Where are the current carbon sources and sinks located on the land and
how do European sinks compare with other large continental areas?
This question is most often addressed using atmospheric inversion
techniques, which provide estimates of net surface-atmosphere exchanges
of CO2 using an optimisation process. The atmospheric CO2
concentrations from a number of measurement sites constrain the
source/sink areas in the presence of estimated atmospheric transport and
known emissions. Since the typical atmospheric gradient of CO2
from North to South pole is only 3 ppm, the accuracy of the network is
critical. Bousquet et al. (1999) estimate a sink of 0.5+/-0.6 Gt C/yr
for Northern America, a sink of 0.3 +/-0.8Gt C/yr for Europe and 1.3 Gt
C/yr for Siberia. By contrast, Fan et al (1998) conclude that a
Northern hemisphere sink of 1.7 Gt C/yr may exist entirely over North
America. The difference between these two studies is partly caused by
the inclusion of more measurement sites in the Bousquet et al study.
Although inversion studies can provide useful estimates of the net
land-atmosphere CO2 exchange they are unable to
separate this into contributions from accidental effects and land
management. A key motivation for CAMELS is to produce a system which can
use the atmospheric CO2 measurements along with remote-sensing
products to constrain mechanistically-based models of the land carbon
cycle, and which can therefore be used to isolate the causes of sources
In the table below we present the current best estimates of European
sink strength obtained by various methods:
C Sink (Gt/yr)
|0.5 (Forests Only)
|Schulze et al, 2000
| Flux measurements
||0.3 (Forests only)
||Martin et al., 1998
||Bousquet et al., 1999
| Forest inventory
||0.1 (Forests only)
||Nabuurs et al., 1997
All methods indicate a land carbon sink but there is a factor 5 between
the highest and lowest estimates. A likely reason for these
discrepencies is that the various methods use different scaling
techniques to transform information obtained at one spatial and
temporal scale to another.
This motivates the CAMELS first
Objective : to provide a consistent estimate of the European
land carbon sink by making intelligent use of all of the existing
data-sources, which provide both small-scale constraints (e.g. flux
measurements, inventory estimates) and large-scale constraints (e.g.
atmospheric CO2 measurements, remote sensing products).
Why do these sources and
sinks exist, i.e. what are the relative contributions of CO2
fertilisation, nitrogen deposition, climate variability, land
management and land-use change?
This is a key scientific question because it relates to the causes of
changes in net land carbon storage. It also has great policy relevance
since the Kyoto protocol only allows nations to claim carbon credits for
carbon accumulation due to direct land-use management.
Contributions to the land carbon sink due to other accidental
environmental changes such as CO2 fertilisation, nitrogen
deposition and climate variability need to be quantified and excluded.
This itself is a far from a trivial matter since the cause of the
land carbon sink in each location is not known. Small scale manipulative
experiments broadly show increased photosynthesis (and water-use
efficiency) under enhanced CO2, but the applicability of this
to real resource-limited ecosystems is uncertain. For example, a recent
analysis of tree-rings in 20,000 forest plots in the eastern USA showed
no evidence of increased growth rates due to rising CO2
(Caspersen et al., 2000). Some have suggested that the Northern
hemisphere sink is instead due to nitrogen deposition which acts to
enhance growth in nitrogen-limited systems. However, recent
manipulative experiments also cast doubt on this mechanism (Nadelhoffer
et al., 1999). The sensitivity of the terrestrial carbon cycle to
climate change and climate variability (Dai and Fung, 1995, Bousquet et
al., 1999) make it especially difficult to isolate the causes of
land carbon uptake from measurement alone.
This motivates the CAMELS second
Objective : to elucidate the mechanisms responsible for the
contemporary land carbon sink (in Europe and elsewhere), isolating the
contribution due to direct land management.
How could we make optimal
use of existing data sources and the latest models to produce
operational estimates of the European land carbon sink?
As outlined above, there are a number of independent data sources which
provide valuable information on the terrestrial carbon cycle. These
include flux-measurements (e.g. from the CarboEurope cluster of
projects), forestry carbon inventories, atmospheric CO2
concentration measurements (as used in inversion studies) and satellite
products (e.g. the fraction of absorbed photosynthetically active
radiation, fAPAR, which controls the rate of photosynthesis). At
the same time terrestrial ecosystem models (TEMs) have been developed
based on the physiological and ecological processes underlying
land-atmosphere CO2 exchange. Observational estimates of the
carbon sink differ partly because the different data sources are
indicative of different time and space scales, while TEM estimates
differ largely because they contain internal parameters and stores (e.g.
leaf nitrogen, leaf area index, respiring soil carbon) which have not
been properly constrained by the observations. To produce a best
estimate of carbon uptake we need to make use of the all of the
constraints implied by the different data sources, as well as the
physiological and ecological constraints embodied in the TEMs. In
otherwords, we need to simultaneously use the observations to constrain
the internal parameters of the TEMs, whilst using the TEMs to
interpolate the observations to produce useful large-scale estimates of
the carbon sink and its causes. This is essentially a Data Assimilation
(DA) problem, requiring a system similar to those used to initialise
weather forecast models.
This motivates CAMELS third Objective
: to develop a carbon cycle data assimilation system (CCDAS) which
optimally combines data and models to produce operational estimates of
the European carbon sink and its constituent contributions