Responses of nutrient loadings entering coastal waters to climate-driven hydrological changes worldwide

Good water quality is essential for society and ecosystems, but it has been a pressing issue in many rivers and coastal waters. Harmful algal blooms resulting from eutrophication are examples of such matters. Eutrophication is often linked to excessive nutrient loadings and climate change (e.g. temperature and precipitation changes). Global water quality models can be used to understand better how nutrients respond to changes in climate and socio-economic developments. Changing climates result in changing hydrological cycles including river discharges. Nutrient flows from land to seas are, in turn, dependent on these hydrological cycles.  Increased runoff, resulting from increases in long-term precipitation or precipitation intensity, may transport more nutrients from land to rivers. As a result, climate change may further exacerbate nutrient problems in the future. However, our understanding of how nutrients in coastal waters respond to uncertainties in climate-driven hydrological changes on land is limited at large scales. This especially holds for global water quality models projecting nutrient loadings to coastal waters worldwide. Water quality models rely on hydrological projections using global hydrological models (GHMs), which are further driven by Global Climate Models (GCMs). Numerous GCMs exist, each simplifying complex systems, and adding uncertainty to their projections. Uncertainties may then propagate through the modelling chain, potentially affecting the robustness of global water quality model results.

Here, we aim to better understand how future nutrient exports by rivers respond to hydrological changes driven by different GCMs and how this affects model reliability. For this, we use a soft-coupled model system accounting for water quantity (VIC model¹) and water quality (MARINA-Multi model²) under a rapid urbanisation and high global warming scenario. Then, we introduce an approach to compare projected trends of nutrient loadings to coastal waters for 2050 across five selected GCMs, which diverge in their climate forcings. This study contributes to the first global-scale water quality model intercomparison effort as initiated by the Water Quality sector of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP). Preliminary results reveal that climate-driven hydrological changes will mainly add uncertainties to projections in arid regions. Nevertheless, a vast majority of the global surface areas agree on trends in nutrient exports by rivers for at least three out of five GCMs. Yet, agreements may differ across sea regions. Our approach can be used to identify robust trends in future nutrient loadings to seas under climate-driven hydrological changes, enhancing the reliability of global water quality models. This aids in identifying effective solutions for coastal water pollution worldwide under climate change.    

¹Van Vliet, M. T. H., Van Beek, L. P. H., Eisner, S., Flörke, M., Wada, Y., & Bierkens, M. F. P. (2016). Multi-model assessment of global hydropower and cooling water discharge potential under climate change. Global Environmental Change, 40, 156-170. https://doi.org/10.1016/j.gloenvcha.2016.07.007

²Micella, I., Kroeze, C., Bak, M. P., Tang, T., Wada, Y., & Strokal, M. (2024). Future scenarios for river exports of multiple pollutants by sources and sub-basins worldwide: Rising pollution for the Indian Ocean. Earth's Future, 12, e2024EF004712. https://doi.org/10.1029/2024EF004712