Multi‐Century Changes in the Ocean Carbon Cycle Controlled by the Tropical Oceans and the Southern Ocean. Issue 12 (24th November 2021) - Chikamoto, Megumi O.
Abstract: The oceanic absorption of anthropogenic carbon dioxide (CO2 ) is expected to continue in the following centuries, but the processes driving these changes remain uncertain. We studied these processes in a simulation of future changes in global climate and the carbon cycle following the RCP8.5 high emission scenario. The simulation shows increasing oceanic uptake of anthropogenic CO2 peaking towards the year 2080 and then slowing down but remaining significant in the period up to the year 2300. These multi‐century changes in uptake are dominated by changes in sea‐air CO2 fluxes in the tropical and southern oceans. In the tropics, reductions in upwelling and vertical gradients of dissolved carbon will reduce the vertical advection of carbon‐rich thermocline waters, suppressing natural outgassing of CO2 . In the Southern Ocean, the upwelling of waters with relatively low dissolved carbon keeps the surface carbon relatively low, enhancing the uptake of CO2 in the next centuries. The slowdown in CO2 uptake in the subsequent centuries is caused by the decrease in CO2 solubility and storage capacity in the ocean due to ocean warming and changes in carbon chemistry. A collapse of the Atlantic Meridional Overturning Circulation (AMOC) predicted for the next century causes a substantial reduction in the uptake of anthropogenic CO2 . In sum, predicting multi‐century changes in the global carbon cycle depends on future changes in carbon chemistry along with changes in oceanic and atmospheric circulations in the Southern and tropical oceans, together with a potential collapse of the AMOC. Plain Language Summary: The ocean is absorbing and storing a significant fraction of anthropogenic carbon dioxide (CO2 ), but we do not know if this effect will weaken in subsequent centuries with important consequences for predicting long‐term climate changes. We investigated these changes in a simulation of future climate and the global carbon cycle in which atmospheric CO2 concentrations continue to increase following a "business as usual" scenario. The simulation shows that the oceanic uptake of CO2 will peak towards the end of this century, then slow down, but remain substantially high through the year 2300. The long‐term increase in uptake is maintained by exchanges of dissolved carbon between the surface and the deep ocean in the tropical and southern oceans. The slowdown in uptake in subsequent centuries is caused by reduced solubility of CO2 due to ocean warming and reduced ability of the ocean to absorb CO2 due to changes in carbon chemistry. This effect is enhanced as more carbon dissolves and accumulates in the ocean. In sum, under high emissions, multi‐century changes in the global carbon cycle will depend on future changes in ocean carbon chemistry as well as atmospheric and ocean changes in the tropics and the Southern Ocean. Key Points: In a high emission scenario, oceanic CO2 uptake is predicted to peak by 2080, then slow down and remain at about half of its peak by 2300 The multi‐century changes in uptake are expected to be dominated by changing air‐sea CO2 fluxes in the Southern and the tropical oceans The slowdown in uptake is caused by the decrease in CO2 solubility and storage capacity due to changes in temperature and carbon chemistry