Caspian Sea: State of the Environment 2019

decline in sea level occurred from 1931 to 1940, decreasing by 1.7 metres, with the fastest rise tak- ing place from 1978 to 1995, when it increased by 2.5 metres. From 1996, the sea level decreased, most noticeably by almost a metre during the 2006–2015 period, before stabilizing in 2016– 2017 (Figure 3.5). In the time period considered, fluctuations in the Caspian Sea level were largely due to changes in its water balance, which have been calculated for each year, starting from the mid-nineteenth cen- tury. Average per year, approximately 1,000 mm of water (approximately 400 km 3 ) evaporates from the surface of the Caspian Sea (including Du- zly-Bogaz-Gol), though it is compensated by river run-off (about 750 mm or 300 km 3 of water) and atmospheric precipitation (about 250 mm or 100 km 3 ). The water balance is positive in the North- ern Caspian, where the Volga and Ural Rivers flow into the sea, and is negative in the Middle and Southern Caspian due to meagre run-off. River run-off is largely responsible for water balance variations. Four fifths of river run-off into the Cas- pian Sea originates from the Volga River, meaning that sea level fluctuations are mainly determined by the river’s water content. These fluctuations are therefore indicative of the transfer of moisture from the Atlantic Ocean to the Volga River basin.

Sea. While this was once the Caspian Sea’s nat- ural rhythm, it has now been affected by global warming, resulting in dry, warm years during the 1996–2015 period, with the 2006–2015 pe- riod providing particularly unfavourable condi- tions. The impact of climate change on the Cas- pian Sea’s ecological situation is unknown, since biochemical processes resulting from changes during 2006–2015, along with bacterial marine products and short-lived species, were found to have contributed to the self-purification of the seawater from pollution (CASPCOM 2017). As a closed water body, considerable fluctuations in water level are common in the Caspian Sea. Observations of the water level began in the first half of the nineteenth century, with coastal ob- servation data from 1900 onwards included in CASPCOM’s General Catalogue of the Caspian Sea Level. Satellite-based sea level observations from 1992 to present day are available and acces- sible online. Although fluctuations are normal in the Caspi- an Sea level, they can have considerable impact, with faster changes resulting in severer conse- quences. In the twentieth century, the most rapid 3.3.3. Changes in sea level

Figure 3.5: Fluctuations of the Caspian Sea level 1900–2017 Note: Acco rding to the CASPCOM data, «0» on the graph corresponds to a mark below 28.0 metres of the Baltic Sea level. Source: Lebedev (2014).

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