Rising atmospheric CO(2) leads to large impact of biology on Southern Ocean CO(2) uptake via changes of the Revelle factor

The Southern Ocean is a key region for global carbon uptake and is characterized by a strong seasonality with the annual CO(2) uptake being mediated by biological carbon drawdown in summer. Here we show that the contribution of biology to CO(2) uptake will become even more important until 2100. This is the case even if biological production remains unaltered and can be explained by the decreasing buffer capacity of the ocean as its carbon content increases. The same amount of biological carbon drawdown leads to a more than twice as large reduction in CO(2(aq)) concentration and hence to a larger CO(2) gradient between ocean and atmosphere that drives the gas exchange. While the winter uptake south of 44°S changes little, the summer uptake increases largely and is responsible for the annual mean response. The combination of decreasing buffer capacity and strong seasonality of biological carbon drawdown introduces a strong and increasing seasonality in the anthropogenic carbon uptake. KEY POINTS: Decrease of buffer capacity leads to stronger summer CO(2) uptake in the future. Biology will contribute more to future CO(2) uptake in Southern Ocean. Seasonality affects anthropogenic carbon uptake strongly;