A team of Russian researchers studied the reduction in the area of the Caspian Sea and the Kara-Bogaz-Gol Bay over the period from 1993 to 2025. As reported by the Russian Science Foundation (RSF), the results of the work, supported by a grant from the organization, were published in the journal “Modern Problems of Remote Sensing of the Earth from Space.”

The level of the Caspian Sea has been continuously declining since 1995 at a rate of about 10 cm per year. During this period, it has fallen by almost 3 m. The scientists developed a methodology based on the analysis of high-resolution satellite data, which made it possible to obtain up-to-date hypsometric curves that will help assess the actual loss of water surface area. These data are necessary for accurately calculating the water balance and evaporation from the reservoir’s surface.

“We needed to obtain up-to-date hypsometric curves for the Caspian Sea and the Kara-Bogaz-Gol Bay separately based on data from Landsat-5, -7, and -8 satellite sensors with a spatial resolution of 30 m for 1993–2025. In addition, we wanted to find out how much the area of the Caspian Sea has actually shrunk and where the shallowing of the Caspian is occurring on the largest scale,” said RSF grant supervisor Andrei Kostianoy, chief researcher at the Shirshov Institute of Oceanology of the Russian Academy of Sciences.

The shallowing has already led to negative consequences for the region. In particular, navigation has become more difficult due to the drop in water level in the Volga-Caspian Sea Shipping Canal and ports, and the shoreline has begun to recede from populated areas. According to experts’ forecasts, by the end of the 21st century the sea level may fall by another 2 to 21 m. The new methodology will make it possible to promptly assess the further reduction in the area of the sea and the bay.

The specialists analyzed data for the period from 1993 to 2025. It was established that in 2025 the Caspian level dropped to –29.4 m. This figure was below the historical minimum of 1977, when the sea level was –29.0 m. The scientists found that the surface area of the Caspian Sea decreased by 28,644 sq. km from 1996 to 2025, which is 7.4% of the level recorded 30 years ago. According to the experts’ conclusions, in recent decades the Caspian has lost an area comparable in size to Albania.

In the course of the work, the researchers obtained regression equations with a high statistical relationship (correlation coefficients of 0.95–0.99). The scientists recommend using them to calculate the area of the Caspian Sea at levels from –26 to –29 m, as well as to assess the condition of the Kara-Bogaz-Gol Bay at levels from –27.1 to –31 m.

“The results obtained are important for assessing the so-called morphometric factor and its role in the water balance of the Caspian Sea, namely in reducing evaporation from the shrinking surface of the Caspian as its level declines. This factor should slow the rate of sea-level decline, all other things being equal,” Kostianoy commented on the results of the work.

However, satellite altimetry data do not yet confirm a slowdown in the process. If in 2005–2019 the sea was losing an average of 8.16 cm in level annually, then in 2020–2024 this rate increased to 22.7 cm per year. According to Kostianoy, the interannual variability of the components of the Caspian’s water balance still significantly exceeds the effect of the reduction in the evaporation area.