Enhanced biological carbon consumption in a high CO₂ ocean: a revised estimate of the atmospheric uptake efficiency

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.