

tion model, Kaplan et al. (2010) suggested that previous

estimates of Holocene carbon emissions as a result of

ALCC were greatly underestimated. This study did not

account for climate variability however, which could

have had confounding or synergistic impacts on the ter-

restrial carbon cycle with respect to ALCC (Pongratz

et al., 2009; Jungclaus et al., 2010).

As a result of the very good spatial coverage of

paleoclimate proxy records and historical observa-

tions, the past climate of Europe can be reconstructed

with reasonably high spatial and temporal resolution.

Over the past two millennia, the climate of Europe is

observed to have been generally cooler and more

variable than the 20th century average (e.g., Buntgen

et al., 2011), and also showed important spatial varia-

tions (Luterbacher et al., 2004; Pauling et al., 2006).

The LIA, and the following transition to the warmer

20th century concurrent with industrialization and

increases in atmospheric CO2 concentrations could

have had significant effects on European terrestrial

ecosystems and their carbon stocks at the present

day and their potential to sequester additional carbon

in the future.

To quantify the effects of ALCC, climate change, and

increasing CO2 on the carbon cycle of Europe over the

past 500 years we assembled datasets of past climate

and ALCC and performed a number of experiments

and sensitivity tests using the Lund-Potsdam-Jena

dynamic global vegetation model (Sitch et al., 2003). We

place our results into the context of other modeling and

observation-based estimates of the current state of ter-

restrial ecosystems in Europe, and we discuss the

potential for future carbon sequestration based on the

trajectory of ecosystems at the present and differences

with our control simulations.

(a) (b)

(c) (d)

Fig. 1 Patterns and trends of anthropogenic land use: (a) KK10

scenarios at year 1500, (b) HYDE at year 1500, (c) HYDE at year

2000, and d, time series of land cover change from 1500 to 2000

for the area shown on the maps: KK10-merged in blue, KK10-

interpolated in orange, and HYDE in red.

© 2011 Blackwell Publishing Ltd, Global Change Biology, 18, 902–914

THE EFFECTS OF LAND USE AND CLIMATE CHANGE 903

With regards to the treatment of human impact in

this study, we only consider the affect of conversion

of land from unmanaged landscapes to agricultural

land and pasture, and the abandonment of land used

for these purposes. While we believe that land con-

version and abandonment are the most important

anthropogenic processes influencing total carbon in

terrestrial ecosystems, other factors not considered in

this study, were widespread in Europe over the past

five centuries and also influenced carbon stocks and

cycling. These processes include forest harvest and

management (coppicing, pollarding), litter collection

and transfer, soil erosion and degradation, draining

of wet soils and peatlands, air pollution and atmo-

spheric deposition of sulfate, nitrate and metals (e.g.,

Ebermayer, 1876; Ellenberg, 1988; Glatzel, 1991).

Nearly all of the European land area was subject to

these processes in addition to basic conversion and

abandonment, and the subject should be the focus of

future research.

Materials and methods

Model setup

LPJ is driven by gridded soils data and time series of tem-

perature, precipitation, cloud cover, and atmospheric CO2

concentrations. We developed a paleoclimate scenario for

LPJ based on the 500-year high-resolution gridded recon-

structions of seasonal temperature and precipitation from

historical observations and biological proxies (Luterbacher

et al., 2004; Pauling et al., 2006). Cloud cover was recon-

structed with a bivariate linear model based on temperature

and precipitation at each gridcell. For model runs with fixed

climate, we detrended the CRU TS2.1 observation-based

dataset and added this to the 1961–1990 mean monthly cli-

matology (Mitchell & Jones, 2005). For model runs with

transient atmospheric CO2 concentrations, we used an annu-

ally resolved time series of CO2 made by a weighted spline

fit through Antarctic ice-core, firn and observational records

(Krumhardt & Kaplan, 2010). For runs with fixed CO2, the

model was run with 280.6 ppm, the approximate mean glo-

bal concentration of atmospheric CO2 at year 1500. Soil tex-

ture, water holding capacity, and saturated hydraulic

conductivity are from the Harmonized World Soil Database

(FAO/IIASA/ISRIC/ISSCAS/JRC, 2009) combined with the

ISRIC-WISE taxo-transfer database (Batjes, 2006). Our ver-

sion of LPJ contains an improved observation-based geo-

graphic distribution of soil organic matter, where the mean

turnover time of soil organic matter is a function of total soil

clay content, following Kaplan et al. (2010). This modification

generally has the effect of reducing the mean turnover time

of soil organic matter in cool-temperate ecosystems relative

to the original LPJ, particularly in regions with sandy or

young soils, e.g., on areas that were ice-covered during the

last glaciation. LPJ was spun up for 1000 years to reach

equilibrium conditions at 1500 for each model run.

Land use scenarios

The principal ALCC dataset used in this study is based on a

recent study (Kaplan et al., 2009), in which the authors created

a new scenario of prehistoric and preindustrial ALCC in Eur-

ope between 1000 BC and 1850. In this study, ALCC was sim-

ulated using historical population estimates and a nonlinear

human population-forest area relationship based on historical

land cover records from several European countries. The

resulting dataset depicts substantially higher levels of prein-

dustrial ALCC than works that rely on population-scaled

extrapolation of 20th century trends in ALCC (Klein Gold-

ewijk et al., 2011). This dataset, referred to hereafter as the

KK10 scenario (Kaplan et al., 2010), was modified for this

study in two ways: (1) We improved the distribution of usable

land and thus the spatial distribution of some of the ALCC,

and (2) we extended the datasets to present day using 20th

century data from the HYDE 3.1 historical land use database

(HYDE; Klein Goldewijk et al., 2011).

Land use in the original KK10 dataset was distributed

based on a datasets of land suitability for agriculture and

usable land, i.e., land with a climate suitable enough for any

use by humans. This results in a fairly reasonable distribution

of ALCC, except in northern areas of Europe (e.g., Scotland

and Scandinavia) where land is too cold, and in desert areas

where land was assigned as too dry for land use, although

irrigation is possible (e.g., along the Nile River). To help make

the distribution of usable land more realistic, in addition to the

original dataset of land suitability for agriculture, we used a

new dataset of usable land: a gridded dataset of the maximum

fractional land use ever assigned in HYDE (Klein Goldewijk

et al., 2011). This results in maps of land cover that are similar

to the original Standard scenario from Kaplan et al. (2009), but

with improved ALCC distribution in cold or dry areas. Fur-

thermore, by redistributing ALCC in this manner, we accom-

modate a smoother merge of the KK10 data, which ends at

1850, to the modern ALCC estimates from the HYDE dataset.

To bring the KK10 land use scenario up to present day, we

developed a strategy to splice the final years of KK10 into the

HYDE data over the period 1850–2000. A description of our

splicing methodology and a conceptual figure (Fig. S1) are

detailed in the supplementary materials. In contrast to this

smooth merging technique, we also included a simple linear

interpolation of the KK10 data at 1850 to HYDE at 2000. To

further quantify the importance of the ALCC dataset used, we

included the standard HYDE dataset (Klein Goldewijk et al.,

2011) as an alternative to the two versions of the KK10

scenario. Therefore, the three ALCC scenarios used in this

study are:

1 KK10 ALCC scenario smoothly merged to modern-day

HYDE data (KK10-merged).

2 KK10 ALCC scenario linearly interpolated from 1850 to

HYDE at year 2000 (KK10-interpolated).

3 HYDE ALCC scenario.


904 KAPLAN e t a l .

The major differences between the HYDE and KK10 scenar-

ios are the amount of land use at 1500 and the subsequent

trends in ALCC (Fig. 1). Their principal similarities, however,

are that they all terminate with the same ALCC fractions of

HYDE at year 2000 and they all have similar maximum levels

of land use in Europe (Fig. 1c and d).

Model runs

To quantify the importance of climate, CO2, and ALCC on the

carbon cycle of Europe, we ran a large series of LPJ model

runs (Table 1). We ran the model with reconstructed paleocli-

mate for the past 500 years and no ALCC, i.e., natural vegeta-

tion change only, fixed climate of the 20th century, fixed CO2,

each of the ALCC scenarios described above, and combina-

tions of the above drivers.

Anthropogenic land cover change was input on an annual

time step and handled with respect to change in carbon using

established methodology (McGuire et al., 2001; Strassmann

et al., 2008; Kaplan et al., 2010). This includes simulating up

to three tiles per gridcell: (1) land that has never been under

land use, (2) land currently used by people, and (3) aban-

doned land that is recovering from past use. Clearance of nat-

ural vegetation results in the transfer of all leaf and

belowground biomass to the litter pool and immediate oxida-

tion of 25% of the above ground woody biomass. The rest of

the woody biomass is transferred to 2- and 20-year exponen-

tial decay pools. Once agricultural land, harvest is simulated

by annually oxidizing 100% of the aboveground biomass on

the agricultural tile; upon harvest the belowground living

root biomass is transferred to the belowground litter pool.

Natural fires are the only form of disturbance simulated in

this version of LPJ.

Results

Our model results demonstrate the various impacts of

millennial-scale climate and ALCC, both separately and

combined, on the state of the terrestrial biosphere of

Europe in the year 2000 and the potential for future car-

bon sequestration. While climate variability resulted in

shorter-term increases in carbon storage, CO2 fertiliza-

tion substantially increased carbon stocks during the

20th century. Conversely, ALCC in Europe has resulted

in major losses of terrestrial carbon during or prior to

the last 500 years. In the following sections, all changes

in carbon storage are described as cumulative sinks or

sources relative to the amounts simulated at the begin-

ning of the model run in 1500 CE.

Climate and CO2 effects on the terrestrial biosphere

Climate change alone over the past 500 years led to

modest fluctuations in carbon storage, which, prior to

1950, generally resulted in a carbon sink. By comparing

model runs with paleoclimate to those with fixed

climate we calculate that carbon sequestration due to

climate reached up to 3 Pg during the past 500 years

(Fig. 2). During the four centuries before 1900, tempera-

tures in Europe were generally below the 20th century

average, with minima centered around 1700 and the

late 19th century (Luterbacher et al., 2004). During these

cold periods carbon sequestration due to climate was

greatest (Fig. 2). Cooler temperatures during the LIA

suppressed primary productivity and led to modest

Table 1 Simulated carbon in living biomass, soil and litter, and total carbon in terrestrial ecosystems of Europe at 1500, 1850, 1950,

and 2000 in LPJ-DGVM runs with fixed climate and reconstructed paleoclimate. Results are shown for the natural vegetation (con-

trol) run and for each of the ALCC scenarios implemented in this study. All runs were performed with transient CO2 concentrations

and all values are in units of Pg C

Climate scenario Living biomass C Soil and litter C Total C

Year 1500 1850 1950 2000 1500 1850 1950 2000 1500 1850 1950 2000

Percent

change

Natural vegetation (no land use)

Fixed 50.6 50.5 53.2 57.4 166.5 168.6 172.2 175.2 217.0 219.2 225.4 232.6 6.7

Paleo 50.5 50.0 52.6 55.7 166.6 170.1 173.0 174.3 217.1 220.0 225.6 230.0 5.6

KK10 land use merged to HYDE (Kaplan et al., 2009; Klein-Goldewijk et al., 2010)

Fixed 33.4 29.7 30.7 34.9 143.6 139.7 139.0 140.7 177.0 169.3 169.7 175.6 �0.8

Paleo 33.4 29.0 29.9 33.3 143.6 140.8 139.6 140.0 177.0 169.8 169.5 173.4 �2.1

KK10 land use interpolated to HYDE (Kaplan et al., 2009; Klein-Goldewijk et al., 2010)

Fixed 33.4 30.3 32.9 36.4 143.5 140.3 142.5 145.4 176.9 170.6 175.4 181.9 2.7

Paleo 33.4 29.7 32.1 34.9 143.6 141.4 143.1 144.8 177.0 171.1 175.2 179.7 1.5

HYDE land use (Klein-Goldewijk et al., 2010)

Fixed 44.2 37.1 31.4 34.6 157.9 151.2 143.3 142.2 202.1 188.2 174.7 176.7 �14.3

Paleo 44.1 36.4 30.7 33.0 158.0 152.4 143.9 141.5 202.1 188.8 174.6 174.5 �15.8



reductions in carbon stored in living biomass. How-

ever, microbial decomposition of litter and soil organic

matter was simultaneously suppressed; this reduction

in heterotrophic respiration generally led to increasing

soil carbon storage and a positive net flux of carbon into

the biosphere (Fig. S2; Table 1).

Increases in atmospheric CO2 concentration from

preindustrial (approximately 280 ppm) to late 20th cen-

tury levels (approximately 370 ppm) had a larger effect

on carbon storage than climate. Results of experiments

we performed with natural vegetation where climate

was held under constant, mean 20th century condi-

tions, and comparisons between model runs with fixed

and transient CO2 clearly show a strong CO2 fertiliza-

tion after 1800. CO2 increases since the mid-19th cen-

tury could have added up to approximately 15 Pg to

the carbon inventory of Europe (Fig. 3; Table 1). This

effect, however, is smaller for runs with simulated

ALCC, as there is less potential for cropland ecosys-

tems to absorb carbon than for forests and annual har-

vest does not allow accumulation of carbon in the

aboveground biomass and litter. These runs with

ALCC forcing indicate that CO2 fertilization could have

led to the sequestration of up to 12 Pg of carbon in

European ecosystems (Fig. 3; Table S1).

ALCC effects on the terrestrial biosphere

The impact of ALCC on the terrestrial carbon balance

of Europe was greater than that of both climate and

CO2 over the past 500 years. All ALCC scenarios

result in carbon storage well below potential natural

values at 1500, followed by further losses of terrestrial

carbon stored in European ecosystems during the first

part of the simulation (Fig. 3; Table 1). Subsequently,

all scenarios also show a reversal from carbon loss to

accumulation caused by a combination of land aban-

donment and CO2 fertilization. Relative to year 1500,

the effects of ALCC alone could have ranged from

5 Pg of carbon uptake to 25 Pg of carbon emissions

over the last 500 years, depending on the ALCC sce-

nario (Fig. 3; Table 1). This large range is mostly due

to the differing amounts of ALCC between the HYDE

and KK10 scenarios at 1500.

Preindustrial land use estimates in KK10 contrast

starkly with those depicted in the HYDE scenario (Kap-

lan et al., 2010; Fig. 1). Most ALCC in the KK10 scenar-

ios occurs prior to the 16th century and therefore a

highly deforested landscape was depicted at the begin-

ning of our runs at 1500. Specifically, the biosphere con-

tained 40 Pg less C than the potential natural state

when using the KK10 scenarios (Fig. 3; Table 1). LPJ

was spun up to equilibrium with year 1500 ALCC esti-

mates and it is likely that European croplands were

nearly in a steady state with respect to carbon since,

according to the KK10 scenario, most areas were defor-

ested during prehistoric times and the carbon in soils

presumably had enough time to equilibrate after con-

version from forest to cropland.

In contrast to the time progression of ALCC in KK10,

the HYDE scenario runs begin at 1500 with a carbon

inventory only 15 Pg C below potential natural levels.

Although HYDE was spun up to equilibrium at year

1500 ALCC estimates, soils could have contained addi-

tional carbon from forests that were cut just prior to

1500, when deforestation in Europe, according the

HYDE scenario, was just beginning to accelerate (Klein

Goldewijk et al., 2011). Nevertheless, carbon losses due

to ALCC after 1500 are rapid for simulations with the

HYDE ALCC relative to those with the KK10 scenario

(Fig. 3).

−4

−3

−2

−1

0

1

Δ C

sto

rage

(P

g)

1500 1600 1700 1800 1900 2000Year

(b)

−1.5

−1.0

−0.5

0.0

0.5

1.0

Ann

ual t

empe

ratu

re a

nom

aly

(°C

)

(a)

Natural vegetation (no land use)KK10 land use, merged to HYDEKK10 land use, interpolated to HYDEHYDE land use

Fig. 2 Climate effects on carbon storage. (a) European mean

annual temperature anomaly from the datasets used to drive

LPJ, the heavy line shows temperature smoothed with a 20 year

Gaussian filter to highlight decadal trends, and (b) simulated

changes in terrestrial carbon storage due to climate variability;

negative values indicate a sink. Net emissions are calculated as

the difference in carbon storage between model runs with fixed

climate and reconstructed paleoclimate.



Combined effects of climate variability, increases inatmospheric CO2, and ALCC

When climate change, transient CO2 concentrations,

and ALCC were combined as input into LPJ, we

observe that terrestrial European carbon stocks at year

2000 are 55 Pg below what they would potentially be

under natural vegetation (Fig. 3); approximately

175 Pg compared to approximately 230 Pg, respec-

tively. In combination with ALCC, climate change

caused one-third less carbon to be sequestered during

cold periods compared to runs with natural vegetation

(Fig. 2). CO2 fertilization integrated with ALCC and

climate change caused nearly 5 Pg of additional carbon

to be sequestered in terrestrial ecosystems by 1950

with an additional 7 Pg of carbon by 2000 (Tables 1

and S1).

Runs with the KK10 land use scenarios reach a mini-

mum in C storage at roughly 170 Pg, whereas the low

point in the HYDE simulations occurs later and with a

slightly larger terrestrial carbon stock of 175 Pg

(Fig. 3). Following these nadirs in terrestrial biomass, a

reversal to carbon accumulation occurs, albeit at differ-

ent times for each of our three ALCC scenarios. The

two types of extensions (merged vs. interpolated) used to

bring the KK10 scenarios to year 2000 resulted in nota-

ble differences for the carbon balance at year 2000. For

the KK10-merged scenario, emissions from ALCC are

fairly stable from 1800 until the mid-20th century, at

which point a reversal to carbon accumulation occurs.

As the last 40 years of the KK10-merged dataset con-

tains the same land use estimates as HYDE, the HYDE

scenarios show the same reversal point, at around

1960.

Carbon fluxes simulated in the combined simula-

tions were highly variable (up to 50 Tg amplitude)

year-on-year; this amount is consistent with other

studies. During the last 50 years of our simulations,

simulated net biome production (NBP) ranged from 50

–450 TgC yr�1. A recent modeling study by Churkina

et al. (2010) accessed the effects of land cover conver-

sion, nitrogen deposition, and climate change on the

carbon balance of Europe, concluding that European

land ecosystems absorbed 56 Tg of carbon per year

on average during the period 1950–2000 and

114 Tg C yr�1 for the early 21st century. Other data-

based studies estimated that European rates of C

sequestration in terrestrial ecosystems in recent decades

range from 111 Tg C yr�1 (Janssens et al., 2003) to

313 Tg C yr�1 (Schulze et al., 2009).

Spatial variability in European carbon stocks

Maps of changing carbon storage aid to illustrate the

effects of ALCC over the past 500 years (Fig. 4). All

scenarios show a general shift in the locus of anthropo-

160

170

180

190

200

210

220

230

1500 1600 1700 1800 1900 2000

Natural vegetationKK10−merged land useKK10−interpolated land useHYDE land use

160

170

180

190

200

210

220

230

1500 1600 1700 1800 1900 2000

(a) (b)

Ter

rest

rial C

sto

rage

(P

g)

Year

Ter

rest

rial C

sto

rage

(P

g)

Year

Fig. 3 Timeseries of carbon storage for runs with transient CO2 (a) and fixed CO2 (b). Each line represents simulated total carbon in

vegetation and soils under different land use scenarios over the time period 1500 to 2000. Heavy lines represent runs with paleoclimate;

model runs with fixed climate are shown with thin lines. The difference in carbon storage at year 1500 reflects the land use map used to

initialize the simulation (see Fig. 1).



genic land use from west to east, but differ in the pat-

tern and timing of carbon emissions and uptake. In the

KK10 scenarios, the 16th and 17th centuries are charac-

terized by a heterogeneous pattern with both highest

emissions and uptake (±5 kg C m�2) in montane areas

of central Europe, partly because the reconstructed cli-

mate anomalies are greatest in mountain areas

(Luterbacher et al., 2004; Pauling et al., 2006). From the

18th century onwards, carbon emissions shift eastward

as increasing population densities and new technolo-

gies opened up the steppe and forest steppe regions of

Eastern Europe to cultivation. Moderate levels of car-

bon uptake (2 kg C m�2) occur during this time period

in western and central Europe in all ALCC scenarios,

with nearly zero emissions occurring in this region

when using the KK10-interpolated scenario (Fig. 4, mid-

dle panels). By the 20th century the carbon-sequester-

ing effects of land abandonment and CO2 fertilization

are apparent in the KK10-interpolated scenario across

nearly all of Europe (1–5 kg m�2 of uptake), whereas

the HYDE scenario still shows widespread conversion

to anthropogenic land use and high carbon emissions

(1–5 kg m�2 of emissions) in the east. The KK10-merged

scenario simulation depicts mostly uptake during the

20th century with only minor emissions occurring in

the mountainous areas of Europe and Ireland.

16th & 17th centuries

KK

10, m

erg

ed t

o H

YD

E

18th & 19th centuries 20th century

KK

10, i

nte

rpo

late

d t

o H

YD

EH

YD

E

−5 −4 −3 −2 −1 0 1 2 3 4 5Terrestrial carbon gain or loss over time period (kg m−2)

Fig. 4 Maps of carbon dynamics. Difference in carbon inventory between the beginning and the end of a time period (1501–1700, 1701–

1900, 1901–2000) for the KK10-merged, KK10-interpolated and HYDE land use scenarios. Positive values indicate carbon emissions. All

simulations were run with paleoclimate forcing.



Discussion

Climate and CO2 effects on carbon

Climate, CO2, and ALCC had contrasting yet sometimes

synergistic effects on the European carbon budget over

the last 500 years. The effect of climate variability alone

was dampened when using an ALCC scenario because

large parts of Europe that would naturally be domi-

nated by carbon-rich temperate forests were replaced

by agricultural land, which contains less biomass and

soil carbon (Fig. 2b). The effect of increasing atmo-

spheric CO2 concentrations had a strong effect on pro-

ductivity and carbon storage in our simulations,

reaching over 15 Pg in the natural vegetation experi-

ment (Fig. 3; Tables 1 and S1). It is possible that this

CO2 fertilization effect could be overestimated, as nitro-

gen availability can limit carbon sequestration caused

by increasing CO2 (Fisher et al., 2010). On the other

hand, anthropogenic N deposition has been widespread

and particularly strong in Europe since industrializa-

tion (Galloway et al., 2004), coincident with the timing

of increasing CO2, and there is empirical evidence that

increasing atmospheric CO2 concentrations over the

past 150 years did positively affect the growth of forest

trees (Gedalof & Berg, 2010). Thus, much of the C-fertil-

ization potential simulated by our model may have

been or yet be realized (Magnani et al., 2007; Zaehle

et al., 2010). We would not necessarily expect this affect

to continue into the future, because although CO2 and

N deposition continues to rise, N loading has become

critical in some regions (e.g., Schulze et al., 1989), and

other mineral nutrients will also limit growth.

ALCC effects on carbon

The time trend of land use in the different ALCC sce-

narios has the strongest effect on carbon storage among

all of the factors we tested. The timing of the reversal

from deforestation to afforestation is particularly

important for the current state of European ecosystems.

That carbon storage in the HYDE and KK10-merged sim-

ulations reaches a low point in the mid-20th century is

an artifact of the methodology used to produce the

HYDE dataset, where FAO statistics on crop and pas-

ture areas only extend back to 1961 (FAO, 2008; Klein

Goldewijk et al., 2011). Conversely, the KK10-interpo-

lated scenario shows a prominent reversal to carbon

accumulation at roughly 1850 – see also our discussion

below on forest transitions – which is a result of the lin-

ear interpolation from KK10 1850 land use to year 2000

land use in HYDE. It is important to note that without

CO2 fertilization, no reversal to carbon accumulation

occurs for any of the ALCC scenarios. However, with-

out land abandonment, carbon storage due to CO2 fer-

tilization would have been less important.

Although the ALCC scenario implemented strongly

affects the simulation of preindustrial carbon balances,

surprisingly, the resulting carbon inventory of Europe

at year 2000 does not differ greatly between ALCC

datasets used in this study (Fig. 3). The KK10-interpo-

lated scenario makes the central assumption that Euro-

pean countries experienced forest transitions during

the mid-19th century (Mather et al., 1998a; Mather,

1999; Mather & Fairbairn, 2000). This results in greater

carbon inventories in Europe 150 years later, at approx-

imately 180 Pg, roughly 5 Pg more than simulations

with our other two ALCC scenarios. As a result of iden-

tical 1961–2000 land use estimates, the small differences

in carbon balances at year 2000 between KK10-merged

scenario and HYDE are due to ALCC prior to 1961

(Fig. 3). For example, fixed climate simulations with

the HYDE scenario result in 1.1 Pg less terrestrial car-

bon at year 2000 than the KK10-merged scenario (Fig. 3;

Table 1). This occurs because according to the HYDE

scenario, lands were deforested more recently and the

soil on previously forested land still contains and con-

tinues to emit carbon even at year 2000 due to the slow

decomposition of soil organic matter. The large discrep-

ancy among ALCC estimates for Europe even over the

last 150 years (Table 2) is a major source of uncertainty

in our results, as was also shown in previous studies (e.

g., Zaehle et al., 2007; Churkina et al., 2010).

Even so, the narrow range in simulated carbon stor-

age at year 2000 in our results indicates that ALCC

occurring over the past one to two centuries following

industrialization had the strongest influence on the

present state of the biosphere, outweighing most effects

of preindustrial ALCC. In our model simulations, the

mean residence time of carbon in European terrestrial

ecosystems is rather short, on the order of 200 years.

As soil organic matter can be stabilized in clay miner-

als (Six et al., 2002), the amount of soil organic matter

entering the slow soil turnover (1000-year) pool may be

strongly influenced by the total clay content of the soil

(Jobbagy & Jackson, 2000). The relatively young soils of

much of Europe north of the Alps have relatively low

clay contents. Thus, in the version of LPJ we used, car-

bon storage in Europe is predominantly in the living

biomass and the fast-turnover SOM pool, both of which

have mean residence times on the order of a few centu-

ries. On the other hand, synthesis of the mean age of

soil organic matter based on radiocarbon dating of bulk

soils (Fig. S3) indicates that much older soil organic

matter can be observed in European ecosystems. It

could be that our version of LPJ, while producing a

geographic distribution of soil organic matter stocks

that is much more in line with observations than previ-



ous versions of this model, requires further modifica-

tion, such as the inclusion of a third soil organic matter

pool (e.g., Baisden & Amundson, 2003) to better simu-

late the temporal dynamics of carbon turnover.

We further explored the assumption we made

regarding the equilibrium state of the biosphere in Eur-

ope at 1500 CE by performing a series of additional

fixed-climate simulations started at 1000 BC (3 ka) and

comparing European carbon storage simulated at year

1500 in a transient run with that of our standard model

runs, where the model was spun-up to equilibrium at

1500 CE (Fig. S4). These experiments show that in most

of Europe, especially when using the KK10 scenario,

carbon storage was close to equilibrium conditions at

1500 despite a long history of land conversion and

abandonment in the preceding millennia. As discussed

above, the residence time of carbon in most European

ecosystems is simulated to be ca. 200 years on average,

so only ALCC in the preceding centuries will have a

substantial impact on carbon storage at 1500. Where

substantial dis-equilibrium is apparent (Fig. S4), it was

caused by rapid land cover change during the two to

three centuries preceding 1500. This dis-equilibrium

appears in the southern Balkans and Russia in the

KK10 scenario and in much of Western Europe in the

HYDE scenario. In these regions, rapid deforestation in

the period 1200–1350 CE was followed by widespread

abandonment after the Black Death (1350–1400) and

renewed expansion of agricultural areas before 1500.

The magnitude of the pre-1500 fluctuations in land use

is particularly strong in the HYDE scenario.

Combined effects

When climate change, transient CO2 concentrations,

and ALCC were combined as input into LPJ it becomes

evident that the effects of ALCC dominate the terres-

trial European carbon cycle over the past 500 years.

Climate variability, both cold periods during the LIA

and the late 20th century warming, caused short-term

fluctuations in total ecosystem carbon, with colder peri-

ods leading to increased storage, and vice versa. As dis-

cussed above, the relatively short residence time of

carbon in most European terrestrial ecosystems means

that the LIA-driven increases in carbon storage do not

persist into the latter half the 20th century. CO2 fertil-

ization over the 20th century led to substantial

increases in carbon stocks and offset some of the carbon

lost due to both warming and ALCC. The trends in car-

bon storage described in the previous section on ALCC

effects on carbon storage are still the predominant con-

trol on European carbon storage even in model experi-

ments driven by variable climate and CO2.

For model runs using the KK10-interpolated ALCC

scenario, all carbon emissions are offset by land aban-

donment and CO2 fertilization and the resulting year

2000 European carbon stock is 1.5% higher than in 1500

(Table 1). However, using fixed CO2 concentrations of

the 16th century, the carbon balance for this simulation

is still 5.4% below the 1500 carbon stock. The HYDE

and KK10-merged scenarios both resulted in a net loss of

carbon over the 500-year time period, but still show a

reversal to carbon accumulation after 1950. Interest-

ingly, by holding CO2 levels at preindustrial levels, the

reversal to carbon accumulation becomes nonexistent

under all ALCC scenarios, demonstrating the impor-

tance that CO2 fertilization has on the carbon balance of

Europe at year 2000 (Tables 1 and S1). Churkina et al.

(2010) found that models that account for carbon-

nitrogen dynamics showed a smaller response to CO2

fertilization, attributed to nutrient limitation. Including

nitrogen dynamics in LPJ could result in a weaker

Table 2 A compilation of published agricultural areas (106 ha) in Europe since 1850. N.B. the exact geographic definition of Eur-

ope varies slightly between studies, but always excludes the former Soviet Union

Area considered

Robertson

(1956):

arable

area

Houghton

(1999):

cropland

CORINE land

cover: Arable

land and

permanent

crops

CORINE land

cover:

Pastures

and mosiacs

KK10-

merged:

land use

HYDE:

crop +pasture

Ramankutty

et al. (2008):

Cropland

Ramankutty

et al. (2008):

Pasture

509 430 373 373 481 492 467 467

Year

1850 61 272 180

1870 140 70 265 190

1900 74 259 209

1910 145 76 257 217

1950 148 82 239 232

1990 73 117 82 225 227

2000 117 82 214 216 125 67



response to increases in atmospheric CO2 over the last

150 years, and therefore lead to a diminished response

of C storage to land abandonment during the last cen-

tury of our model simulations.

The effects of climate variability are low for simula-

tions with natural vegetation, but even lower when

ALCC is simulated as well. Even so, cold periods

during the LIA appear to have had a synergistic

effect with ALCC prior to 1950 by allowing for tem-

porary sequestration of additional carbon in the form

of soil organic matter. However, this carbon is rap-

idly emitted when the climate warms during the 20th

century (Table 1), leaving no long-lasting effect (see

natural vegetation runs in Fig. S2; Table 1). According

to Zaehle et al. (2007), further carbon emissions from

soil respiration are likely to continue with increased

warming over the 21st century. After 1950, in addi-

tion to increased carbon emissions from soils, biomass

production also decreased compared to fixed climate

runs likely due to increased evapotranspiration from

warmer temperatures without significant increases in

precipitation during summer (Pauling et al., 2006;

Table 1). This adversely affects any carbon sequestra-

tion from late 20th century land abandonment occur-

ring in the HYDE and KK10-merged scenarios. Overall,

the effect of climate change combined with ALCC is

very small in our experiments, less than 1 Pg on

average (Fig. 3, difference between thin and thick

lines).

Forest transitions

By comparing the outcomes of model runs using our

different ALCC scenarios, we can speculate on the

importance of the timing of the forest transition. The

HYDE dataset depicts sharply increasing ALCC until

ca. 1960 when a sharp transition to land abandonment

occurs (Fig. 1d). The KK10-interpolated dataset has a vis-

ible transition nearly 100 years earlier, an intentional

artifact of a straight linear interpolation to year 2000

land use fractions, implying a ubiquitous European for-

est transition at 1850. The KK10-merged dataset is essen-

tially a combination of the HYDE and KK10-interpolated

datasets, as KK10 data are blended to HYDE ALCC

fractions. This dataset shows three reversals from

deforestation to afforestation, at year 1850, 1900, and

another one at 1960, as in HYDE. The reversal points in

all three datasets inherently have a strong influence on

the outcome of our LPJ simulations and have a substan-

tial effect on the year 2000 carbon balance. However, a

breakdown of ALCC on a per country basis reveals that

neither the HYDE nor the KK10 ALCC datasets capture

the forest transition in most countries as it was

recorded in contemporary observations (Fig. S5).

Despite numerous studies performed on forest transi-

tion phenomenon (Mather, 1992, 1999; Mather &

Needle, 2000; Rudel et al., 2005; Kauppi et al., 2006), the

timing and magnitude of the forest transition is not

adequately represented in any of the ALCC datasets we

used (Fig. S5). Furthermore, land use inventories dis-

agree over the amount of agricultural land area in Eur-

ope for the last two centuries (Table 2), and all current

land use models underestimate the amount of ALCC at

the time of the transition for all regions with a docu-

mented forest transition (Fig. S5, triangles). Forest tran-

sitions in western and Central Europe occurred on

average around 1861 (area-weighted average of docu-

mented cases for France, Denmark, Switzerland, Ire-

land, the UK, Sweden, Portugal, and Italy). In Eastern

Europe an area-weighted average of Hungary and

European Russia puts the average timing of the forest

transition at 1930. From these years onward, we expect

to see increasing carbon inventories, as agricultural

lands are abandoned and ALCC decreases. If the mag-

nitude of ALCC and the timing of the forest transitions

were captured correctly, as according to the historical

records, then the results of our simulations would

likely be significantly different, possibly showing sub-

stantially more carbon accumulation in European eco-

systems over the past 150 years.

The present and future carbon balance of Europe

Our natural vegetation scenario demonstrates the

potential amount of carbon that could be stored in Eur-

ope in the unlikely scenario that all land is abandoned

and left to regrow with natural vegetation. We found

it more informative to examine only those lands in

Europe under natural vegetation in year 2000 (i.e., not

currently used for crops or pasture). We calculated the

difference between the carbon inventories at year 2000

for these unused areas between the natural vegetation

scenario and the ALCC scenarios. This provides an

indication of the carbon sequestration potential of cur-

rent European natural ecosystems in future decades.

Considering only the area of Europe that is currently

not under ALCC and assuming no future changes in

land use, we estimate that between 5 and 12 Pg of car-

bon remain to be sequestered. As a result of the rela-

tively early forest transition in the KK10-interpolated

scenario, at present, forest ecosystems are closer to sat-

uration with respect to carbon (cf. Albani et al., 2006).

In contrast, model runs using the HYDE and KK10-

merged scenario, where deforestation peaked only in

the second half of the 20th century indicate that terres-

trial ecosystems are still far from equilibrium and

therefore have more potential to absorb carbon. These

results are in line with the results of future scenario



model simulations that indicated similar amounts of

sequestration (Zaehle et al., 2007).

To further clarify the effects of past land use on Euro-

pean carbon stocks at the present day, we compared a

LPJ run spun up to equilibrium with year 2000 land use

and CO2 concentrations with a run spun up to year

1500 and run transiently with the KK10-merged scenario

and transient CO2 concentrations, 1500–2000. The dif-

ference between these two runs demonstrates the effect

of land use on carbon at year 2000 (Fig. 5). ALCC prior

to 2000 caused less carbon to be stored in Ireland,

northern European Russia, and in smaller regions of

western and central Europe (Fig. 5; areas in red). These

areas are depleted with respect to equilibrium carbon

storage of up to approximately 10 kg m�2 C, and there-

fore would have the potential to sequester additional

carbon in the future. In contrast, green areas on Fig. 5

highlight areas where more carbon is stored in ecosys-

tems according to the LPJ run that included ALCC. In

this case, soils contain more carbon from previously

forested areas; this is carbon that was not stored in the

run spun up with year 2000 land use patterns. This car-

bon would continue to be decomposed and emitted

after year 2000. Taken in sum, the whole of our Euro-

pean study region has the potential to take up an addi-

tional 12 Pg C (Fig. 5), not accounting for future ALCC

or CO2 fertilization.

Previous studies support the claim that European

ecosystems have been a net sink of carbon over the past

several decades and will continue to be in the near

future (Vermoere et al., 2000; Janssens et al., 2003;

Churkina et al., 2010). Ciais et al. (2008) published a

study on carbon accumulation in European forests in

which forest inventories and carbon accumulation rates

from the EU 15 countries (plus Switzerland and Nor-

way, excluding Luxembourg) were accessed over the

past 50 years. They report that biomass carbon stocks

and NPP in European forests increased 1.75-fold and

1.67-fold, respectively, from 1950 to 2000. Over the

same time period, in our model simulations we demon-

strate roughly a 1.3-fold increase in carbon stocks and a

1.2-fold increase in NPP. Differences between Ciais

et al. (2008) and our study can be attributed to (1)

uncertainties in historical ALCC data (see previous sec-

tion), (2) the comparatively coarse resolution used in

this study (i.e., a single 30 min gridcell is not typically

entirely forest, even under natural conditions, and

therefore average biomass is typically less than

reported for just forests), (3) nonforested areas included

in our study (e.g., Mediterranean biomes, Eastern Euro-

pean steppe) and (4) lack of forest management in our

simulations, as the net biomass sink reported in Ciais

et al. (2008) is largely attributed to forestry and harvest-

ing rates, rather than increases in forest area. Our

study, conversely, focuses on the overall carbon stocks

in European ecosystems, rather than just forests, and a

substantial reason for the European carbon sink can be

attributed to a combination of CO2 fertilization plus

increases in the area of abandoned agricultural land.

Conclusions and future directions

In this study, we demonstrate that ALCC clearly over-

shadows the effect of climate variability on the Euro-

pean carbon cycle over the past 500 years. After a long

history of net carbon loss to the atmosphere through

deforestation, Europe experienced gradual abandon-

ment of marginal lands and afforestation in the 19th

and 20th centuries. Increasing atmospheric CO2 concen-

trations, supported by forest regrowth, and in some

areas, warmer climate, resulted in the reversal of some

or all of the carbon lost since 1500. The timing of this

reversal, as reflected in the different ALCC scenarios, is

especially important for assessing the trajectory of eco-

systems at present and hence their remaining seques-

tration potential, although all of the ALCC scenarios

result in simulations that indicate European forests still

have the potential to sequester atmospheric carbon in

the future.

KK10 merged at 2000− spinup to 2000

−10 −8 −6 −4 −2 0 2 4 6 8 10

Carbon difference (kg m−2)

Fig. 5 Terrestrial carbon lost or gained as a result of the past

500 years of ALCC. A LPJ run spun up to equilibrium at year

2000 conditions (with respect to ALCC and CO2 concentration)

was subtracted from year 2000 of a 500 year run with transient

CO2 and KK10-merged land use. Areas in red have less carbon,

while areas in green have more carbon as a result of anthropo-

genic land use.



There are several limitations to our current study that

should be addressed in future work on assessing the

state of the European carbon cycle. These include add-

ing the capability to simulate forest management, culti-

vation practices such as tilling, nitrogen deposition,

fertilization, and nitrogen cycle dynamics to LPJ. In

addition, it is absolutely necessary to improve ALCC

datasets to better reflect the forest transitions and

cleared land amounts recorded in the historical record

during the last two centuries.

Our current inability to quantify the effects of indus-

trial forest management on the carbon inventory on

vegetation and soils, e.g., as a result of the large-scale

logging and draining of peat forests that was wide-

spread in Northern Europe over the 20th century (e.g.,

Jandl et al., 2007) could have an important affect on our

final results. Future studies could build on vegetation

models that contain schemes for simulating forest man-

agement (Zaehle et al., 2007). Our calculations of

sequestration potential also do not take into account the

potential for enhanced carbon sequestration as a

result of future CO2 fertilization, although this effect

may be limited by lack of nitrogen or other nutrients

(Thornton et al., 2007; Gedalof & Berg, 2010) and from

tropospheric ozone damage to plants (Sitch et al.,

2007). Nevertheless, this study demonstrates the

importance of accounting for at least centennial-scale

changes in land use, CO2 and climate in evaluating

the current state and potential future behavior of

European ecosystems.

Acknowledgements

This work was supported by a Swiss National Science Founda-tion professorship grant (PP0022_119049) to JOK, FIRB projectCASTANEA (RBID08LNFJ), the EU MILLENNIUM IP (GOCE-017008), and CCES project MAIOLICA. We thank Helga Van-thournout for her comments that improved this manuscript,and Sue Trumbore, Troy Baisden, Jan Esper, and the EPFLARVE Group for discussions.

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Supporting Information

Additional Supporting Information may be found in theonline version of this article:

Figure S1. Extending the KK10 dataset to year 2000.Figure S2. Climate-driven changes in soil and vegetationcarbon.Figure S3. Measured soil carbon ages in Europe.Figure S4. Dis-equilibrium of European carbon storage at1500 CE.Figure S5. European forest transitions.Table S1. Simulated carbon resulting from runs with fixedpreindustrial CO2 (~280 ppm) in living biomass, soil and lit-ter, and total carbon in terrestrial ecosystems of Europe at1500, 1850, 1950, and 2000 in LPJ-DGVM runs with fixed cli-mate and reconstructed paleoclimate.

Please note: Wiley-Blackwell are not responsible for the con-tent or functionality of any supporting materials suppliedby the authors. Any queries (other than missing material)should be directed to the corresponding author for thearticle.



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