The possible reasons for this include:

• increased cloud cover, and changes in wind speed
• effect of oil pollution on the surface of the sea
• increased rainfall over the sea
• possible under-estimation of inflows (mainly discharges from the Volga)

However, information on evaporation loss is one of the most problematic areas in hydrological assessment because of the difficulty in obtaining measurements. It is virtually impossible to measure evaporation directly (except under research conditions) and extensive data on the variables which control evaporation are hard to come by. In this instance, there are no data available for the sea surface.

Over the land:

Potential evapotranspiration over the basin is certainly likely to change as a result of global warming. Such changes may vary significantly over the region, and the manner in which plant growth may alter in response to increased carbon dioxide available for photosynthesis is unknown. (Plant physiology is an area of active international research.) It can be expected, therefore, that changes in evapotranspiration will influence hydrological responses in the basin but perhaps more importantly, irrigation demands may change dramatically.