How we discovered that the ocean’s surface absorbs much more CO₂ than previously thought

The oceans play a pivotal role in drawing down atmospheric carbon dioxide (CO₂) and have so far acted as a brake on the full impact of climate change. Current estimates of the CO₂ from the atmosphere that disappears in the ocean, commonly referred to as the ocean CO₂ sink, suggests that around 25% of all human CO₂ emissions have been taken up by the oceans.

In our recent journal paper in Nature Geoscience, we show that a thin layer at the ocean surface called the “ocean skin”, a layer thinner than a human hair, increases this ocean CO₂ uptake by about 7%. That sounds like a small difference, but this additional uptake is equivalent to the CO₂ absorbed by the entire Amazon rainforest each year.

This long-term uptake of carbon into the ocean has negative implications for ocean health. It is slowly causing the acidification of the oceans – as sea water takes up more CO₂ it is altering the ocean chemistry and lowering its pH, and this cannot easily be reversed.

Since the 1990s, scientists have suggested that a cooler skin would enhance CO₂ uptake by the oceans. As such, estimates of CO₂ absorption that ignore this effect would be inaccurate.

Since then, the sea surface temperature researchers have shown that the ocean skin is slightly cooler than the waters just below. This surface skin is, on average, ~0.17°C cooler. A temperature change like this increases the concentration of CO₂ in this tiny sliver of water. This matters because it’s this water that is in direct contact with the atmosphere.

Because the exchange of CO₂ between the ocean and atmosphere is controlled by the concentration difference between the surface and the layer of water below, this cooler skin increases the absorption of CO₂ into the ocean.

European researchers confirmed these concentration driven processes in 2007. They used equipment similar to a powerful microscope with a camera to visualise oxygen gas concentrations within these tiny layers in a laboratory. In recent years, the impact of the surface layer on global ocean carbon has been evaluated using theory, modelling and satellite-based observations, but until now, nobody had actually measured this effect in the sea.

To carry out our research, the European Space Agency helped us put specialist measurements on board two research ships taking part in the annual Atlantic Meridional Transect scientific cruises that each year hosts UK and international scientists.

In 2018, we collected data from our kit on board the royal research ship James Clark Ross as it travelled approximately 9,000 miles (14,500km) from Harwich in south-east England to Port Stanley on the Falklands.

In 2019, the equipment was installed on the royal research ship Discovery which went from Southampton, UK, to Puntas Arenas, Chile. This ship sailed through very rough seas in the North Atlantic and near the Falklands, but experienced a mirror glass ocean with no real waves near the Equator, so our measurements reflected a wide range of different sea conditions.

Up, up and away?

On each voyage, two sets of measurements were taken. For one set of measurements, we used a micro meteorological system to measure wind speed and air temperature, combined with atmospheric gas measurements. Collectively, this is known as the “eddy covariance system” and it tracks how much CO₂ gas is in the air moving up (away from the surface) as opposed to that moving down. This tells us how much CO₂ is being absorbed or emitted by the ocean.

The second set of measurements sampled water collected from an inlet pipe on the ship. From this, we measured the gas in the water and its temperature. We then combined this with a high specification thermal camera that measures the temperature of the ocean skin.

Together, both sets of measurements should provide the same result if the ocean skin had no effect. Any differences between them revealed how the ocean skin was affecting the ocean’s CO₂ sink.

Accurate estimates of the CO₂ absorbed by the oceans are critical to calculating global carbon budgets. These budgets quantify how carbon moves around global systems and are used to guide international policy on reducing emissions.

The ocean and atmosphere are the two primary reservoirs of carbon that can be accurately observed. Accurately estimating these, constrains all other parts of the global carbon budget and allows us to assess what is termed the “remaining budget”. This identifies how much more carbon can be emitted before a specific climate target is breached. Importantly, we cannot estimate the carbon absorbed by all the land on Earth without first estimating the carbon absorbed by the oceans. Therefore, the ocean CO₂ uptake being approximately 7% higher will have implications for the whole global carbon budget and Earth’s carrying capacity for further emissions.

As the UN’s climate summit, Cop29, approaches in Azerbaijan, this research helps define the problem of CO₂ emissions more accurately. Climate experts will need to reassess the global carbon budget to reflect our new findings and this additional ocean uptake will cause an imbalance in the budget, potentially indicating that the land-based carbon sink is smaller than currently thought, so less effective in helping remove atmospheric emissions.

The oceans sucking up more of our carbon emissions than previously thought sounds positive. But this news means that climate change, along with other human activities, such as over fishing and pollution, are putting increased pressure on ocean health. It could also imply that the land’s capacity to absorb CO₂ has been overestimated, and that more attention should be paid to conserving ocean ecosystems.

As the need to reduce emissions and meet reduction targets ramps up, insights about how the ocean skin works will help scientists understand how the ocean will respond to our emissions. Unfortunately though, it won’t let anyone off the hook.

Don’t have time to read about climate change as much as you’d like?
Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. Join the 40,000+ readers who’ve subscribed so far.

Daniel Ford receives research funding from the European Space Agency and the EU Horizons programme and UK Research and Innovation.

Ian Ashton received funding from the European Space Agency

Jamie Shutler receives research funding from the European Space Agency, the EU Horizons programme, UK Research and Innovation, and Convex Insurance.