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¹ Marine Hydrophysical Institute, Kapitanskaya St. Sevastopol, 99011, The Crimea, Ukraine DOI : 10.4194/1303-2712-v12_2_27 Viewed : 4399 - Downloaded : 3774 The article’s goal is to assess the rate of acidification of the Black sea upper layer in XX century using historical data since 1924. It is shown that statistically significant century-scale acidification cannot be extracted, while decadal-scale reduction of pH has been really observing in 1960’s and between 1980 and 2000 in spite of high noise level and intense interannual pH variability. The rate of acidification for these periods reached 0.4 (0.2) pH units per decade at the surface (10 m depth). Such high level of acidification of the Black sea upper layer is mostly due not to the rise of carbon dioxide concentration in the atmosphere, but to natural quasi-periodical decadal-scale intensification of upward motions in the subsurface layer of the Black sea transporting the water with low-pH to the surface. Rough assessment of recent century-scale rate of acidification of the surface Black sea layer shows that its likely magnitude lies in the range -0.1 to -0.5 pH units per 100 years. Keywords : pH variability, quasi-periodical decadal-scale intensification of vertical motion in the Black sea, space-time inhomogeneity of pH observations