Long-term response of oceanic carbon uptake to global warming via physical and biological pumps

Abstract. Global warming is expected to significantly decrease oceanic carbon uptake and therefore increase atmospheric CO₂ and global warming. The primary reasons given in previous studies for such changes in the oceanic carbon uptake are the solubility reduction due to seawater warming and changes in the ocean circulation and biological pump. However, the quantitative contributions of different processes to the overall reduction in ocean uptake are still unclear. In this study, we investigated multi-millennium responses of oceanic carbon uptake to global warming and quantified the contributions of the physical and biological pumps to these responses using an atmosphere–ocean general circulation model and a biogeochemical model. We found that global warming reduced oceanic CO₂ uptake by 13 % (30 %) in the first 140 years (after 2000 model years), consistent with previous studies. Our sensitivity experiments showed that this reduction is primarily driven by changes in the organic matter cycle via ocean circulation change and solubility change due to seawater warming. These results differ from most previous studies, in which circulation changes and solubility change from seawater warming are the dominant processes. The weakening of biological production and carbon export induced by circulation change and lower nutrient supply, diminishes the vertical DIC gradient and substantially reduces the CO₂ uptake. The weaker deep-ocean circulation decreases the downward transport of CO₂ from the surface to the deep ocean, leading to a drop in CO₂ uptake in high-latitude regions. Conversely, weaker equatorial upwelling reduces the upward transport of natural CO₂ and therefore enhances the CO₂ uptake in low-latitude regions. Because these effects cancel each other out, circulation change plays only a small direct role in the reduction of CO₂ uptake due to global warming but a large indirect role through nutrient transport and biological processes.