Accounting for the uncertainty in nitrogen deposition estimates in support of policy

Deposition of reactive nitrogen (N_r) onto sensitive habitats in exceedance of Critical Load (CL) thresholds can drive biodiversity loss and affect ecosystem function. N_r deposition is a highly complex process that is difficult to measure and model, leading to large uncertainties. We assess the implications for policy development and target setting of the large range in estimates provided by different modelling approaches. We considered three UK models (UKIAM, EMEP4UK, CBED), used to inform national policy and responses to the UN-ECE Air Convention. We used a scaling method to project the range in current estimates to future scenarios, and a risk-based approach to provide a probabilistic assessment of exceedances. We considered two future scenarios, a 2040 baseline and a 2040 high ambition technological measures scenario, in relation to a 2018 baseline. The 2018 baseline CL exceedances are highly dependent on the model used - Average Accumulated Exceedance of 1.3-9.1 kg.N.ha^-1.yr^-1 across all habitats. The relative reduction in exceedances for future scenarios also depends on the model, with a range of 30-66 % achieved by 2040 for the high ambition scenario, posing a challenge for target setting. Despite this, it's clear that a much greater level of ambition is required to protect the majority of habitat areas. Our risk-based approach shows that implementing only technological measures is likely to leave most areas in exceedance in 2040. This uncertainty in the assessment of N_r deposition and the benefits of abatement measures poses a challenge for policy development that is not unique to the UK.