The Caspian Sea, renowned for its diverse flora and fauna, plays a crucial economic role for the coastal states. The socio-economic development in the coastal regions is heavily influenced by the long-term fluctuations in the Caspian Sea's water level. A significant drop in the sea level can severely impact oil and gas production and the fishing industry in these coastal areas. Additionally, it creates operational challenges for international and domestic ferry transport and hinders the docking of passenger and cargo ships. Fish habitats are also disrupted.

Due to decades-long cycles of rising and falling sea levels, the Caspian Sea's ecosystem and the economies of the coastal nations must adapt to these significant changes.

Instrumental measurements of the Caspian Sea level have been recorded since 1840. The highest recorded level was in 1882 at -25.2 meters (Baltic System), and the lowest was in 1977 at -29.0 meters, showing a fluctuation amplitude of 3.8 meters.

In the 20th century, three distinct periods of low-level fluctuations were observed. From 1900 to 1929, the level was relatively stable at around -26.3 meters. From 1930 to 1977, the level steadily decreased, dropping by 1.75 meters between 1930 and 1938, reaching -29.0 meters in 1977, and reducing the sea's area by 48,000 km². From 1978 to 1995, the level rose sharply by 2.0 meters. However, towards the end of the 20th century and into the early 21st century, the level began to decline again. Between 2020 and 2023, the sea level in the Azerbaijani sector of the Caspian dropped by 75 cm, reaching -28.7 meters as of January 1, 2024.

Various factors influence long-term sea level changes and can be categorized into four groups: cosmo geophysical forces, geological and geodynamic processes, hydrometeorological processes, and anthropogenic factors. Analyzing these cause-and-effect relationships reveals that the long-term variability of the Caspian Sea level is an indicator of large-scale moisture exchange within the ocean (North Atlantic) - atmosphere - land surface (Volga River basin) system. In the 20th century, about 64% of the level change was due to river waters from the Volga River basin. Including contributions from approximately 130 other rivers, such as the Kura, Ural, Terek, and Sulak, this figure rises to 70%. The remaining 30% is primarily due to evaporation, with the average annual air temperature in the Caspian region (sea and adjacent areas) rising by 0.03°C annually, or 1.2°C overall, accelerating evaporation.

Another significant factor is the subsidence of the sea floor due to tectonic movements, which currently affects the Mediterranean, Black, and Azov seas that were once connected to the Caspian Sea. Unlike these seas, the Caspian lacks water exchange with the Atlantic Ocean, making it more susceptible to level drops. The destruction of the dam separating the Kara-Bogaz-Gol Bay (Qara-Boğaz-Qol körfəzi) from the Caspian in 1992 has also contributed to the level decline, as water from the sea flows into the bay and evaporates. The strait connecting the sea and the bay has a bottom height of -30.6 meters, and once the sea level drops to this point, the flow to the bay will stop, potentially stabilizing the Caspian Sea level.

The coastal states are highly concerned about the rapid decline in the Caspian Sea level. In July 2022, an international conference in Baku addressed these fluctuations, with experts from coastal and foreign countries and international organizations participating.

The current priority is to develop adaptation measures based on realistic medium- and long-term sea level change forecasts. Despite numerous prediction methods, an accurate and reliable method has yet to be established, as the mechanisms driving multi-year level fluctuations remain unclear.

Farda Imanov, Professor of the Department of Hydrometeorology of the Faculty of Geography of BSU