Large anthropogenic emissions of carbon dioxide don’t only cause global warming, but also acidification of the oceans. In the Baltic Sea pH is expected to decrease between 0.1 and 0.4 units during this century depending on future emissions. The effects of ocean acidification on organisms have already been visible and are expected to increase, but not for the reason previously thought.
Sam Dupont, University of Gothenburg. Photo: Lisa Bergqvist
The oceans absorb about 25 percent of the global carbon dioxide emissions. One effect of this is that pH in the water is decreasing – the oceans are being acidified.
“The global trend is currently a pH decrease of 0.02 units per decade”, explains Erik Gustafsson, oceanographer and researcher at Stockholm University Baltic Sea Centre.
Future acidification is directly related to the development of carbon dioxide emissions. If the goals of Paris agreement (to limit global warming to well below 2 degrees) are met, the pH decrease will level out and pH begin to increase again during this century. A more moderate mitigation of emissions would lead to a continuous decrease and in the worst-case scenario, where emissions continue to increase significantly, pH would decrease by up to 0.4 units until the end of this century.
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Complex situation in coastal seas
In coastal seas, like the Baltic Sea, however, the development of pH is much more complex and variable compared to in the open oceans.
“In coastal seas we have strong interactions between land, and sea and there are other processes that can counteract and enhance ocean acidification, such as properties of the freshwater input, changes in run-off and salinity and changes in production and respiration patterns over time”, says Erik Gustafsson.
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Future development in the Baltic Sea
Modelling of the future development in the Baltic Sea shows a mean pH decrease similar to the one in the open oceans – about 0.1 units 2100 with moderate CO2 mitigation, and 0.3-0.4 in the worst-case scenario. But the large seasonal variations also means that the exposure time for harmfully low pH levels increases.
In a scenario where the Baltic Sea has recovered from eutrophication, the mean pH decrease is larger, but the seasonal variations are smaller, which altogether means less exposure time for harmful pH levels.
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Ecological effects of acidification
Sam Dupont, senior lecturer at the Department of Biological and Environmental Sciences at University of Gothenburg, has been studying the consequences of ocean acidification for species and ecosystems for many years.
He highlights that extensive ocean acidification has occurred before. At the end of the Permian period (250 million years ago), there was a change in volcanic activity and a lot of carbon dioxide was released into the atmosphere, which led to climate change and ocean acidification.
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Stockoholm University, 19 March 2024. Press Release.
