Abstract
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.
Highlights
Since the beginning of the industrial era, global oceans have reduced the atmospheric CO2 concentration and mitigated climate change by taking up approximately 25 % of anthropogenic CO2 emissions (Ciais et al, 2013)
We investigated multi-millennium changes in the ocean carbon cycle under a quadrupling of atmospheric carbon dioxide using an atmosphere–ocean general circulation models (AOGCMs) and an offline biogeochemical model
At the end of the simulation, the cumulative CO2 uptake is 2028 PgC and global warming reduces the uptake by 30 %; this primarily occurs in the tropical and subtropical regions
Summary
Since the beginning of the industrial era, global oceans have reduced the atmospheric CO2 concentration and mitigated climate change by taking up approximately 25 % of anthropogenic CO2 emissions (Ciais et al, 2013). Yamamoto et al.: Long-term response of oceanic carbon uptake to global warming are major contributors to the reduction in oceanic carbon uptake (Sarmiento et al, 1998; Joos et al, 1999; Matear and Hirst, 1999; Plattner et al, 2001; Matsumoto et al, 2010) In such previous studies, changes in the biological pump associated with ocean circulation change were regarded as a second-order process even though the biological pump plays a crucial role in the natural carbon cycle. Previous studies using Earth system Models of Intermediate Complexity (EMICs) show that global warming decreases oceanic CO2 uptake continuously over 1000 years or more (Plattner et al, 2008; Schmittner et al, 2008; Archer et al, 2009a) These millennial-scale simulations are feasible with an AOGCM due to increased computer power.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
