Global scenarios for ecosystem health : nutrient pollution and toxic stress

The Global Environment Monitoring Unit in the Science Division of the United Nations Environmental Programme (UNEP) provides support to the coordination of the World Water Quality Alliance (WWQA) by assessing key environmental issues and providing focused outlooks to strengthen science-based policy making. In this project, global scenarios of water quality were modelled for two indicators: nutrient pollution and toxic stress. Calculations of spatially explicit global scenarios of nutrient pollution (total nitrogen and total phosphorus) are based on the IMAGE model framework including the IMAGE-Global Nutrient Model (IMAGE-GNM) by PBL and UU, with consistent combinations of socioeconomic scenarios regarding population, economy, attitude towards sustainability and environment (socioeconomic pathways or SSPs) and climate (Representative Concentration Pathways or RCPs). Intermediate model results with respect to socio-economic drivers have been provided to Deltares as input to the scenario runs for toxic stress. In all SSPs except SSP1, the inputs, delivery and export of N and P increase rapidly. The natural nutrient sources will continue to decline in all SSPs in future decades due to massive land use transformations, while agriculture, human sewage and aquaculture are becoming increasingly dominant (globally up to 80% of nutrient delivery). To achieve improved water quality, more efforts are needed than those employed in SSP1. Global scenarios of toxic stress, consistent with the scenarios for nutrient pollution with respect to the used socioeconomic drivers and climate input were calculated by Deltares. Especially innovative is the method used to quantify toxic stress which calculates the cumulative impact of the full mixture of chemical substances, and which is expressed in terms of biodiversity loss using the indicator msPAF. This Toxic Stress indicator for the expected loss of aquatic species can be linked to the SDG Indicator 6.3.2 “Proportion of bodies of water with good ambient water quality”. We distinguish two different options for the calculations with the HIWAI indicator: a “best case” and a “worst case” calculation. For the “best case” option the percentage of land cover grid cells containing water bodies with a “good quality” (based on msPAF values <0.05, meaning less than 5% of species are expected to be lost) on a global scale in 2010 is 91% and shows a decrease in 2050 of 6% in the RCP6.0/SSP2 scenario and an even higher decrease of 8% in the more extreme RCP8.5/SSP5 scenario. For the “worst case” option the percentage of water bodies with a “good quality” on a global scale is much lower: 71% in the Baseline 2010 scenario and 62% in the RCP8.5/SSP5 scenario.