Foto: Roel Wijtmans/ Azote.

The glaciers are melting faster than expected, sea levels may rise by as much as a meter during this century, and the seas are becoming warmer and more acidic, threatening both ecosystems and people's well-being. That was stated in a special report by the United Nation's Panel on climate change, IPCC, in September. During the twentieth century the sea level rose by approximately 15 centimetres, but it rises twice as fast today.

If carbon dioxide emissions continue to increase, the level may rise as much as 60-110 centimetres by year 2100, the IPCC writes in the report. During storms and extreme weather, this has dramatic consequences. The reason for the rising sea level is that both glaciers and permafrost are melting at an increasing rate and that the water temperature is rising, which also increases the volume of the water.

– In the northern and central parts of the Baltic Sea, sea level rise is mitigated by the land rise, but in the southern parts sea level rise will be evident. This also affects the important influences of salt water into the Baltic Sea and can lead to a stronger stratification of the water column and, in the long run, lead to lack of oxygen, says Christoph Humborg, scientific director at the Baltic Sea Centre.

Substantial impact on the Baltic Sea

A small and shallow sea such as the Baltic Sea will be affected faster by climate change than the other seas. Through the scientific collaboration; Baltic Bridge, Christoph Humborg's research group together with colleagues at Tvärminne Zoological Station in southern Finland studied temperatures and gas exchanges in the Baltic Sea. The measurements show that the average temperature in the Baltic Proper has increased in some places by as much as two degrees in the surface water since the 1990s.

– It is very worrying and can have major effects on ecosystems, he says.

A higher temperature means that less oxygen can be bound in the water, which some plants and animals will have difficulties to cope with. The situation can be further aggravated when extreme weather events occur and these are expected to become more common as the climate changes. During the heatwave in 2018, the record temperature of 21 degrees was measured at a depth of 30 meters right outside Tvärminne.

– At such temperatures some species such as seagrass meadows and mussels die immediately. It is a great stress factor for them, says Christoph Humborg.

The rising temperature in the Baltic Sea is worrying for many organisms, says Christoph Humborg, scientific director at the Baltic Sea Centre.

At the same time, climate change is expected to lead to increased rainfall in the Northern Hemisphere and as more rainwater reaches the Baltic Sea, the salinity decreases. The water that flows into the sea also carries loose organic material, which reduces the secchi depth, i.e. the depth where light reaches.

– It makes life harder for fish and plankton, while promoting the bacterial community. These processes also lead to increased eutrophication.

Eutrophication has long been regarded as the most important issue in the Baltic Sea environment. However, powerful policy measures to reduce emissions of nutrients have had an effect, and model simulations from the Baltic Sea Center show that nitrogen and phosphorus are no longer accumulated in the Baltic Sea.

– When it comes to eutrophication as such, the Baltic is on the road to improvement. But climate change is slowing down that progress.

From sink to source

The seas have so far served as a buffer for climate change and absorbed both large amounts of excess heat and carbon dioxide. The IPCC report shows that the seas have until now absorbed more than 90 percent of the excess heat in the climate systems and since the 1980s they have absorbed 20-30 percent of carbon dioxide emissions. When carbon dioxide dissolves in the water, however, acidification of the seas occurs, which is problematic for many species, such as coral reefs, and acidification is also expected to increase in the future, according to IPCC.

- In the Baltic Sea surface water, eutrophication has a dampening effect on the acidification, but in the long term it will affect us as well, says Christoph Humborg.

And even though eutrophication in the Baltic Sea is slowing down acidification, the study from Tvärminne shows that it can at the same time contribute to aggravate climate change.

– As the temperature rises, carbon dioxide and methane are released from the eutrophicated seabed. The sea goes from being a carbon sink to a carbon source, says Christoph Humborg.

Higher levels of environmental toxins?

The problems with environmental toxins in the Baltic Sea are also affected by climate change, says Emma Undeman, environmental chemist and researcher at the Baltic Sea Center. Exactly how, however, is difficult to know because the different effects of the changing climate can even each other out.

– Higher temperatures, for example, causes environmental toxins stored in soil to evaporate to a greater extent, but at the same time the molecules can be broken down more quickly in the air. Increased wind speeds make the environmental toxins more efficiently transported from the air to the surface water, but warmer surface water also evaporates a larger amount of environmental toxins, she says.

Most importantly, however, the changing climate is expected to have an indirect impact on environmental toxins, as emissions themselves increase.

– This could be, for example, increased by the need to use pesticides in agriculture and because of the large number of forest fires that lead to emissions of, for example, aromatic hydrocarbons and dioxins, says Emma Undeman.

Calculations for the Baltic Sea show that in most climate scenarios these effects lead to a doubling of the levels of certain chemicals.

Action is required

The IPCC special report on the seas and the cryosphere emphasizes that climate effects can be slowed down. The risks to society and to ecosystems are significantly lower if we manage to stop emissions than if they are allowed to increase.

The Baltic Sea Center's head of policy Gun Rudquist sees the COP25 in Madrid as an arena for linking climate and ocean issues. The interaction of climate change and the state of the oceans, for instance the Baltic sea, must be addressed further.  

– Another important process is the UN Ocean Conferences, next up in Lisbon in 2020, where further global initiatives for the oceans will be discussed.

Text: Lisa Bergqvist och Nastassja Ekelöf

The UN COP25 Climate Change Conference started this week in Madrid. Photo: UNclimatechange.