Ocean carbon uptake under aggressive emission mitigation

Abstract

Nearly every nation has signed the UNFCC Paris Agreement, committing to mitigate anthropogenic carbon emissions so as to limit the global mean temperature increase above pre-industrial levels to well below 2 °C, and ideally to no more than 1.5 °C. A consequence of emission miti gation that has received limited attention is a reduced effi ciency of the ocean carbon sink. Historically, the roughly exponential increase in atmospheric CO2 has resulted in a proportional increase in anthropogenic carbon uptake by the ocean. We define growth of the ocean carbon sink exactly proportional to the atmospheric growth rate to be 100 % ef ficient. Using a model hierarchy consisting of a common reduced-form ocean carbon cycle model and the Community Earth System Model (CESM), we assess the mecha nisms of future change in the efficiency of the ocean carbon sink under three emission scenarios: Aggressive mitigation (1.5 °C), intermediate mitigation (RCP4.5), and high emis sions (RCP8.5). The reduced-form ocean carbon cycle model is tuned to emulate the global-mean behavior of the CESM and then allows for mechanistic decomposition. With inter mediate or no mitigation (RCP4.5, RCP8.5), changes in ef ficiency through 2080 are almost entirely the result of future reductions in the carbonate buffer capacity of the ocean. Un der the 1.5 °C scenario, the dominant driver of efficiency de cline is the ocean s reduced ability to transport anthropogenic carbon from surface to depth. As the global-mean upper ocean gradient of anthropogenic carbon reverses sign, carbon can be re-entrained in surface waters where it slows further removal from the atmosphere. Reducing uncertainty in ocean circulation is critical to better understanding the transport of anthropogenic carbon from surface to depth and to improv ing quantification of its role in the future ocean carbon sink.