pH Changes in the Black Sea Surface Waters from 1956 to 2020

The increased concentration of the greenhouse gases (first of all, carbon dioxide—CO2) is a global problem. Compared to the pre-industrial era, the content of CO2 in the atmosphere has increased by about 40%. In the global carbon cycle, the oceans at least 30% of anthropogenic CO2 from the atmosphere. As a result, the acidity of the World Ocean waters is increasing. This process has a negative impact on marine organisms and biogeochemical cycles. A similar problem is relevant for the Black Sea. In the present study, the Black Sea data of the Institute of Natural and Technical Systems (IPTS) for the period from 1956 to 1996 were used to analyze the deep-sea pH time series and to predict the decadal-scale pH value. First of all, the pH time series were divided into two segments. These segments are characterized by different data quality and different sign of long-term pH trend. In interannual and seasonal pH variations, four (quasi)-periodic components of variability with periods of about 6.7, 2.5, 1, and ½ years were identified. The superimposition of trend, multiplicative seasonal, and interannual cyclical components indicates a decrease in pH after 1977 with an average speed of about 0.017 units pH/10 years which is close to the acidity trends in the other parts of the World Ocean. The pH forecast after 1996 using obtained decomposition was verified based on the data of 2010, 2019, and 2020.