Cod in the Baltic Sea are struggling, even without climate change and ocean acidification. Image: / Raw Pixel LTD CC BY

Climate change and other threats to marine life in the Baltic Sea

Researchers conclude that climate change represents the overarching factor, affecting almost all the other natural and human-induced factors in the Baltic Sea.

A very long list could be made of the harmful environmental effects that are the result of historic and ongoing human activities, including the devastating consequences of climate change. The effects on aquatic ecosystems include eutrophication, overfishing and pollution, to name a few. Clearly, to understand the overall consequences of such detrimental processes, a multitude of factors need to be analysed simultaneously. For the Baltic Sea region, such an analysis was recently published in the journal Earth System Dynamics1, based on an extensive review of available literature.

This Acid News article highlights some of the concurrent environmental effects of a selection of these factors and those caused specifically by greenhouse gases (climate change and ocean acidification), partly in relation to other recent publications.

In the case of climate change the analysis in Earth System Dynamics states: “Due to its proximity to the northern polar region, the Baltic Sea region is warming faster than the globe”. It is also noted that the air temperature change is already among the strongest signals of climate change in Europe, with an observed land surface temperature rise of approximately 1°C over the past century. Projections to the end of this century range from 1.5 to 4.3°C.
Sea water temperatures are following suit and have also started to rise. Projections for 2100 range between 1.6 and 3.2°C.

Other clear changes include a drastic projected decrease in sea ice cover, as well as sea level rise. The latter is reported to be caused by thermal expansion (as the volume of water masses increases with warming) as well as global ice sheet melting and atmospheric circulation changes. The effect of sea ice melting is perhaps surprisingly coupled more to Antarctic sea ice than to that in northern polar regions. Globally, sea levels are expected to rise by 43–84 cm, whereas predictions for the Baltic Sea (corrected for land lift) range from 80 to 100 % of the global figures.

Changes that are reported to be more difficult to predict include those for wind and precipitation. For the latter, however, an increasing trend is suggested for the entire region in winter, and the northern part in summer.
Ocean acidification is treated as one of the “other factors”, since it is strictly speaking not a climate change effect, although caused by the greenhouse gas carbon dioxide. (In fact, ocean acidification is sometimes called “the equally evil twin of climate change”.) As reviewed earlier by AirClim2, understanding ocean acidification in the Baltic Sea is not as straightforward as in the oceans. In the Baltic Sea, several factors affect the buffering capacity (or alkalinity) of the sea water. Generally speaking, alkalinity is lower in the Baltic Sea than in the oceans, which would make it more vulnerable. However, there is great variability in the buffering capacity. For instance, geological conditions vary in such a way that the buffering capacity is higher in the southern parts and decreases northwards. Additionally, the buffering capacity has increased. The analysis in Earth System Dynamics mentions that the explanations for this increase are not conclusive, but suggested reasons include processes in the sediment (sediment is material deposited on the sea floor) and increased weathering.

In AirClim’s report, the role of factors such as eutrophication in this process is also discussed. Most importantly, however, an increase in buffering capacity has not compensated for all the fossil-derived carbon dioxide and should not be interpreted as offering protection from future acidification.

As for the effects of ocean acidification, the analysis in Earth System Dynamics mentions the important work done in mesocosms but concludes that ecosystem effects of ocean acidification still constitute a knowledge gap, i.e. that further studies are needed. It can also be noted that if other kinds of experimental studies are included (e.g. single-species studies), it becomes evident there are several species in the Baltic Sea that are sensitive to ocean acidification (see citations in AirClim’s report), including some species of mussels and zooplankton, as well as cod (the western Baltic population). For cod, a significant population decrease has been predicted. Consequently, there is good reason to further address the knowledge gap in order to better understand how different sensitivities (and tolerances) at species level translate into ecosystem effects. This is especially true as many of the species are facing so many other environmental threats at the same time. Cod are a good example, as they are simultaneously threatened by oxygen depletion, accelerated by eutrophication (see below), and overfishing.

Regarding the effects of climate change on other factors, the analysis reveals that sea level rise could increase the inflow of sea water through the Danish straits. This sea water is saltier than the brackish water in the Baltic Sea, and therefore has a higher density. Consequently, the stratification of the Baltic Sea, which consists of a saltier deep water layer and a surface layer of fresher water, would be reinforced. This is bad news, as oxygen from the surface is not mixed into the deeper water layer, and oxygen is ultimately depleted. This oxygen depletion is enhanced by eutrophication because organic matter is produced in excess, and organisms that consume this organic matter when it reaches the sea floor use up the oxygen in this process.

Another factor that is affected by climate change via sea level rise is erosion, which is predicted to increase. The analysis considers a number of other factors in relation to climate change, including nutrients, which are of course a major factor as eutrophication is such a serious problem. Nutrient leakage is generally believed to increase due to climate change, but the authors conclude that there are uncertainties in predictions for nutrient levels, taking into account aspects such as precipitation changes and changes in agricultural processes. Nevertheless, increased erosion could lead to an increase in nutrient-rich materials entering coastal waters if erosion occurs on nutrient-rich land. The internal nutrient load is believed to increase even more due to the release of phosphorous from the sea floor when oxygen is depleted.

Chemical contaminants are also believed to be affected by climate change, and interestingly a possible connection between acidification and a release of contaminants is discussed. This connection would stem from changes in marine chemistry.

To be fair to the authors of the analysis, it should be emphasised that this Acid News article only briefly touches on a few of the factors included in the analysis. Nevertheless, this article hopefully allows agreement with the authors when they conclude that “The main message from this analysis is that climate change represents the overarching factor, affecting almost all the other natural and human-induced factors”.

Marko Reinikainen

1: Reckermann, M. et al. 2022. Human impacts and their interactions in the Baltic Sea region. Earth System Dynamics 13(1). Https://
2: Vehmaa, A. & Reinikainen, M. 2021. Ocean Acidification in the Baltic Sea (report). Air pollution and Climate Series 40. Https://

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