Sofia Saraiva1,2, H. E. Markus Meier3,1, Helén Andersson1, Anders Höglund1, Christian Dieterich1, Robinson Hordoir1, Kari Eilola1
1 Department of Research and Development, Swedish Meteorological and Hydrological Institute, 60176 Norrköping, Sweden.
2 Department of Mechanical Engineering, Technical University of Lisbon, 1049-001 Lisboa, Portugal.
3 Department of Physical Oceanography and Instrumentation, Leibniz Institute for Baltic Sea Research Warnemünde, 18119 Rostock, Germany.
Many coastal seas worldwide are affected by human impact like eutrophication causing, inter alia, oxygen depletion and extensive areas of hypoxia. Depending on the region, global warming may reinforce these environmental changes by reduced air-sea oxygen fluxes, intensified internal nutrient cycling and increased riverborne nutrient loads. The development of appropriate management plans to protect more effectively the marine environment, requires projections of future marine ecosystem state. However, projections with regional climate models commonly suffer from shortcomings in the driving global General Circulation Models (GCMs). The differing sensitivities of GCMs to increased greenhouse gas emissions may considerably impact regional projections. In this study, we focused on one of the most threatened coastal seas, the Baltic Sea, and estimated uncertainties in projections due to GCM deficiencies relative to uncertainties caused by unknown future greenhouse gas emissions and nutrient load scenarios and by natural variability. To address the latter, transient simulations for the period 1975- 2098 were performed using the initial conditions from an earlier performed reconstruction with the same Baltic Sea model (starting in 1850). To estimate the impacts of GCM deficiencies, dynamical downscaling experiments with four driving global models were carried out for two greenhouse gas emission scenarios, RCP 4.5 and 8.5, and for three nutrient load scenarios covering the plausible range between low and high loads. Results of primary production, nitrogen fixation, and hypoxic area show that differences caused by the various nutrient load scenarios are larger than the uncertainties due to global model deficiencies, unknown future greenhouse gas emissions and natural variability. In all scenario simulations, a proposed nutrient load abatement strategy, the Baltic Sea Action Plan, will lead to a significant improvement in the overall environmental state. However, projections cannot provide detailed information on the timing and the reductions in future hypoxic area due to uncertainties in salinity projections caused by uncertainties in projections of the regional water cycle and of the global mean sea level rise.