Water levels in the Caspian Sea dropped nearly seven centimetres (cm) per year from 1996 to 2015, or nearly 1.5 metres total, researchers said.
The current Caspian Sea level is only about a metre above the historic low level it reached in the late 1970s.
Increased evaporation over the Caspian Sea has been linked to increased surface air temperatures.
Researchers, including those from the University of Texas Austin in the US, found that the average yearly surface temperature over the Caspian Sea rose by about one degree Celsius between the two timeframes studied, 1979-1995 and 1996-2015.
These rising temperatures are likely a result of climate change, according to the researchers.
Evaporation brought about by warming temperatures appears to be the primary cause of the current drop in sea level and the decline will likely continue as the planet warms, researchers said.
The Caspian Sea, located between Europe and Asia, is about 371,000 square kilometres across in size. It has experienced substantial changes in its water level over the past several hundred years, but previous studies were unable to nail down the exact causes of the sea level changes.
The Caspian Sea is bordered by five countries and contains an abundance of natural resources and diverse wildlife.
Researchers compiled information about water level changes observed by satellites, records of precipitation and drainage into the sea from rivers, and estimations of precipitation and evaporation from climate models.
They then assembled a record of how much each of these factors contributed to observed changes in the Caspian Sea level from 1979 to 2015.
They found Caspian Sea levels increased by about 12 cm per year from 1979-1995. But in 1996, sea levels began to drop, and declined by an average of nearly seven cm per year through 2015.
From 1996-2015, the Caspian Sea level dropped nearly 1.4 metres, according to sea level records.
Evaporation contributed to about half of that decline, while the combined effects of precipitation and river discharge changes contributed to the other half.
The observed evaporation rates are associated with increased surface air temperature and other climate factors such as surface humidity and wind.
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