Long‐term alkalinity trends in the Baltic Sea and their implications for CO 2 ‐induced acidification

Anthropogenic CO 2 emissions currently decrease open ocean pH, but on multi‐millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity ( A T ) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such A T dynamics, due to its direct link to terrestrial processes, short water residence time and long history of A T measurements dating back to the early 20 th century. We compiled an extensive A T data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water A T levels increased throughout the Baltic Sea. The rates of change were highest in the low‐saline, northern areas and decreased gradually toward constant levels in the North Sea. The A T increase observed in the Central Baltic Sea (+3.4 µmol kg −1 yr −1 ) and the Gulf of Bothnia (+7 µmol kg −1 yr −1 ) has compensated CO 2 ‐induced acidification by almost 50% and 100%, respectively. Further, the A T trends enhanced the CO 2 storage capacity and stabilized the CaCO 3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the A T trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO 2 , agricultural liming and internal A T sources.