Analysis of Caspian Sea Water Storage Dynamics Over the Past 22 Years Using Multi-Source Remote Sensing Observations

Accurately monitoring Caspian Sea Water Storage (CSWS) dynamics has become important for regional water security, ecological preservation, and climate change adaptation, given its status as Earth’s largest enclosed water body with historically volatile water levels. This study employs multi-source remote sensing observations to (I) quantify recent CSWS variations from 2002 to 2024, (II) identify driving mechanisms, and (III) assess implications for water resource management. By integrating multi-mission altimetry and gravimetry observations, we show that the detrended and deseasonalized monthly CSWS anomalies from the two independent approaches are highly consistent (R2=0.94, 2002–2023). This agreement demonstrates strong interannual coherence between the complementary spaceborne techniques and provides robust dual-sensor support for monitoring and trend analysis of large-scale water bodies. Our analysis reveals an alarming CSWS decline of 759 ± 46 Gt over the past 22 years, equivalent to 2.11 ± 0.13 mm global sea level rise, with an accelerating loss rate of 4.7 ± 0.6 Gt y−2 since 2018 (mean of altimetry- and gravimetry-based estimates: 5.0 ± 0.5 Gt y−2 and 4.4 ± 0.8 Gt y−2, respectively). This 33 ± 2 Gt y−1 net loss correlates strongly with regional meteorological trends. The primary driver emerges as a climate-driven hydrological imbalance, where temperature-driven evaporation increases (+0.4 K decade−1) outweigh precipitation inputs, particularly in the sea’s shallow northern basin, exhibiting accelerated lake shrinkage compared to southern regions. Our findings underscore the critical vulnerability of terminal lakes to climate warming, demonstrating CSWS’s sensitivity as a climate change indicator. The observed acceleration suggests that the current long-term decline in CSWS is expected to continue in the recent years, necessitating urgent adaptive strategies. This dual-sensor monitoring framework offers a paradigm for closed-basin water budget assessments, providing essential data for transboundary water governance and refining global hydrological models under warming scenarios.