IVL Swedish Environmental Research Institute

This study illustrates how a temporally consistent deposition reanalysis can be used 1/ as input data to ecosystem impact assessments, 2/ to understand past trends, and 3/ for validation of ecadal to century-scale model scenarios. We have constructed a multi-decadal (1983–2013) reanalysis of nitrogen deposition (NDEP) to northern Europe, including the Baltic Sea and the candinavian Mountains, using a combination of observations and modelling.

We expanded the period with an operational annual reanalysis applying chemistry transport modelling, resulting in a fused dataset using the MATCH Sweden system for the period 1983–2021, and compared this to multi-century model scenarios of nitrogen deposition. Since the 1980s, NDEP has decreased until early 2010s in northern Europe including the Baltic Sea (by 32 %) and the Scandinavian Mountains (by 19 %). Present NDEP is on pair with the levels in the 1950s, after peaking in 1980–1990. We project continued decrease in oxidized NDEP until 2050, but still exceeding the pre-industrial levels. We also project an increase in reduced NDEP from present to mid-21st century, with stronger signal compared to previous estimates.

This results in a weakening of the annual reduction of NDEP, stabilizing to the levels of the 1940s to 1950s by mid-21st century, resulting in approximately twice as high NDEP compared to pre- industrial times. The projected NDEP decrease will likely not be sufficient to avoid future effects on sensitive ecosystems. Thus, there is a need for continued efforts to further decrease nitrogen emissions to the atmosphere for protection of terrestrial and aquatic environments, not the least as ecosystems are under additional pressure of climate hange and intensive management. The NDEP trends and levels in our model scenarios compare well to the reanalysis results (including fused observations).