Anthropogenic carbon storage and its decadal changes in the Atlantic between 1990–2020

Abstract. The Atlantic inventory of anthropogenic carbon (Cant) and its changes between 1990 and 2020 are investigated by applying the transit time distribution (TTD) method to anthropogenic tracer data. In contrast to previous TTD applications, here we take into account the admixture of old waters free of anthropogenic tracers. The greatest difference from other methods based on direct carbon observations is the higher Cant storage in the deep ocean. Estimations of the decadal Cant increase based on direct carbon observations yield in general a smaller share of Cant storage in the North Atlantic and a larger share in the South Atlantic compared to our results. Changes in oceanic circulation and/or ventilation have significant impacts on the Cant inventory on the regional scale. The enhanced upwelling of older water in the Southern Ocean and the variability in the convection depth in the Labrador Sea lead to deviations in the inferred Cant increase between 1990 and 2020 from the rate equivalent to a steady-state ocean. For the total Atlantic Cant inventory, however, decadal ventilation variability of individual water masses partially compensates for each other. In addition, its impact on the Cant storage is small due to the much higher flushing time for the whole Atlantic of the order of hundreds of years. The total Cant inventory increases from 43.0 ± 7.3 Pg C in 1990 to 68.2 ± 10.8 Pg C in 2020, almost in unison with the rising CO2 in the atmosphere. So far, ventilation changes have impacted the Cant concentrations only on the regional scale, especially in the subpolar North Atlantic and the Southern Ocean.