Long term impacts of societal and climatic changes on nutrient loading to the Baltic Sea

Marianne Zandersen1, Sampo Pihlainen2, Kari Hyytiäinen2, Hans Estrup Andersen1, Mohamed Jabloun6, Erik Smedberg3, Bo Gustafsson3, Alena Bartosova4, Hans Thodsen1, H.E. Markus Meier5,4, Sofia Saraiva4,7 , Jørgen E. Olesen1, Dennis Swaney3, Michelle McCrackin3


1 Aarhus University, Denmark

2 University of Helsinki, Finland

3 Stockholm University, Sweden

4 Swedish Meteorological and Hydrological Institute, Sweden

5 Leibniz-Institute for Baltic Sea Research Warnemünde (IOW), Germany

6 University of Nottingham, UK

7 University of Lisbon, Portugal


The Baltic Sea is a vulnerable ecosystem that is greatly influenced by human activities and the climatic system due to a combination of natural conditions and multiple, interacting anthropogenic pressures. Excessive nutrient loading is one of the main pressures altering the state of the Baltic Sea. Policy-makers and stakeholders around the riparian countries can benefit from a systematic exploration of how alternative global developments in climate and socio-economic factors are likely to affect the amount of nutrient loading to the Baltic Sea in the long term. Such load projections can be used for estimating the additional nutrient abatement efforts needed to reach the goals of marine protection, such as under HELCOM BSAP.

We develop long-term quantitative scenarios of nutrient loading under different combinations of RCPs and SSPs over the period 2010-2100. The SSPs include the pathways Sustainability (SSP1), Middle of the Road (SSP2), Fragmentation (SSP3), and Fossil-fuelled development (SSP5); RCPs cover intermediate scenario (RCP4.5) and very high GHG emissions (RCP8.5). The analysis accounts for all major sources of nitrogen and phosphorus from non-point source loading (i.e. agricultural load, loads from forest and background loading), point source loading (i.e. load from wastewater treatment plants) and the atmospheric deposition. The impacts of inventory uncertainty and structural uncertainty are addressed for scenarios.

Quantitative assumptions of drivers of nutrient emissions are developed based on an extension of the SSP narratives to the Baltic Sea and applied in a spatially explicit model framework combining agricultural management and land use, wastewater treatment levels and scale, and atmospheric deposition based on sectoral developments. Climate impacts are integrated via the HYPE riverine flow model, altering nutrient loads from land compared to no climate change.

Results show that nutrient loads overall are on a declining trend for SSP1, SSP2, SSP3 but increase for SSP5 with respect to both nitrogen and phosphorus loads. With sustainable development and moderate climate change (SSP1/RCP4.5), N loads could be halved and P loads decrease to around 60% of the current level by the end of the century. In a fossil-fuelled world (SSP5) and extreme climate change (RCP8.5) the nutrient loads would increase by around 25% from the current level, necessitating substantial policy efforts in nutrient abatement.

Non-point sources are the biggest contributor to the external nutrient loading of the Baltic Sea. Furthermore, their share of the total load will increase over time for all climate and socioeconomic scenarios considered. Point source loading of both N and P will decrease, but for different reasons for different SSPs. Main determinants include technological development and urbanization (SSP1 and SSP2), investments in treatment technology (SSP1, SSP2 and SSP5), and decreasing population (SSP3).