Hydrodynamics and Salinity of the Caspian Sea Under Future Climate Change

Hydrodynamic simulations with the Delft3D model for CMIP6 climate scenarios show that during the 2021–2100 period, the Caspian Sea would experience considerable changes in water level, surface circulation, and horizontal salinity distribution. The calibrated model was able to reproduce historic water-level fluctuations and spatial patterns of salinity, instilling confidence in its predictive skill. The hydrodynamic response of the basin is mainly controlled by the evaporation-precipitation-river discharge balance, focusing on the Volga River, which delivers over 80% of the freshwater inflow. Under the low-emission SSP1-2.6 scenario, the sea level remains almost unchanged with no substantial increase in salinity. Under SSP2-4.5, a moderate fall in water level will result in hydrological isolation of Kara-Bogaz-Gol Bay and shoreline retreat at the northern and eastern parts. Salinity changes remain low but detectably spatial. The model of SSP3-7.0 simulates a drop of approximately 9.5m and salinities rise to about 16.6 ppt in this scenario. These changes result in severe desiccation in shallow northern and eastern zones and high salt concentration in the eastern basin. The greatest impacts will be in SSP5-8.5. A decrease in sea level by about 13 m will occur, salinity levels will rise to about 16.8 ppt. In this extreme drying scenario, the eastern coast is severely dried out, with Kara-Bogaz-Gol Bay completely disconnected in most of the simulations. The hydrodynamic results overall suggest that under high-emission pathways, the Caspian Sea is likely to evolve to a shallower and more saline system, with dire consequences for coastal environments, ecosystems, and the long-term management of water resources.