Reduced CO2 Uptake and Growing Nutrient Sequestration from Slowing Overturning Circulation
We examine output from a suite of thirty-six CMIP6 models to quantify the impacts of climate warming on deep ocean circulation, as reflected in the overturning rates of the Atlantic Meridional Overturning Circulation (AMOC) and the Southern Meridional Overturning Circulation (SMOC). We then study how climate-driven slowing of both AMOC and SMOC impacts ocean anthropogenic CO2 uptake, and the ocean distributions of carbon and the key nutrients nitrate and phosphate.
Our results show there is a drastic reduction in both overturning cells with climate warming, even under the moderate-warming SSP1-2.6 scenario. The slowing deep circulation weakens uptake of anthropogenic CO2 by the ocean solubility pump, proving a positive feedback, that could extend or intensify peak-warmth climate conditions. The reduced uptake by the solubility pump is partially compensated by a more efficient biological pump, that transfers carbon and nutrients to the deep ocean. On multi-century timescales this will increasingly sequester nutrients in the deep ocean, leading to declining global-scale net primary production (NPP). The Integrated Assessment Models (IAMS) used to generate the CMIP6 climate scenarios likely overestimated ocean CO2 uptake by not properly accounting for the slowing deep circulation. Thus, emissions reduction goals for climate stabilization may need to be revised.
The ocean biological pump refers to organic matter sinking from surface waters to decompose in the interior ocean. This acts to move carbon and key nutrients from the surface to the deep ocean. The deep circulation must eventually return these exported nutrients to the surface to maintain biological productivity. We demonstrate that climate-driven slowing of the deep circulation weakens this return flow to the surface, allowing nutrients to increasingly accumulate in the deep ocean. The slowing circulation also decreases the transport of absorbed anthropogenic CO2 into the deeper ocean, weakening the ocean capacity to remove anthropogenic CO2 from the atmosphere, providing a positive feedback for climate warming.