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| Titel |
A model-based assessment of the TrOCA approach for estimating anthropogenic carbon in the ocean |
| VerfasserIn |
A. Yool, A. Oschlies, A. J. G. Nurser, N. Gruber |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 2 ; Nr. 7, no. 2 (2010-02-24), S.723-751 |
| Datensatznummer |
250004499
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| Publikation (Nr.) |
 copernicus.org/bg-7-723-2010.pdf |
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| Zusammenfassung |
| The quantification of the amount of anthropogenic carbon (Cant) that
the ocean has taken up from the atmosphere since pre-industrial times
is a challenging task because of the need to deconvolute this signal
from the natural, unperturbed concentration of dissolved inorganic
carbon (DIC). Nonetheless, a range of techniques have been devised
that perform this separation using the information implicit in other
physical, biogeochemical, and man-made ocean tracers. One such
method is the TrOCA approach, which belongs to a group of
back-calculation techniques, but relative to other methods employs a
simple parameterization for estimating the preformed, pre-industrial
concentration, the key quantity needed to determine Cant. Here we
examine the theoretical foundation of the TrOCA approach and test its
accuracy by deconvoluting the known distribution of Cant from an
ocean general circulation model (OGCM) simulation of the industrial
period (1864–2004). We reveal that the TrOCA tracer reflects the
air-sea exchange of both natural and anthropogenic CO2 as well as
that of O2. Consequently, the determination of the anthropogenic
CO2 flux component requires an accurate determination not only of
the contribution of the natural (pre-industrial) CO2 flux
component, but also of the O2 flux component. The TrOCA method
attempts to achieve this by assuming that the concentration changes
invoked by these two air-sea flux components scale with temperature
and alkalinity. While observations support a strong exponential
scaling of the oxygen flux component with temperature, there exists no
simple relationship of the natural CO2 flux component with
temperature and/or alkalinity. This raises doubts whether the sum of
these two components can be adequately parameterized with a single
function. The analyses of the model support this conclusion, even
when Cant is deconvoluted using parameter values that were optimized
on the basis of the synthetic dataset from the model. Application of
an optimal, but globally uniform set of parameters for the estimation
of Cant results in a global positive bias in the inventory of more
than a factor of two, suggesting that a "universal" TrOCA
parameterisation is not achieveable. Even the application of
regionally specific sets of parameters causes, on average, a global
positive bias of more than 50%. This is substantially larger than the
potential positive bias of 7% identified for the ΔC*
method using a similar model-based assessment method. |
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