Scientific studies of the last 100 years indicate that the eutrophication trend in the Baltic Sea has been broken. More nitrogen and phosphorus are disappearing from the sea than are added. The marine ecosystem is slowly recovering and some places are already seeing improvements. However, positive developments risk being slowed down by the next major challenge for the Baltic Sea: climate change.
In the 21st century, the extent of the negative impact of climate effects
on the Baltic Sea will largely be determined by two things:
- global climate policy to reduce anthropogenic greenhouse gas emissions.
- society’s ability to reduce nutrient inputs to the sea.
Scientific analyses using Baltic Sea-specific climate models show that
if nutrient supply continues to decline in the coming decades, it will lead
to improved oxygen conditions in the Baltic Sea – even in the most pessimistic climate scenario. At the same time, the ecosystem resilience
to negative climate impacts will be strengthened.
Climate can amplify the effects of eutrophication
Ongoing climate change is causing changes in the physical marine environment, such as temperature, oxygen conditions and salinity. These changes have direct impacts on the plants and animals of the Baltic Sea. But there are also indirect impacts. Higher water temperatures and increased precipitation can amplify the effects of eutrophication and thus lead to an even worse oxygen depletion, among other threats.
At present, it is difficult to estimate exactly how large these climate effects will be as many parameters are still quite uncertain. However, it is clear that effects from eutrophication and climate change are linked. The greater the eutrophication, the greater the potential damage of climate change, because eutrophication makes the marine environment more sensitive to other stressors.
Various scenarios for climate and nutrient emissions
In a 2018 study, researchers from the Swedish Meteorological and Hydrological Institute (SMHI) and the Stockholm University Baltic Sea Centre show various possible future scenarios for the Baltic Sea by the year 2100, depending on how much climate change and eutrophication are mitigated.
The study was based on global climate scenarios - based on the emission scenarios (RCPs) for greenhouse gas emissions used in research evaluated by the UN’s IPCC climate panel - and scaled them down to a more detailed, regional level for the Baltic Sea:
- RCP 4.5 - significantly reduced greenhouse gas emissions
by the year 2100.
- RCP 8.5 - significantly increased greenhouse gas emissions
by the year 2100.
None of the scenarios meet the so-called “two-degree target” of limiting global warming to well below two degrees compared to the pre-industrial level.
The emission scenarios are modelled with three possible scenarios for nitrogen and phosphorus emissions from land to sea:
- BSAP - the supply of phosphorus is continuously adjusted downwards to the levels in the Helcom Baltic Sea Action Plan (BSAP).
- Reference scenario - no further measures are taken to reduce emissions from, e.g., agriculture and water treatment. Climate change gradually increases the supply of nutrients to the sea through more precipitation and increasing inputs from watercourses.
- Worst case scenario - the nutrient supply steadily increases
and is further strengthened by climate change.
Illustration: Robert Kautsky/Azote
Improved oxygen situation and less dead environments
The results indicate that the measures currently employed to cut nutrient emissions from land (Reference) are already sufficient to improve the oxygen situation. If nutrient supply is reduced according to the BSAP scenario, the situation will be significantly better in year 2100 compared to what it was in year 2000 - regardless of which of the two climate scenarios is applied.
Even if greenhouse gas emissions increase according to the RCP 8.5 scenario, the situation with oxygen-free bottom environments in the Baltic Sea will gradually improve, as long as the Baltic Sea countries continue – and preferably intensify – efforts to reduce nutrient supply from land.
Only in one of the scenarios does the oxygen situation get worse in the Baltic Sea: continued increasing greenhouse gas emissions (RCP 8.5) combined with an increase in nutrient supply (Worst case scenario). Such a development would lead to an expansion of oxygen depleted (hypoxic) areas in the Baltic Sea.
Strong action increases the resilience of the sea
Over the last 50 years, the Baltic Sea countries have made great efforts
to reduce the nutrient supply, especially from sewerage and agriculture. Counteracting eutrophication is a slow process. In most cases, it takes decades for the effects of various measures to become visible in the marine environment. However, thanks to long-term and patient action on land, improvements are now starting to be visible in the Kattegat and in some coastal areas, although they are not yet visible in the extent of hypoxic and anoxic areas in the open Baltic Sea.
According to the researchers, the best measures for the Baltic Sea to combat eutrophication – despite climate impact – is that the Baltic Sea countries achieve and maintain the levels of nutrient supply in the BSAP until the year 2100. In such a case, the climate’s impact on the oxygen situation in the sea will be relatively small even in the most pessimistic climate scenario. However, all the Baltic Sea countries still have some way to go before they reach the BSAP targets. At the same time, carbon dioxide emissions are increasing globally from year to year, according to the annual Global Carbon Budget report. This makes it even more important to prioritise regional efforts to reduce the flow of nutrients from land.
With stronger measures such as wastewater treatment, wetland construction and more efficient agricultural practices (e.g., better manure management), the Baltic Sea countries can face the negative impact of climate change together and achieve a better marine environment in the future than today.
This policy brief is a co-production between the Swedish Meteorological and Hydrological Institute (SMHI) and Stockholm University. It is based on analyses of different Baltic Sea ecosystem models, and in particular the study Baltic Sea ecosystem response to various nutrient load scenarios in present and future climates (2018). The analyses were conducted within the context of the BONUS project BalticApp, by researchers at SMHI and the Stockholm University Baltic Sea Centre. The results are summarised in the Baltic Sea Centre’s report series 2/2020, “Framtidens Östersjön – påverkan av övergödning och klimatförändringar” (“The Baltic Sea of the Future – the impact of eutrophication and climate change”), which was commissioned by the Swedish All Party Committee on Environmental Objectives in 2019/20.
Top photo: Martin Almqvist/Azote