Baltic Sea Centre's eutrophication model can provide knowledge about greenhouse gases
The BALTSEM model is a well-used tool for studying the effects of nutrient inputs to the Baltic Sea and has been used in the HELCOM Baltic Sea Action Plan. The modelling tool has now been further developed to include methane fluxes, which can provide important knowledge about the climate impact of the Baltic Sea.
Emissions of methane from the Baltic Sea to the atmosphere can be significant and make the sea a net source of greenhouse gases. However, measurements of methane in the Baltic Sea region are still relatively few and limited to certain areas.
“Modelling can be an important tool to understand the large-scale fluxes and help to reduce the knowledge gaps”, says researcher Erik Gustafsson, Stockholm University Baltic Sea Centre, who is the lead author of a new study on methane dynamics in the Baltic Sea.
The BALTSEM model was developed by researchers at the Baltic Sea Centre, among others, and has proved to be able to reproduce the nutrient dynamics in the Baltic Sea. The model has been used within the Baltic Sea co-operation HELCOM for calculations of how much the nutrient supply must be reduced in order to achieve the goal of a Baltic Sea unaffected by eutrophication, as well as in various research studies.
Erik Gustafsson and his colleagues in CoastClim, a collaboration between the Universities of Stockholm and Helsinki, have now developed the model to get a picture of the extent of methane fluxes between the Baltic Sea's sediments, water and air. The new BALTSEM-CH4 model includes methane of different flavours, known as isotopes. The composition of isotopes is affected by processes that take place when methane is formed and decomposed.
“The isotope composition therefore becomes a kind of fingerprint, which can provide a guide to how much different processes contribute to methane dynamics", explains Erik Gustafsson.
Methane leaks from the sediments
The model calculations show that the methane supply to the Baltic Sea is largely dominated by methane formed by decomposition processes in oxygen-free sediments and then leaking into the water above. The total amount of methane produced in the Baltic Sea each year is just over 120 000 tonnes, according to the modelling, which should be regarded as a first rough estimate. A small portion is also transported to the sea by the rivers.
“Most of the methane is oxidised in the water and converted to carbon dioxide and water. About 16 000 tonnes are released into the atmosphere", says Erik Gustafsson.
Even if the emission to the atmosphere is small in relation to production, it can be significant for the climate, as methane is a powerful greenhouse gas. Calculated as carbon dioxide equivalents, the methane emissions amount to just under 450 000 tonnes in a 100-year perspective or just under 1.3 million tonnes in a 20-year perspective, according to the latest conversion table from the Intergovernmental Panel on Climate Change (IPCC).
“It is becoming increasingly urgent to reduce greenhouse gas emissions to limit global warming. It is therefore important to increase the knowledge about methane emissions, including those from natural environments, and to try to limit them”, says Erik Gustafsson.
'Hotspots' can make important contributions
The BALTSEM-CH4 model includes the entire Baltic Sea, but due to the coarse resolution, the calculations are not fully representative of shallow areas near the coast, explains Erik Gustafsson. At the same time, these areas are probably the most important in terms of how much methane reaches the atmosphere.
“Emissions to the atmosphere are probably greater than these calculations indicate. In shallow coastal areas, methane concentrations have been measured at much higher levels than in the open Baltic Sea. It is not yet known how important such local 'hotspots' for methane formation are, compared to the low emissions from large areas.”
The study is probably the first to include methane isotopes in a physical-biogeochemical model. The goal of the CoastClim researchers is now to conduct more measurements in different types of coastal areas and then develop the model further.
"This study is a first step to assess methane emissions from the Baltic Sea, but more studies from coastal areas are needed to get a more comprehensive picture", concludes Erik Gustafsson.
Read the publication in Geoscientific Model Development:
Text: Lisa Bergqvist
Last updated: March 26, 2024
Source: Baltic Sea Centre