Decadal‐Scale Increases of Anthropogenic CO2 in Antarctic Bottom Water in the Indian and Western Pacific Sectors of the Southern Ocean.

We determined decadal‐scale increases of anthropogenic CO2 in the water column using data sets collected 17 years apart (1994–1996 and 2012–2013) along a transect at nominal 62°S in the Indian and western Pacific sectors of the Southern Ocean. Large increases of anthropogenic CO2 (up to 9.1 ± 1.5 μmol/kg), closely following atmospheric CO2 increases, were found in Antarctic Bottom Water (AABW), previously considered a small sink of anthropogenic CO2. Vertical distributions of anthropogenic CO2 increases showed significant positive correlations with those of changes in CFC‐12 and SF6, implying that the distributions were mainly controlled by physical processes such as ventilation and circulation. Calculated uptake rates of anthropogenic CO2 by AABW were between 0.29 and 0.39 mol·m−2·yr−1 in five longitudinal segments of the transect. In accounting for the large increase of anthropogenic CO2 in AABW, sea surface conditions in the formation region of AABW are important. Plain Language Summary: The ocean absorbs about 30% of CO2 emitted into the atmosphere as a result of human activities, which we term "anthropogenic CO2." The Southern Ocean undertakes about 40% of anthropogenic CO2 entering the ocean. Therefore, to investigate Southern Ocean CO2 uptake is an important task for the prediction of global warming caused by anthropogenic CO2 increases in the atmosphere. Here we focus anthropogenic CO2 in the bottom layers below approximately 2,500 m deep, where storage of anthropogenic CO2 has been considered to be little. Using high‐quality data for oceanic CO2, we find significant increases of anthropogenic CO2 in the bottom layers. Our findings highlight the importance of anthropogenic CO2 stored in the bottom layers in estimates of the global carbon budget, because the anthropogenic CO2 in the bottom layers is transported into the entire world ocean through the ocean circulation. Key Points: Anthropogenic CO2 increased up to 9.1 ± 1.5 μmol/kg over 17 years in Antarctic Bottom Water (AABW), previously considered a small CO2 sinkCO2 increases correlate with increased CFC‐12 and SF6, implying that ventilation and circulation control anthropogenic CO2 uptake by AABWFor a high uptake of anthropogenic CO2 by AABW, sea surface conditions in the formation region of AABW are important [ABSTRACT FROM AUTHOR]