The Caspian Sea, the largest inland body of water on Earth, is shrinking. Not fluctuating, not entering another natural cycle, but shrinking.

For decades, scientists and policymakers treated changes in the Caspian as part of the basin’s natural variability. Water levels in the sea have always risen and fallen.

But our new study shows something far more troubling: the current decline is increasingly driven by human decisions to dam and divert rivers, and by fragmented decision-making across five countries that border this body of water.

Using satellite observations together with ground-based hydrological records from rivers across all five shoreline states (Iran, Russia, Turkmenistan, Azerbaijan and Kazakhstan), we found that flow into the Caspian Sea has declined sharply over the past three decades.

The main reason is not declining rainfall. In fact, rain over the Volga Basin, which supplies roughly 80% of the Caspian’s inflow, has slightly increased. That finding matters because it overturns one of the most common assumptions surrounding the Caspian crisis. The common narrative has been straightforward: climate change increases evaporation, rainfall declines, and the sea shrinks.

Climate change certainly plays a role: our analysis confirms that evaporation across the Caspian has increased significantly as regional temperatures rise. But evaporation alone explains only about 40% of the observed water loss since the mid-1990s.

The remaining decline points overwhelmingly toward human activity. The Volga River has been heavily engineered for decades. Dams, reservoirs, use for irrigation, industrial consumption and........

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