A Streams of Thought contribution by Hayat Nasirova

The Caspian Sea (CS) is the largest lake on Earth. It is located at the intersection of Europe and Asia. It is called “sea” because it has an ocean-type earth layer at the bottom as being a remnant of the Tethys Ocean and has the dimensions of a sea. The water level is variable, currently it is approximately -28 meters below the ocean level.

The current worry of some hydrologists is whether the CS will follow the same fate as the  Aral lake. NASA’s Global Water Monitor, which uses radar altimetry data collected by multiple satellites, shows changes in water levels in the CS since the mid-1990s (Figure 1).

The Caspian water level changes occur due to river flooding, meteorological forcing, steric changes, tsunami, tidal effects, mud volcanic activities and water budget. 130 large and small rivers flow into the Caspian Sea. The income of the water balance (80%) is made up of river flows, of which up to 85% is accounted for by the waters brought by the Volga River. According to the Ministry of Ecology and Natural Resources of the Republic of Azerbaijan, observations show that river flows undergo changes in a large interval compared to the average value. The reason for this is the hydrometeorological processes that take place in the very large catchment area of ​​the sea (about 20% of the area of ​​the northern hemisphere). During the last 3500 years the sea level change was 15 m, and in the last 200 years it was 3.5 m. 

In terms of precipitation, it is unevenly distributed on the shores of the CS and the most of it falls on the south-western coast of the sea, which amounts to 1700 mm per year. The northern and eastern coasts of the sea are characterized by low rainfall which is up to 100-200 mm. On the western shores of the Middle Caspian, 200-400 mm of precipitation falls during the year. It snows in the northern regions of the sea in November-March, and in the southern regions in December-February. Since 1995 the rising water level has stopped and a descending trend prevailed with a noticeable fall in 2010 due to a summer drought in the northern part (Lahijani et al., 2010; Arpe et al., 2012).

The factors that cause currents in the CS are mainly winds, the relief of the seabed, the configuration of the coastline, the temperature difference of the water of different areas of the sea, rivers flowing into the sea. Based on long-term studies, wind is the most important determining factor. The flow scheme of the CS was first given by N. M. Knipovich in 1921 and it shows the circulation of water in the Middle Caspian and South Caspian. Part of the water from the Volga River flows in the North-West direction to the Middle Caspian, and the other part moves in the North-East direction, creating an anticyclonic water cycle, and its intensity is higher in summer than in winter. 

Mud volcanoes in the CS are more widespread in the South Caspian Basin and up to 500 mud volcanoes have been discovered in this basin. In the sea, they appear in the form of islands, ridges and underwater heights. After some time, some of the islands formed by the eruption are washed away by the sea water and here you can read about the last eruption. The eruption frequency of volcanoes is different and that makes it even harder to understand the connection between the CS volcanic eruptions and its level change.

According to paleogeographic, archeological and historical data, the amplitude of the change of the level of the CS in the last 3 thousand years was close to 15 meters. The basis of observations in this field was laid by E. Lents in 1837 in Baku. The highest level recorded during the observation period (-25.2 meters) was recorded in 1882, and the lowest (-29.0 meters) level was recorded in 1977. Since 2005, there has been a decrease in the level of the CS. In the last 3 years, stabilization of the sea level has been recorded. Thus, the observed regular decrease in the level has stopped since 2015, and during these 3 years, the level of the CS with the Baltic system was equal to -27.67 m.

The first instrumental observations of the level began in 1837 at the Baku station (Bayil area) and are continued to this day at 15 stations on the entire CS, and at 7 observation stations in the Azerbaijani sector of the sea. Based on the Baku gauge data, the CSL was 1.9 m lower at the end of 2021 than that in 1995 which is confirmed by satellite altimetry data (hydroweb.theia-land.fr), followed by a continuous decrease between July 2005 and July 2022.

In addition to multi-year fluctuations, the level of the CS also changes seasonally. Thus, in the warm months of the year, the level in the CS is higher than in the cold months, the minimum in January and February, and the maximum in July and August (Figure 2).

Why should we care about the level change in the Caspian Sea?

It does not only have a regional importance, economically and climatically, but also a global one. It affects precipitation and surface air temperature inertia across seasons, and also world’s climate by teleconnections via an impact on the Jet Stream (Farley-Nicholls and Toumi, 2014).

Despite the importance of the Caspian coast, stationary observations with common methodology are limited by meteorological parameters. Coastal environmental parameters are sometimes monitored by methods whose reliability can be challenged. The same applies for the activities in mud volcanoes. These activities’ contributions are not studied well. In order to be prepared for future changes and Aral lake outcomes, it is important to look at a longer time perspective than the instrumental record with different characteristics.

References:

Chen J. L.,Wilson C. R.,Tapley B. D.,Save H., Jean-Francois Cretaux, Long-term and seasonal Caspian Sea level change from satellite gravity and altimeter measurements, 2017

Nicholls J. F., Toumi R., On the lake effects of the Caspian Sea, 2013

K. Arpe, S. A. G. Leroy, H. Lahijani, and V. Khan, Impact of the European Russia drought in 2010 on the Caspian Sea level, 2012

Lahijani H., Leroy S.A.G., Arpe K.,Crétaux F., Caspian Sea level changes during instrumental period, its impact and forecast: A review, 2023 

About the author

Hayat Nasirova is a geographer and teacher with a background in education. She originally completed her Bachelor’s degree in Earth Science from Azerbaijan State Pedagogical University and is currently a Master’s student in Physical Geography: Climate and Environmental Sciences at the University of Bremen. She is an experienced science communicator, especially with respect to the intersection of science and contemporary art, having led masterclasses and workshops integrating and articulating scientific phenomena on art and canvas. She runs a series of courses called “The History of Science” and “Sci-Art”, as part of the Pint of Science festival. 

LinkedIn profile: www.linkedin.com/in/hayat-nasirova