The Caspian has characteristics common to both seas and lakes. It is often listed as the world's largest lake, although it is not freshwater: the 1.2% salinity classes it with brackish water bodies.
It contains about 3.5 times as much water, by volume, as all five of North Americas Great Lakes combined. The Volga River (about 80% of the inflow) and the Ural River discharge into the Caspian Sea, but it has no natural outflow other than by evaporation. Thus the Caspian ecosystem is a closed basin, with its own sea level history that is independent of the eustatic level of the worlds oceans.
The sea level of the Caspian has fallen and risen, often rapidly, many times over the centuries. Some Russian historians claim that a medieval rising of the Caspian, perhaps caused by the Amu Darya changing its inflow to the Caspian from the 13th century to the 16th century, caused the coastal towns of Khazaria, such as Atil, to flood. In 2004, the water level was  below sea level.
Over the centuries, Caspian Sea levels have changed in synchrony with the estimated discharge of the Volga, which in turn depends on rainfall levels in its vast catchment basin. Precipitation is related to variations in the amount of North Atlantic depressions that reach the interior, and they in turn are affected by cycles of the North Atlantic oscillation. Thus levels in the Caspian Sea relate to atmospheric conditions in the North Atlantic, thousands of kilometres to the northwest.
The last short-term sea-level cycle started with a sea-level fall of  from 1929 to 1977, followed by a rise of  from 1977 until 1995. Since then smaller oscillations have taken place.
A study by the Azerbaijan Academy of Sciences estimated that the level of the sea was dropping by more than six centimetres per year due to increased evaporation due to rising temperatures caused by climate change.