The global ocean absorbs about a quarter of human-caused carbon emissions, helping to mitigate climate change. However, this absorption increases ocean acidity, threatening marine ecosystems.
Understanding the processes behind sea-air carbon exchanges and improving carbon sequestration estimates are vital for accurate climate assessments.
Researchers believe that the ocean’s “skin”—a thin layer of cooler water—enhances carbon dioxide absorption, as cooler water is more effective at taking in the gas. This phenomenon had not been thoroughly measured at sea until recently.
New research, partially funded by ESA, shows that the cool “ocean skin” increases the ocean’s ability to absorb atmospheric carbon dioxide more than previously believed. These findings could improve global carbon assessments and inform more effective emission-reduction policies.
Researchers assessed in situ measurements taken from ships as they traversed the Atlantic Ocean. Measurements taken by flux systems detected small differences in carbon dioxide levels in the air near the ocean surface, alongside precise temperature readings of the thin ocean skin. The findings confirm that the cooler temperature of this surface layer enhances carbon absorption.
As a result, the ocean absorbs about 7% more carbon dioxide annually than previously estimated. While this might seem modest, it amounts to one and a half times the carbon captured by annual forest growth in the Amazon rainforest when considered across all oceans.
Global estimates of air-sea carbon dioxide fluxes typically ignore the importance of temperature differences in the near-surface layer.
Array of sensors aboard the RSS Discovery research ship Daniel Ford, from the University of Exeter, said, “Our findings provide measurements that confirm our theoretical understanding about carbon dioxide fluxes at the ocean surface.”
“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 used to guide emission reductions.”
Ian Ashton, also from the University of Exeter, said, “This work is the culmination of many years of effort from an international team of scientists. ESA’s support was instrumental in creating such a high-quality measurement campaign across an ocean.”
Gavin Tilstone, from Plymouth Marine Laboratory, added, “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.”
ESA’s Craig Donlon noted, “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.”
Journal Reference:
- Ford, D.J., Shutler, J.D., Blanco-Sacristán, J. et al. Enhanced ocean CO2 uptake due to near-surface temperature gradients. Nat. Geosci. (2024). DOI: 10.1038/s41561-024-01570-7
