Projected changes in Caspian sea level under CMIP6 climate change scenarios: probabilistic and deterministic approaches

The Caspian Sea (CS), the world's largest inland body of water, faces diverse challenges due to the rapid fluctuations in its water level induced by climate change. Projecting future Caspian Sea level (CSL) changes is crucial for developing effective mitigation and adaptation strategies to address these fluctuations. Climate-induced changes in the CSL during the 21st century are studied by projecting the CS water budget components using 22 Global Climate Models (GCMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6). In the deterministic approach, Multi-Model Ensemble (MME) and bias correction techniques are applied to reduce associated uncertainties and correct biases in the CMIP6 outputs. Using a probabilistic framework, future projections of the CS water budget components are constructed as probability density functions based on a weighted combination of the top-ranked GCMs. All ensemble members of selected individual models are combined through Monte Carlo simulations. This study provides the following median estimates (25th to 75th percentile) of CSL changes in 2100 relative to 2021: 0.3 m (-12.2 to 10.2) for the low emission scenario, -1.8 m (-6.7 to 3.1) for the medium emission scenario, -3.3 m (-9.3 to 2) for the medium-to-high emission scenario, and - 4.4 m (-11 to 1.6) for the high emission scenario. The high likelihood of a significant decline in the CSL puts the shallow northern CS and Kara-Bogaz-Gol Bay at serious risk of drying up. The probabilistic projections provide information on the likelihood of the CSL remaining within specific ranges, which can be highly beneficial for policymakers.