The Largest Inland Body Of Water On Earth Has Dramatically Shrunk Since The '90s, And Scientists Think They Finally Know Why

This decline isn’t the product of natural climate fluctuations or temperamental rains, but the meddling of humans. According to a new international study, the Caspian's decline is increasingly driven by human activity, specifically the construction of dams and the diversion of rivers that feed it. 

“Fluctuations in the Caspian Sea's volume have been documented, with historical records indicating changes exceeding three [meters] over the years. However, the sustained decline observed over the past 30 years appears to be different in both character and cause, driven not only by climate variability but also, and perhaps more critically, by intensive human activities,” the study authors write in the conclusion. 

What is the Caspian Sea?

The Caspian Sea is located at the crossroads between Europe and Asia, with shores bordering five countries: Iran, Russia, Turkmenistan, Azerbaijan, and Kazakhstan. Despite being completely landlocked, it's classified as a sea rather than a lake, although that distinction is somewhat open to interpretation. Its waters are highly saline, yet it has no direct connection to any ocean or other sea.

Whatever way it's classified, it’s evident that it's shrinking and, until now, the causes of its plight have not been fully understood.

What's causing the Caspian Sea to shrink?

To close that gap, researchers used satellite data and river flow measurements to track where the water was actually going. Rainfall has more or less stayed the same since the mid-1990s, when the Caspian Sea started to suffer. 

In some areas, it has even increased. The Caspian Sea receives around 80 percent of its water from the Volga Basin in Russia, a region that has actually seen an increasing amount of rain in recent decades. 

Rising temperatures and the resulting increase in evaporation account for less than 40 percent of the total volume lost. That's a meaningful chunk, but far from the whole story.

“Contrary to recent climate modeling studies that attribute Caspian Sea decline primarily to Climate-driven evaporation increase, our results indicate that the observed water level decrease over the recent years cannot be fully explained by atmospheric forcing alone,” the study authors write. 

Instead, the researchers point the finger at a decreasing water flow from the Volga River, the longest river in Europe that serves as its main water source. This behemothic river has seen a growing number of dams and reservoirs built along it in recent years, and more of its flow is being diverted for farming and industry. 

According to the study, this is the single biggest factor behind the sea's decline. 

Big implications of the Caspian Sea's decline

That conclusion suggests the only solution is international action, especially from Russia, through which the entire Volga flows. But this needn't be done out of altruism or feel-good environmentalism – there’s also a practical, self-interested reason to act quickly. 

The Volga–Don Canal uses water from the Don River, but Russia also adjusts the Volga's flow to help keep the canal and shipping routes running smoothly.

This canal matters a lot strategically. It allows Russia to move its naval fleet between the Caspian Sea and the Black Sea, eventually leading to the Mediterranean Sea and the rest of the oceans. On top of that, major trade routes depend on the Caspian Sea as a key link by water: Russia's North-South trade corridor and China's Belt and Road Initiative both rely on it. 

If water levels in the Caspian Sea keep falling, the consequences for shipping could be serious and felt worldwide: less cargo capacity, costlier dredging operations, and growing difficulty keeping ports functional. 

The study is published in the journal Earth’s Future.