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

Abstract

Abstract. Global warming is expected to significantly decrease oceanic carbon uptake and therefore increase atmospheric CO 2 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 CO 2 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 CO 2 uptake. The weaker deep-ocean circulation decreases the downward transport of CO 2 from the surface to the deep ocean, leading to a drop in CO 2 uptake in high-latitude regions. Conversely, weaker equatorial upwelling reduces the upward transport of natural CO 2 and therefore enhances the CO 2 uptake in low-latitude regions. Because these effects cancel each other out, circulation change plays only a small direct role in the reduction of CO 2 uptake due to global warming but a large indirect role through nutrient transport and biological processes.

Sensitivity of atmospheric CO<sub>2</sub> to regional variability in particulate organic matter remineralization depths

“…A disadvantage to using a steady-state circulation is that we cannot quantify impact of the CO 2 -climate feedback on ocean circulation and atmospheric CO 2 . Studies exploring the simultaneous effects of warming temperatures on circulation and biology in response to anthropogenic CO 2 emissions show that changes in circulation could be as important as bi-ological changes (Cao and Zhang, 2017;Yamamoto et al, 2018). Quantifying the regional sensitivity with a dynamic ocean is therefore an important focus for future work.…”

Description of the MIROC-ES2L Earth system model and evaluation of its climate–biogeochemical processes and feedbacks

“…Global warming induces loss of carbon from the land to the atmosphere by accelerating ecosystem respiration (Arora et al, 2013;Todd-Brown et al, 2014;Friedlingstein et al, 2014), while ocean 75 surface warming reduces the solubility of CO2 in seawater. Intensification of upper-ocean stratification and weakening of the biological pump by global warming also prevent effective transportation of dissolved carbon into the deeper ocean (Frölicher et al, 2015;Yamamoto et al, 2018). Global warming might lead to localized intensification of the natural carbon sink (e.g., lengthening of the growing season and exposure of the ocean surface through melting of sea ice).…”

Glacial CO<sub>2</sub> decrease and deep-water deoxygenation by iron fertilization from glaciogenic dust

“…The oceanic carbon cycle has been proposed as a driver of glacial-interglacial CO 2 change; however, the magnitude of glacial CO 2 reduction of 80-100 ppm has yet to be fully reproduced by numerical model simulations using both an ocean general circulation model (OGCM) and a biogeochemical model (Ciais et al, 2013). The oceanic soft-tissue biological pump, by which the photosynthetic production, sinking, and remineralization of organic matter store dissolved inorganic carbon in the deep ocean, is among the mechanisms controlling glacial-interglacial as well as future atmospheric CO 2 change (Sarmiento and Gruber, 2006;Sigman et al, 2010;Yamamoto et al, 2018). During glacial periods, the efficiency of the biological pump would have been enhanced by biogeochemical processes (e.g., dust-borne iron fertilization, Martin, 1990; and an increase in nutrient inventory associated with a sea-level drop, Broecker, 1982;Wallmann et al, 2016), leading to the transfer of carbon from the atmosphere to the deep ocean.…”