ateam - Potsdam Institute for Climate Impact Research

ATEAM final report Section 5 and 6 (2001-2004) 29
NBE (sources plus sinks) over time. Net carbon uptake (negative NBE) is valued as an ecosystem
service to reduce carbon dioxide concentrations in the atmosphere. Net carbon emission (positive NBE)
is regarded as an ecosystem disservice, adding to the atmospheric carbon dioxide concentration. The
amounts of carbon that can be efficiently stored in terrestrial vegetation over long periods of time need
to be considered in terms of absolute numbers, in relation to other pools and fluxes (atmospheric
concentration, anthropogenic emissions, uptake by the oceans) and within the political context.
A modified version of LPJ was used to assess the effect of the ATEAM scenarios on the European
carbon balance, representing the actual land cover and land cover dynamics. Based on land use data
from remotely sensed sources (CORINE/PELCOM), and a reanalysis of historical trends of the land
cover classes (FAO 2000, Ramankutty and Foley (1999)) each grid cell is subdivided into land cover
classes of potential natural vegetation (standard LPJ), managed grasslands, a generic cropland scheme
(Bondeau et al. in prep.), managed forest (Zaehle et al., in prep), and barren land. The model is spun up
to equilibrium using the 10' CRU4-climatology (1901-1930 recycled), and reconstructed land use data
from 1900. LPJ is then run in a transient mode with the 20 th century 10' CRU climatology and the
reconstructed land use, which is updated annually.
The results confirm that Europe's terrestrial biosphere currently acts as a sink, mainly due to carbon
sequestration in expanding forest areas (e.g. Nabuurs et al. 2003). Although the size of the sink
modelled with LPJ is notably smaller than estimated elsewhere (e.g. Janssens et al. 2003), the results
are within the uncertainty bounds of these studies. In an experiment to estimate the effect of historic
forest management changes on the increase in forest carbon stock we found a strong impact on the
current carbon balance. However, the effect of historic forest management on future forest vegetation
carbon is small compared to the effect of future climate, land use and forest management change.
The variation between climatic change derived from different GCMs was evaluated using different
climate models driven with the same radiative forcing (the emission scenario A2). All model runs show
an increasing sink strength up to the mid of the 21 st century (Figure 18A, negative values denote fluxes
into the terrestrial system, positive values denote fluxes out of the terrestrial system into the
atmosphere). Thereafter, increased warming (mainly in winter) in Northern Europe enhances soil
respiration more than net primary production in all scenarios , therefore decreasing the sink or even
turning into a net carbon source towards the end of the century. In Southern Europe the climate models
differ more strongly both in terms of warming and increase in drought stress. Particular mild and wet
scenarios (PCM) lead to an increase in net carbon uptake, whereas hotter and dryer scenarios (such as
HadCM3) lead to more carbon neutral conditions (Figure 18B).
Land use change alone has a positive effect on carbon storage, while climate change can counteract
this effect (Figures 19, negative values denote fluxes into the terrestrial system, positive values denote
fluxes out of the terrestrial system into the atmosphere). The differences between different storylines
(A1f, A2, B1, B2) represent the opportunity of choice between different worlds. The effect of such
choices on the European terrestrial carbon balance was examined using combined land use and climate
change scenarios based on the four storylines. Generally it is found that reforestation, particularly on
previously agriculturally used soils has a potential to sequester a notable amount of carbon, however,
the uptake is never as strong as 10% of the EU's 1990's emission (Figure 20A, compare effect of
climate change and land use change driven NBE, upper maps, with NBR driven by climate change only,
lower maps). Climate change will interfere with this trend (as do other land use choices such as
increasing urban area and regional shifts in agricultural areas). The variation over different climate
model runs (A2 with four climate models) and the variation over four socio-economic storylines (A1f, A2,
B1, B2 with climate model HadCM3) is similar (variation represented by standard deviation, Figure 20B).
This indicates that a considerable amount of the variation in Europe’s terrestrial carbon balance lies in
the choices we make concerning our socio-economic future.
In our simulations climate change dominates the overall trend in European wide net carbon exchange.
However, the EFISCEN model used in the forestry sector to simulate the growth of managed forests