Mechanisms of ocean carbon cycle variability in the 21st Century

The ocean is an enormous and variable sink of carbon dioxide gas (CO2) for the atmosphere, and a detailed knowledge of the drivers of uptake variability is needed to predict future climate change. Here, a leading-edge ocean computer model is used to attribute 21st Century ocean carbon cycle variability to underlying causal physical, chemical, and biological mechanisms. First, North Atlantic carbon flux variability across a range of timescales is attributed to each component of the flux equation: the air-sea concentration gradient (the difference of partial pressures, ∆pCO2), the gas transfer velocity (which quantifies how environmental factors e.g. wind enhance gas exchange, k), and the solubility coefficient (which quantifies how temperature and salinity affect gas dissolution, α). Both ∆pCO2 and k are strong controls on interannual flux variability, but the longer decadal and multidecadal changes are dominated by just ∆pCO2. Next, the drivers of North Atlantic Dissolved Inorganic Carbon (DIC) inventory changes are identified. Interannual variations in temperature and preformed alkalinity cause almost all the basin’s year-to-year DIC fluctuations. Decadal variability is attributed to saturation and anthropogenic carbon forcing. Multidecadal cycles and the trend up to the year 2100 are dominated by anthropogenic carbon uptake. Finally, the global DIC inventory variance is quantified, highlighting Pacific up-welling of remineralised carbon as the main driver of interannual variability. Anthropogenic carbon is the largest single contributor to variability on longer timescales up to 2100, with other processes playing secondary or negligible roles