Baltic Sea Gas Exchange: Lower Transfer Velocities Explained

The Baltic Sea, a crucial waterway for regional trade and a sensitive ecosystem, is exhibiting lower rates of gas exchange with the atmosphere than other ocean regions, a phenomenon with potential implications for carbon dioxide cycling and broader climate dynamics. Recent research indicates this reduced exchange is linked to the sea’s unique characteristics – limited wind fetch and elevated surfactant levels – and could affect the accuracy of current climate models.

Gas transfer velocity, denoted as ‘k’, is a key metric used to determine the rate at which gases move between the ocean and the atmosphere. Lower ‘k’ values mean less gas is exchanged. Studies, including work published in March 2019 in Ocean Science, demonstrate that the Baltic Sea’s coastal waters have significantly lower ‘k’ values compared to other oceanic areas experiencing similar wind speeds. This isn’t simply a matter of differing wind conditions; the Baltic Sea’s inherent physical properties are playing a substantial role.

One primary factor is wind fetch. Fetch refers to the distance over which wind blows across a body of water. The Baltic Sea, being a relatively shallow and enclosed sea, has limited fetch in many areas. This restricts the development of larger waves, which are crucial for enhancing gas exchange. Larger waves create more surface area and turbulence, facilitating the transfer of gases. Without sufficient fetch, the wind’s energy is not fully transferred to the water, resulting in smaller waves and reduced gas exchange.

Adding to the complexity is the presence of surfactants. These are compounds that reduce the surface tension of water. The Baltic Sea is known to have high concentrations of naturally occurring surfactants, largely due to the presence of microalgae. While surfactants can have various effects, in the context of gas exchange, they tend to suppress the formation of bubbles and dampen wave breaking, both of which contribute to gas transfer. A recent study, detailed in a preprint published on January 19, 2026 in EGU Sphere, suggests that elevated surfactant concentrations may contribute to a 25% reduction in gas transfer velocity.

The implications of these findings extend beyond purely academic interest. Accurate modeling of air-sea gas exchange is vital for understanding the global carbon cycle and predicting future climate change scenarios. The Baltic Sea, while relatively small compared to the world’s oceans, plays a disproportionately important role in regional carbon dynamics. If current models overestimate gas exchange in the Baltic Sea – as these findings suggest – they may also be overestimating the ocean’s overall capacity to absorb carbon dioxide from the atmosphere.

Researchers are employing various techniques to measure gas transfer velocities in the Baltic Sea. The active controlled flux technique (ACFT) has been used to measure heat transfer velocities, which can then be scaled to estimate gas transfer velocities using laboratory-derived Schmidt number exponents. Eddy covariance (EC) and dual-tracer techniques are also being utilized to independently determine ‘k’ values. Notably, the January 2026 research indicates that EC-based CO2 transfer velocities are, on average, 33% lower than those reported in open ocean EC studies, highlighting the unique conditions present in the Baltic Sea.

The study published in EGU Sphere emphasizes the need to move beyond simple wind speed-based parameterizations of gas transfer velocity. Traditional models often rely solely on wind speed as the primary driver of gas exchange. However, the Baltic Sea case demonstrates that factors like fetch and surfactant concentrations are equally, if not more, important, particularly in fetch-limited and surfactant-abundant environments. Resolving the variability of sea surface CO2 concentration and incorporating these mechanisms into gas transfer velocity parametrizations is crucial for improving the accuracy of climate models.

Further research is needed to fully quantify the combined effects of fetch and surfactants on gas exchange in the Baltic Sea and other similar coastal environments. Understanding these complex interactions is essential for refining our understanding of the global carbon cycle and developing more accurate climate projections. The Baltic Sea serves as a valuable natural laboratory for studying these processes, offering insights that can be applied to other regions facing similar challenges.

The lower gas transfer velocities observed in the Baltic Sea also have potential implications for other gases beyond carbon dioxide. Dimethyl-sulfide (DMS) emissions, for example, are influenced by aerosolization of microalgae and bubbling, processes that are also affected by surface conditions and wave dynamics. Recent research, as reported by Google News, highlights the link between microalgae aerosolization and DMS emissions during bubbling events, further emphasizing the interconnectedness of physical and biological processes in the Baltic Sea.