Estuaries, the transition zones between rivers and the sea, are vital socio-economic and ecological systems. Their salinity distribution, a key characteristic, is influenced by factors like river flow, tidal pumping, and estuarine circulation. Salt intrusion length, defined as the distance of the 2-psu isohaline from the estuary mouth, is a critical measure of salinity extent. Previous research, often focusing on individual estuaries, has highlighted the increased risk of salt intrusion due to drought, sea-level rise, land subsidence, and channel deepening. These events severely impact drinking water supplies and food production, causing significant economic losses. The need for reliable, decadal-scale projections of salt intrusion is therefore crucial for developing effective mitigation and adaptation strategies. Climate change is expected to significantly impact freshwater availability through altered precipitation, evapotranspiration, and snowmelt patterns. While global and continental-scale projections of freshwater availability exist, the connection between macro-hydrological models and estuarine salt intrusion processes remains understudied. Existing studies utilizing detailed hydrodynamic models for salt intrusion projections are limited by computational costs, often focusing on single realizations of future climate variables and specific locations. This lack of comprehensive, statistically robust projections across multiple estuaries hinders our understanding of the future risks associated with salt intrusion.

Prior research on salt intrusion has predominantly focused on individual case studies, often limited by computational constraints and the inability to comprehensively quantify uncertainties associated with climate variability. Studies have shown increased saltwater intrusion in various locations due to factors such as extended drought periods, sea level rise, anthropogenic land subsidence, and channel deepening. These impacts can cause severe problems, including disruptions to drinking water supplies and food production, ultimately resulting in substantial economic losses. However, a continental-scale assessment of the statistical characteristics of future salt intrusion events across multiple estuaries in Europe has been lacking, highlighting the need for the current study.

This study utilizes the CESM-LE2, a large ensemble simulation from the Community Earth System Model 2, under the SSP 3-7.0 high CO2 emission scenario. CESM-LE2 provides daily river discharge data, which is corrected using Quantile Delta Mapping (QDM) to account for systematic biases. The study focuses on 9 representative partially or well-mixed European estuaries selected to avoid complex configurations and strongly stratified systems. A time-dependent deterministic salt transport model is employed to determine the statistics of present and projected future salt intrusion lengths for these estuaries, utilizing observed salt intrusion lengths from the literature to calibrate model parameters (eddy viscosity and diffusion). The model calculates time-averaged salt intrusion lengths and the return periods of extreme events, defined as 1 in 100-year events during summer months in the present climate. The return periods of extreme salt intrusion are computed using the Generalized Extreme Value (GEV) theory, fitting the parameters using the maximum likelihood method. The analysis focuses on changes in the 35-year mean and return periods of extreme events for future salt intrusion, considering both the entire year and the summer months separately.

The study projects a substantial decrease (10-60%) in summer river discharge (June-August) in most major European river basins by the end of the 21st century under the SSP 3-7.0 scenario. This reduction in freshwater flow leads to a projected 10-30% increase in the 35-year mean salt intrusion length in the 9 selected estuaries, primarily during the summer months. Critically, the analysis indicates that the frequency of extreme salt intrusion events (those with a 100-year return period in the present climate) will increase by more than five times. This increase is most pronounced in the Tamar Estuary (UK), where the return period of such extreme events is projected to decrease from 100 years to approximately 3 years. Similar decreases in return periods are observed in other estuaries, indicating that events previously considered extreme will become far more frequent. For example, the return periods of extreme salt intrusion events decrease to approximately 4 years in Guadalquivir, 9 years in Gironde, 42 years in Loire, 41 years in Thames, 4 years in Rhine-Meuse, and 12 years in Elbe. The analysis also considered the impact of sea-level rise, but this impact was deemed a lower-bound estimate in comparison to changes in the river discharge.

The findings highlight the significant threat posed by climate change to European estuaries through increased salt intrusion. The substantial reduction in summer river discharge, a key driver of salinity distribution, is projected to dramatically alter the frequency and severity of extreme events. The more than five-fold increase in the frequency of extreme salt intrusion events emphasizes the urgent need for adaptation and mitigation strategies. While the study focused on the effects of reduced river discharge, the impact of sea-level rise is acknowledged as a relevant factor to be further investigated. The observed changes in the return periods of extreme events have critical implications for coastal communities and ecosystems, underscoring the need for robust coastal management plans. The methodology developed in this study provides a valuable framework for assessing salt intrusion projections in other estuaries worldwide.

This study provides a continental-scale projection of increased salt intrusion risks in European estuaries under a high CO2 emission scenario. The significant reduction in summer river discharge and the resulting more frequent extreme salt intrusion events necessitate immediate attention from policymakers and stakeholders. Future research should focus on refining the models to incorporate additional factors such as regional sea level variations and the combined effects of various climate and anthropogenic stressors. Further research is also needed to integrate socio-economic factors into the risk assessment for a comprehensive understanding of the impacts and effective planning for adaptation and mitigation strategies.

The study focuses on a high CO2 emission scenario (SSP 3-7.0) due to data limitations, and may not fully capture the potential impact under other scenarios. The model employed assumes a constant cross-sectional area along the estuary, potentially leading to an overestimation of the impact of sea-level rise on salt intrusion. Regional differences in sea-level change were not considered. The analysis emphasizes changes in river discharge as the primary driver of salt intrusion, but additional factors like changes in storm surges and tides, as well as human modification of estuaries, could further exacerbate the problem.