The world's oceans take in about 25% of carbon emissions from human activities, a crucial contribution to mitigating climate change. However, this absorption also has a downside: as oceans sequester more carbon, they become more acidic, posing risks to marine ecosystems.

Refining our understanding of the factors behind sea-air carbon exchanges and accurately estimating the ocean's carbon uptake is essential for precise carbon budgeting and targeted climate action.

Scientists previously theorized that the ocean skin - an ultra-thin, 0.01 mm layer cooler than the water immediately below - might enhance carbon dioxide absorption from the atmosphere, as cooler water is generally more efficient at this process. The gas concentration differential between this topmost layer and the water just a few millimeters below it drives the carbon exchange between air and sea.

However, extensive measurements of these dynamics had not been conducted at sea until now.

Through research supported by ESA, scientists from the University of Exeter, Plymouth Marine Laboratory, and the University of Southampton conducted measurements in the Atlantic Ocean. Their work involved flux systems on ships, detecting minute differences in carbon dioxide levels in the air moving towards and away from the ocean surface, along with precise temperature readings of the thin ocean skin.

The findings, published in *Nature Geoscience*, confirm that the cool temperature of the ocean skin indeed boosts carbon absorption. According to the study, oceans are absorbing approximately 7% more carbon dioxide annually than previously estimated due to the presence of this cooler surface layer. While seemingly modest, this additional carbon uptake across global oceans is equivalent to about 1.5 times the annual carbon sequestered by the Amazon rainforest.

Current global air-sea carbon dioxide flux estimates often overlook the role of near-surface temperature variations in this absorption process.

"Our findings provide measurements that confirm our theoretical understanding about carbon dioxide fluxes at the ocean surface," said Daniel Ford from the University of Exeter. "With the COP29 climate change conference taking place next month, this work highlights the importance of the oceans, but it should also help us improve the global carbon assessments that are used to guide emission reductions."

Ian Ashton, also from the University of Exeter, emphasized, "This work is the culmination of many years of effort from an international team of scientists. ESA's support was instrumental in putting together such a high-quality measurement campaign across an entire ocean."

Gavin Tilstone from Plymouth Marine Laboratory noted, "This discovery highlights the intricacy of the ocean's water column structure and how it can influence carbon dioxide draw-down from the atmosphere. Understanding these subtle mechanisms is crucial as we continue to refine our climate models and predictions. It underscores the ocean's vital role in regulating the planet's carbon cycle and climate."

Craig Donlon of ESA added, "Measurements of the cool skin of the ocean and precision atmosphere-ocean fluxes made together aboard a ship is an incredibly challenging task. The implications of these results are profound in terms of carbon accounting - which currently pays little attention to the role of the ocean surface. With the issue of climate change more pressing than ever, these results will help improve our understanding and assessment of the complex role that the oceans play in regulating the climate, and to take action."

Funding for this research was provided by ESA's Science for Society initiative, Horizon Europe, and the UK Natural Environment Research Council. The ship measurements were part of the Atlantic Meridional Transect project led by Plymouth Marine Laboratory.

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