Sliver of cool surface water helps the ocean absorb more carbon

Subtle temperature differences at the ocean surface allow more carbon dioxide (CO₂) to be absorbed, new research shows.

Scientists have thought that the ocean skin – a 0.01 mm sliver of surface water, thinner than a human hair, which is typically fractionally cooler than the water below – should increase the amount of carbon dioxide being absorbed from the atmosphere.

This is because cooler waters are more efficient at absorbing carbon dioxide. The gas concentration between this layer and the water some 2 mm deeper is what controls the exchange of the gas between the atmosphere and the ocean.

Theoretical and lab work have suggested this temperature difference should increase the amount of CO₂ absorbed by the ocean – but this had never been successfully observed at sea before.

The new study – led by researchers from the University of Exeter’s Penryn Campus in Cornwall – used precision measurements to confirm that the temperature of the ocean skin does indeed aid carbon absorption.

Carried out in the Atlantic, the findings suggest this ocean absorbs about 7% more CO₂ each year than previously thought. It might sound small, but when applied across all oceans this additional carbon absorption is equivalent to one and half times the carbon captured by annual forest growth in the Amazon rainforest.

The global ocean absorbs about a quarter of humanity’s carbon emissions, slowing climate change whilst also harming the ocean, and the new findings help improve our understanding of these processes.

“Our findings provide measurements that confirm our theoretical understanding about CO₂ fluxes at the ocean surface,” said lead author Dr 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.”

And the team have now included this advancement within their data submission to this year’s Global Carbon Budget assessment.

The ship observations – from two European Space Agency projects – were taken by a CO₂ flux systems that measured tiny differences in CO₂ in air swirling towards the ocean surface and away again, along with high-resolution temperature measurements.

Until now, global estimates of air-sea CO₂ fluxes typically ignore the importance of temperature differences in the near-surface layer.

Dr 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. The European Space Agency’s support for science was instrumental in putting together such a high-quality measurement campaign across an entire ocean.”

Dr Gavin Tilstone, from Plymouth Marine Laboratory (PML), said: “This discovery highlights the intricacy of the ocean’s water column structure and how it can influence CO₂ 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.”

The study’s international partners included sea temperature measurement experts from the European Space Agency and the university of Southampton.

It was funded by the European Space Agency, Horizon Europe and the Natural Environment Research Council.

The ship cruises were part of the Atlantic Meridional Transect (AMT) project led by PML.

The paper, published in the journal Nature Geoscience, is entitled: “Enhanced ocean CO2 uptake due to near surface temperature gradients.”