Environment Counts | CO2 sequestered by the Southern Ocean can be reventilated into the atmosphere
Author: Geoff Zeiss – Published At: 2012-07-31 20:25 – (1081 Reads)
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The ocean plays a crucial role in mitigating the effects of anthropogenic CO2 on the climate system, sequestering 20 to 35 per cent of anthropogenic CO2 emissions. Considerable uncertainties remain as to the distribution of anthropogenic CO2 in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in anthropogenic CO2 sequestration. The Southern Ocean contributed over 40 per cent of the anthropogenic CO2 inventory in the ocean in 2008. A recent analysis demonstrates that anthropogenic CO2 enters the ocean in specific locations and spreads equatorwards along well-defined transport pathways, rather than uniformly around the circumpolar belt. Physical transport processes also reventilate anthropogenic CO2 contained in the ocean interior, hence estimates of net sequestration must therefore account for both subduction and reventilation. Nature Geoscience July 2012
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The release of fossil fuel CO2 to the atmosphere by human activity has been implicated as the predominant cause of recent global climate change. The ocean plays a crucial role in mitigating the effects of this perturbation to the climate system, sequestering 20 to 35 per cent of anthropogenic CO2 emissions. Considerable uncertainties remain as to the distribution of anthropogenic CO2 in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in anthropogenic CO2 sequestration. Recent research has reconstructed an observationally based, spatially resolved, time-dependent history of anthropogenic carbon in the ocean over the industrial era. The results indicate that ocean uptake of anthropogenic CO2 has increased sharply since the 1950s. The inventory and uptake rate of anthropogenic CO2 in 2008 has been estimated at 140 plusminus 25 Pg C and 2.3 plusminus 0.6 Pg C yr-1, respectively. The Southern Ocean is the primary conduit by which this CO2 enters the ocean, contributing over 40 per cent of the anthropogenic CO2 inventory in the ocean in 2008.
Reconstruction of the history of anthropogenic CO2 concentrations in the ocean
The problem of estimating anthropogenic carbon C(ant) in the ocean is challenging because
- Anthropogenic carbon is not a directly measurable quantity. It has to be estimated using indirect means.
- The anthropogenic signal in the ocean is only a few percent of the (unknown) natural background of dissolved inorganic carbon (DIC).
- Carbon in the ocean participates in rather complex in situ biogeochemistry.
- Due to long transport time scales, the Cant distribution in the ocean is highly heterogeneous.
C(ant)is typically estimated using the so-called “back calculation” methods that attempt to separate the small anthropogenic perturbation from the large background. The essential idea is that C(ant) can be estimated by correcting the measured total inorganic carbon for changes due to biological activity. The basic equation involves the measured DIC and the change in DIC due to biological processes, soft tissue remineralization and carbonate dissolution.
Reconstruction of the Ocean Sink of Anthropogenic Carbon
A study has compared three different data-based approaches for estimating the distribution of C(ant) in the ocean:
- a recently developed method based on the composite Tracer Combining Oxygen, Inorganic Carbon, and Total Alkalinity (TrOCA)
- the “historical†back-calculation method, the so-called ΔC* method
- the preformed dissolved inorganic carbon method.
All three methods were applied to data collected at the Indian-Atlantic boundary, where significant transient tracer concentrations were observed in deep and bottom waters. North of 50°S, distribution and inventories of Cant were found to be coherent with previous data-based and model estimates, but larger storage of C(ant) south of 50°S as compared to the midlatitude region was found. These results disagree with most previous estimates and suggest that the global inventory of anthropogenic CO2 in the Southern Ocean could be much larger than what is currently believed.
Distribution and inventory of anthropogenic CO2 in the Southern Ocean: Comparison of three data-based methods
A very recent study has found that C(ant) enters the ocean in specific locations and spreads equatorwards along well-defined transport pathways, rather than uniformly around the circumpolar belt. Physical transport processes in the Southern Ocean also reventilate C(ant) contained in the ocean interior, hence estimates of net sequestration must therefore account for both the subduction and reventilation of C(ant). Two often neglected physical mechanisms—eddy transport and lateral induction by the mean flow—make a significant contribution to the magnitude and distribution of the subduction of C(ant) and must be considered when investigating the upper-ocean carbon budget.
Figure The C(ant) inventory on selected isopycnal surfaces: a, 26.8, b, 26.9, c, 27, d, 27.1 and e, 27.2. White patches highlighted with a thick blue line are regions of subduction maxima. White patches with a thick green line are regions of reventilation maxima.
The analysis demonstrates that the subduction of C(ant) in the Southern Ocean depends on physical variables that are sensitive to climate variability and change, including wind stress, eddy fluxes, surface currents and mixed-layer depth. The results provide an observationally based estimate of the spatial distribution and magnitude of C(ant) transport from the surface to the ocean interior that can be used to assess models.
Localized subduction of anthropogenic carbon dioxide in the Southern Hemisphere oceans
