Ocean Carbon Uptake Under Agressive Emission Mitigation

Nearly every nation has signed the Paris Agreement, committing to reduce global anthropogenic carbon (C_ant) emissions and limit temperature increase to 1.5˚C. Though it is clear that strong mitigation will reduce the magnitude of the ocean carbon sink, the mechanisms of this decline has not been studied in detail. Historically the ocean sink has grown at a rate that is approximately proportional to the atmospheric CO₂ perturbation, and we define this proportionality as 100% efficiency. Deviation from this expected relationship can be defined as a change in efficiency. The efficiency is controlled by a variety of effects related to reduced air-sea disequilibrium (concentration effects) and reductions due to surface warming (warming effects). We evaluate the impacts of warming effects and concentration effects by comparing future uptake efficiency to historical uptake efficiency in the CESM Large Ensemble suite and the CMIP6 multi-model ensemble. These simulations are forced with SSP585 (low mitigation), SSP245 (moderate mitigation), and SSP126 (high mitigation). Efficiency losses are greatest in the subpolar regions of the North Atlantic and Southern Ocean, where intermediate waters outcrop. The greatest loss of uptake efficiency occurs in the aggressive mitigation scenario, resulting in a near-zero sink in 2100. This efficiency loss is due to the reemergence of C_ant from intermediate waters, and is not offset by a relatively higher buffer capacity and reduced warming. Because most anthropogenic carbon is stored in the upper ocean, a trajectory wherein atmospheric pCO2 peaks sharply and then is followed by a rapid drawdown leads to intermediate waters holding more Cant than is in equilibrium with the atmosphere during the drawdown phase. The equilibration of these waters leads to a severe reduction of the ocean carbon sink through 2100.