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Biogeosciences An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/bgd-6-8101-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
 
10 Aug 2009
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A revision of this discussion paper for further review has not been submitted.
Enhanced biological carbon consumption in a high CO2 ocean: a revised estimate of the atmospheric uptake efficiency
R. Matear1 and B. McNeil2 1Centre for Australian Weather and Climate Research (CAWCR), A partnership between CSIRO and the Bureau of Meteorology, Sydney, Australia
2Climate Change Research Centre, Faculty of Science, University of New South Wales, Sydney, Australia
Abstract. A recent mesocosm study under high CO2 conditions has found phytoplankton carbon consumption is elevated beyond typical Redfield ratios (Riebesell et al., 2007). We investigate the efficacy of this elevated biological carbon consumption to increase global oceanic CO2 uptake from the atmosphere in an ocean general circulation model (OGCM). In the OGCM, elevated biological carbon consumption throughout the ocean increased oceanic CO2 uptake by 46 Pg C during 1800 to 2100 period, which is less than half the value estimated by (Riebesell et al., 2007). Our study's lower ratio of oceanic CO2 uptake from the atmosphere caused by enhanced biological carbon consumption (export production) is due to a more realistic 3-D circulation and the resulting spatial patterns in the re-supply of carbon from the interior ocean to the surface. In our OGCM simulations, despite increased biological carbon export to the ocean interior, some regions like the eastern equatorial Pacific and Southern Ocean actually take up less CO2 from the atmosphere. This is due to the pooling of exported carbon at intermediate depths within these regions (analogous to nutrient trapping) and its subsequent re-supply back to the surface that exceeds the enhanced biological carbon export in the high CO2 world. Thus large-scale increases in biological carbon export can lead to some areas where surface ocean pCO2 increases more rapidly than atmospheric CO2. Furthermore, our results demonstrate that enhancing biological carbon export via other means such as iron fertilization is inefficient in regions like the Southern Ocean because of the rapid vertical re-supply of carbon-rich waters. This vertical resupply of carbon-rich waters in the Southern Ocean dampens the oceanic CO2 uptake efficiency due to enhanced biological carbon consumption to be only 16% and suggests a very low efficacy of biological fertilization in the region.

Citation: Matear, R. and McNeil, B.: Enhanced biological carbon consumption in a high CO2 ocean: a revised estimate of the atmospheric uptake efficiency, Biogeosciences Discuss., 6, 8101-8128, https://doi.org/10.5194/bgd-6-8101-2009, 2009.
R. Matear and B. McNeil
R. Matear and B. McNeil

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