Past, current, and future freshwater inflows to the Baltic Sea under changing climate and socioeconomics

The Baltic Sea is suffering from eutrophication caused by nutrient discharges from land to sea. These freshwater inflows vary in magnitude from year to year as well as within each year due to e.g. natural variability, weather patterns, and seasonal human activities. Nutrient transport models are important tools for assessments of macro-nutrient fluxes (nitrogen, phosphorus) and for evaluating the connection between pollution sources and the assessed water body. While understanding of current status is important, impacts from changing climate and socio-economics on freshwater inflows to the Baltic Sea also need to be taken into account when planning management practices and mitigation measures.

Continental to global scale catchment-based hydrological models have emerged in recent years as tools e.g. for flood forecasting, large-scale climate impact analyses, and estimation of time-dynamic water fluxes into sea basins. Here, we present results from the pan-European rainfall-runoff and nutrient transfer model E-HYPE, developed as a multi-purpose tool for large-scale hydrological analyses. We compared current freshwater inflows from land with those from dynamic modelling with E-HYPE under various climate and socioeconomic conditions. The socioeconomic conditions (land use, agricultural practices, population changes, dietary changes, atmospheric deposition, and wastewater technologies) were evaluated for 3 additional time horizons: 2050s using the Shared Socioeconomic Pathways, 1900s using historical data, and a reference period using a synthetic “no human impact” scenario. An ensemble of 4 climate models that preserves the range of projected changes in precipitation and temperature from a larger ensemble was selected for analysis of climate impacts in 2050s.  

We show that while climate change affects nutrient loads to the Baltic Sea, these impacts can be overshadowed by the impacts of changing socioeconomic factors. Historical nitrogen loads were estimated as 43% and 33% of the current loads for the 1900s and the “no human impact” scenarios, respectively. Average nitrogen loads are projected to increase by 4-10% (8% on average) as a response to climate change by 2050s. Purely mitigation measures that did not address the magnitude of the nutrient sources reduced the total nitrogen load by <5%, with local efficiencies being reduced through retention processes. However, changes in the socioeconomic drivers led to significant changes in the future loads with the range of impacts spanning 30% of the current load depending on the socioeconomic pathway to be followed. This means that policy decisions have by far the largest impact when managing eutrophication in the Baltic Sea region.

Bartosova, A., Capell, R., Olesen, J.E. et al. (2019). Future socioeconomic conditions may have a larger impact than climate change on nutrient loads to the Baltic Sea. Ambio 48, 1325–1336 doi:10.1007/s13280-019-01243-5