The fate of the Caspian Sea under projected climate change and water extraction during the 21st century

Sifan A Koriche,Joy S Singarayer,Hannah L Cloke

doi:10.1088/1748-9326/ac1af5

Abstract

The Caspian Sea (CS) delivers considerable ecosystem services to millions of people. It experienced water level variations of 3 m during the 20th century alone. Robust scenarios of future CS level are vital to inform environmental risk management and water-use planning. In this study we investigated the water budget variation in the CS drainage basin and its potential impact on CS level during the 21st century using projected climate from selected climate change scenarios of shared socioeconomic pathways (SSPs) and representative concentration pathways (RCPs), and explored the impact of human extractions. We show that the size of the CS prescribed in climate models determines the modelled water budgets for both historical and future projections. Most future projections show drying over the 21st century. The moisture deficits are more pronounced for extreme radiative forcing scenarios (RCP8.5/SSP585) and for models where a larger CS is prescribed. By 2100, up to 8 (10) m decrease in CS level is found using RCP4.5 (RCP8.5) models, and up to 20 (30) m for SSP245 (SSP585) scenario models. Water extraction rates are as important as climate in controlling future CS level, with potentially up to 7 m further decline, leading to desiccation of the shallow northern CS. This will have wide-ranging implications for the livelihoods of the surrounding communities; increasing vulnerability to freshwater scarcity, transforming ecosystems, as well as impacting the climate system. Caution should be exercised when using individual models to inform policy as projected CS level is so variable between models. We identify that many climate models either ignore, or do not properly prescribe, CS area. No future climate projections include any changes in CS surface area, even when the catchment is projected to be considerably drier. Coupling between atmosphere and lakes within climate models would be a significant advance to capture crucial two-way feedbacks.

Highlights

The Caspian Sea (CS) is the largest land-locked lake in the world, with a surface area currently larger than Japan
Water budget of the CS basin and its relation to the CS area representation in coupled model intercomparison projects (CMIP) models We find a considerable spread in the annual mean water budget of the CS catchment (P–E) between climate models in both CMIP5 and CMIP6, with some models displaying a positive net water balance and some a negative balance
In this study we have investigated water budget variation in the CS basin and its potential impact on the CS level during the 21st century using projected climate change from selected CMIP6 and CMIP5 models (Taylor et al 2012, Eyring et al 2016)

Summary

Introduction

The Caspian Sea (CS) is the largest land-locked lake in the world, with a surface area currently larger than Japan. As the water level changes it substantially alters CS surface area. ±>70% change from its current surface area occurred during various time periods in the late Quaternary. Such changes in surface area impact the climate in the regional catchment due to feedbacks with evaporation, precipitation, and wind patterns, as well as the large-scale atmospheric circulation in the northern hemisphere (Arpe et al 2019, Koriche et al 2020b). CS climate impacts extend eastward, modifying summer precipitation over central Asia, and even potentially influencing sea-ice concentrations over the northwestern Pacific (Koriche et al 2020b)

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Conclusion