Natural variability is a large source of uncertainty in future projections of hypoxia in the Baltic Sea

Abstract

Coastal seas worldwide suffer from increasing human impact. One of the most severe environmental threats is excessive nutrient pollution from land, which causes oxygen depletion and harmful algal blooms. In 2018, the semi-enclosed Baltic Sea was determined to contain the largest hypoxic area among the world’s coastal seas, with a size equal to the Republic of Ireland. In this study, ensemble modelling was used to investigate whether climate change will intensify hypoxia in the Baltic Sea and whether nutrient load abatement strategies would counteract this scenario. We analysed the largest ensemble of scenario simulations for the Baltic Sea currently available (including different boundary conditions) and estimated the magnitude of various sources of uncertainty. The results showed that natural variability was a larger source of uncertainty than previously considered. The earliest time and appropriate location to detect a trend above the background noise were estimated. A significant decrease in hypoxia can be achieved by further reductions in nutrient loads implemented in combination with existing measures.

Oceanographic regional climate projections for the Baltic Sea until 2100

“…The large spread in river discharge did not decrease between the various studies ranging between -8 and +26% (Meier et al, 2006;2021). As global sea level rise projections were corrected in more recent assessments towards higher rates (e.g., IPCC, 2019a;Bamber et al, 2019), recent scenario simulations for the Baltic Sea also considered sea level rise (Meier et al, 2021). As a consequence of compensating effects of the competing drivers of salinity changes, i.e.…”

“…There is a notable difference in salinity projections between the first two assessments (BACC Author Team, 2008;BACC II Author Team, 2015) and recent scenario simulations (Meier et al, 2021). While the first Baltic Sea scenario simulations driven by nine RCMs and five GCMs showed a pronounced negative ensemble mean change in salinity because two of the involved GCMs showed a significant increase in the mean west wind component (Meier et al, 2006), such pronounced changes in the large-scale atmospheric circulation were not observed in later studies anymore (Saraiva et al, 2019a).…”

Retracing hypoxia in Eckernförde Bight (Baltic Sea)

“…In 2008 this number already increased to 400 (Diaz and Rosenberg, 2008). The implications can be substantial, including mass mortality of (commercial) fish, loss of blue carbon (associated with seagrass habitat loss), degradation of touristic and recreational assets, and release of the potent greenhouse gas N 2 O (e.g., Naqvi et al, 2010).…”

“…For example, it has been illustrated in a global model that deficiencies in biogeochemical model components may be compensated for by deficiencies in circulation model components (Löptien and Dietze, 2019), thereby obscuring even the sign of the sensitivity of the (global) warming to come. This raises the question if it is actually feasible to reliably (i.e., getting the right answer for the right reason) simulate lowoxygen events in systems such as the Baltic Sea that are (1) infamous for their natural variability (Meier et al, 2021) and (2) subject to antagonistic effects of improved management of water resources and climate change on oxygen concentrations (e.g., Lennartz et al, 2014;Hoppe et al, 2013), which is notoriously difficult to deconvolve (Naqvi et al, 2010).…”

Human impacts and their interactions in the Baltic Sea region

“…Hence, it is vital to know how salinity will change in the future. It may increase through intensified inflows (as sea level rise would widen the passages in the Danish Belts and Sounds), but conversely, it may be reduced by increased runoff (Meier et al, 2021c). Currently it is 3090 unclear which effect is prevailing.…”