Petersen, Arthur; (2023) Expert judgement on the underpinning of the assessment threshold used in project-specific calculations of nitrogen depositions. (Nitrogen research project ). De Nieuwe Denktank: The Hague, Netherlands.

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Abstract

When using a model for a specific (policy) goal, it must first be determined what the scientific application range of a model is. The application range indicates where the model can be used to make reliable statements (given the purpose for which the calculation results are used). If the results are too uncertain (there is a ‘false certainty’), then the model is insufficiently reliable (not valid) for use. There is always a limit to a model. In the context of deposition modelling of individual sources, the scientifically accepted limit below which results are insufficiently reliable lies between approximately 1 and 35 mol/ha/year. Calculation results lower than 1–35 mol/ha/year are scientifically insufficiently reliable for use in decision-making (the model system is then not fit for purpose). Theoretical and empirical considerations, the agreement with other models and peer consensus do not allow room for policy – due to false certainty – to scientifically calculate nitrogen depositions from individual sources and to attribute effects where the deposition is lower than 1–35 mol/ha/year. The current assessment threshold in the Netherlands of 0.005 mol/ha/year cannot therefore be maintained from a scientific, legal and policy perspective; the new assessment threshold to be chosen should be at least 200 times higher. Which assessment threshold between 1 and 35 mol/ha/year is ultimately chosen is not up to science, because no clear answer can be given, and non-scientific factors such as the precautionary principle also play a role. How this precautionary principle is implemented is not a question that can be answered by science. However, the room for policy here is constrained by the scientifically substantiated range of 1–35 mol/ha/year. In Germany, an assessment threshold of 21 mol/ha/year (more than 4,000 times higher than the current assessment threshold in the Netherlands) was chosen based on the precautionary principle. The Netherlands could follow this approach, but from a scientific point of view, higher or lower assessment thresholds are also possible (provided they are chosen within the substantiated range).

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