Estimates of probable change in water balance parameters of the Black and Azov Seas drainage basin under the global warming by 1°C were obtained. A steady-state hydrological model has been developed for evaluation of river runoff and evaporation. The paleoclimatic reconstruction for global warming on 1°C has been used as empirical scenario. This scale of climate change corresponds to the warm epoch of the past, considered as an analogue of the future climate: the Holocene Thermal Maximum (7-6 ka BP). The estimates indicate possible increase in the total river inflow into the Black and Azov Seas by about 17 km³ per year and increase of the mean annual evaporation by 27 mm per year relative to the period of 1961-1990 years. Estimation was made notwithstanding water consumption.