Caspian Sea Level Decline by 2035: Mathematical and Regional Assessment

The Caspian Sea is the world’s largest endorheic water body and has experienced substantial fluctuations in water level during the twentieth and twenty-first centuries. Recent climate projections indicate that increasing temperature, enhanced evaporation, and changes in riverine inflow may intensify the long-term decline in Caspian Sea level. This study builds on the mathematical and geometrical framework presented by Ghorbani (2025) for assessing shoreline and geometric changes along Iran’s Mazandaran coast and applies a 3-m water-level decline by 2035 as a regional scenario for the entire Caspian Sea. The main objectives are to estimate the potential changes in the total surface area and water volume of the Caspian Sea and to assess the potential consequences for coastal geometry, navigation, ports, and offshore oil and gas infrastructure in the five littoral states: Iran, Kazakhstan, Russia, Turkmenistan, and Azerbaijan. The present Caspian Sea surface area is approximately 389,000 km 2 and its volume is approximately 78,000 km 3 . Because the relationship between water level and surface area is strongly nonlinear, a 3-m decline cannot be converted into an area loss using a simple linear relationship. Nevertheless, historical observations and recent bathymetric studies suggest that a decline of this magnitude could result in the loss of several tens of thousands of square kilometers of water surface, particularly in the shallow northern Caspian. In this study, an initial scenario range of approximately 30,000-45,000 km 2 of surface-area loss is proposed, while the corresponding reduction in water volume is estimated at approximately 1,000-1,200 km 3 . The consequences are not spatially uniform. Kazakhstan and Russia are expected to face the greatest direct risks to shallow-water offshore energy infrastructure because of fields such as Kashagan and Vladimir Filanovsky. Turkmenistan may also face substantial risks because of offshore developments in the Cheleken area and the shallow eastern Caspian. In Azerbaijan, major fields such as Azeri-Chirag-Gunashli are located in considerably deeper waters, making their direct exposure to a 3-m reduction relatively limited; however, ports, coastal terminals, navigation routes, and logistics infrastructure may be affected. In Iran, the principal impacts are expected to involve shoreline retreat, ports, wetlands, fisheries, tourism, and coastal infrastructure rather than large-scale offshore hydrocarbon production. The results indicate that a 3-m decline, although substantially smaller than the 8-14 m best-fit projections and the upper-end projections of up to 21 m by the end of the twenty-first century, should be considered an important early-warning scenario for regional infrastructure and energy planning. The study further demonstrates the potential value of extending the mathematical shoreline framework developed by Ghorbani (2025) toward a basin-wide model of energy-infrastructure vulnerability.