A Threefold Increase in Observed Summertime Atmospheric Dimethyl Sulfide over the Austral Southern Ocean
New atmospheric observations from Kennaook-Cape Grim reveal a threefold increase in summertime dimethyl sulfide over the Austral Southern Ocean between 1993–96 and 2023–26, indicating that changing marine sulfur emissions and atmospheric processing are significantly reshaping the regional sulfur cycle under a changing climate.
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The air above the Southern Ocean is not empty; it is filled with a faint, invisible mist of sulfur compounds released by microscopic marine plants. These plants, drifting in the sunlit surface waters, produce a gas called dimethyl sulfide. When this gas escapes into the atmosphere, sunlight and chemical reactions transform it into tiny particles of sulfate aerosol. These particles act as seeds for cloud droplets, influencing how much sunlight the clouds reflect back into space. Because the Southern Ocean is so remote and vast, it plays a massive role in the Earth's natural cooling system, yet scientists have struggled to track how these natural emissions change over time. Understanding whether the ocean is releasing more or less of this gas is crucial for predicting future climate shifts, as these natural processes interact with human-made pollution to shape the planet's temperature.
For decades, a monitoring station on the windswept cliffs of Tasmania has served as a sentinel for this remote air. Known as Kennaook-Cape Grim, the station sits where the prevailing winds blow directly from the Southern Ocean, carrying air that has traveled thousands of kilometers without touching land. Researchers recently compared new measurements taken between 2023 and 2026 with historical records from the 1990s. They discovered a startling change: the concentration of dimethyl sulfide in the summer air has tripled. The air that arrived at the station in the 2020s carried three times as much of this gas during the summer months as the air did in the 1990s. This is not a minor fluctuation but a substantial shift in the baseline of the atmosphere, suggesting that the ocean's biological engine is running differently than it did thirty years ago.
The team, led by scientists from CSIRO and international partners, ruled out several common explanations for this surge. They checked if the winds had changed, bringing air from different, more productive parts of the ocean, but the air masses followed similar paths in both eras. They examined if the gas was simply lingering longer because the chemical "sinks" that usually break it down had weakened, but the data showed no evidence of such a slowdown. They also looked at whether the air was more stagnant, trapping the gas near the surface, but wind speeds and atmospheric mixing remained consistent. The most likely cause, the researchers conclude, is that the ocean itself is producing more of the gas. This increase appears linked to changes in the upper ocean, specifically how deep the water mixes and how much sunlight reaches the phytoplankton, which are the tiny plants responsible for the emissions.
Alongside the dimethyl sulfide, the researchers also measured a related compound called methanethiol, which had not been tracked in the 1990s. They found that methanethiol levels rose in step with dimethyl sulfide, particularly during the spring and summer when marine life is most active. This co-occurrence suggests that the entire process of sulfur production in the ocean has intensified. However, a surprising twist emerged when the team looked at the final product: the sulfate aerosol particles. Despite the massive increase in the gas precursors, the total amount of sulfate aerosol in the air did not rise proportionally. In fact, the seasonal pattern of these particles has sharpened. The winter background levels of sulfate have dropped, likely because human-made pollution from ships and industry has decreased, while the summer peak has stayed steady or risen slightly due to the surge in natural emissions.
This disconnect between the gas and the resulting particles suggests that the atmosphere is processing the sulfur differently than before. The extra gas is not simply turning into extra particles in a straightforward way. The researchers suggest that the chemical pathways are becoming more complex, possibly influenced by the changing balance between natural and human-made sulfur sources. The findings highlight that the Southern Ocean's natural climate engine is not static; it is shifting in response to a changing environment. As the ocean warms and its chemistry evolves, the amount of sulfur it releases is changing, which in turn alters the clouds that cover the Southern Hemisphere. These observations provide the first direct evidence of a threefold increase in this critical gas over the mid-latitude Southern Ocean, offering a new piece of the puzzle for understanding how the Earth's climate system is adjusting to the modern world.
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