Abstract
Many coastal seas worldwide are affected by human impacts such as eutrophication causing, inter alia, oxygen depletion and extensive areas of hypoxia. Depending on the region, global warming may reinforce these environmental changes by reducing air-sea oxygen fluxes, intensifying internal nutrient cycling and increasing river-borne nutrient loads. The development of appropriate management plans to effectively protect the marine environment requires projections of future marine ecosystem states. 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 concentrations affect regional projections considerably. In this study, we focused on one of the most threatened coastal seas, the Baltic Sea, and estimated uncertainties in projections due to climate model deficiencies and due to unknown future greenhouse gas concentration, nutrient load and sea level rise scenarios. To address the latter, simulations of the period 1975-2098 were performed using the initial conditions from an earlier reconstruction with the same Baltic Sea model (starting in 1850). To estimate the impacts of climate model uncertainties, dynamical downscaling experiments with four driving global models were carried out for two greenhouse gas concentration scenarios and for three nutrient load scenarios, covering the plausible range between low and high loads. The results of primary production, nitrogen fixation, and hypoxic areas show that uncertainties caused by the various nutrient load scenarios are greater than the uncertainties due to climate model uncertainties and future greenhouse gas concentrations. In all scenario simulations, a proposed nutrient load abatement strategy, i.e., the Baltic Sea Action Plan, will lead to a significant improvement in the overall environmental state. However, the projections cannot provide detailed information on the timing and the reductions of future hypoxic areas, due to uncertainties in salinity projections caused by uncertainties in projections of the regional water cycle and of the mean sea level outside the model domain.
Highlights
Regional projections of future climate based on dynamical downscaling of global model results using regional climate models (RCMs) suffer from considerable uncertainties caused by (1) shortcomings of global and regional climate models, (2) unknown future greenhouse gas concentrations, (3) natural variability, and (4) experimental design (e.g., Hawkins and Sutton, 2009; Kjellström et al, 2011; Déqué et al, 2012; Mathis et al, 2018)
The impacts of the changing climate within the range of the considered greenhouse gas concentration scenarios (RCP 4.5 and Representative Concentration Pathways (RCPs) 8.5) on biogeochemical cycles will be smaller than the impacts of the considered nutrient load changes (BSAP, Reference, Worst Case)
(3) Substantial uncertainties of future projections for the Baltic Sea are caused by the driving General Circulation Models (GCMs)
Summary
Regional projections of future climate based on dynamical downscaling of global model results using regional climate models (RCMs) suffer from considerable uncertainties caused by (1) shortcomings of global and regional climate models, (2) unknown future greenhouse gas concentrations, (3) natural variability, and (4) experimental design (e.g., Hawkins and Sutton, 2009; Kjellström et al, 2011; Déqué et al, 2012; Mathis et al, 2018). They have still significant shortcomings on regional scales, inter alia, because their horizontal grids are too coarse to resolve details of the orography and the land-sea mask, which might be important to the regional climate (Stocker et al, 2013). To overcome the limitations of global climate models for regional climate studies, limited-area RCMs with high resolution were developed for the region of interest, driven by data from GCMs or ESMs at the lateral boundaries of the model domain (e.g., Giorgi and Mearns, 1991) With such an experimental setup, scenario simulations were carried out, with the aim to study the impact of climate change and to develop climate adaptation strategies for selected regions (e.g., Räisänen et al, 2004). As the socioeconomic development in the catchment area of the coastal sea is unknown, future nutrient loads from land and atmospheric depositions of nitrogen and phosphorus are speculative, contributing to the uncertainties of the projections of the marine ecosystems (e.g., Meier et al, 2011a)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
