The Ocean Acts as a Giant Sponge for Carbon Dioxide

The ocean acts as a giant sponge for carbon dioxide (CO₂), playing a crucial role in fighting climate change. Every year, the ocean absorbs about 26% of the CO₂ released into the air from activities such as burning fossil fuels to make electricity, cutting down forests, and manufacturing cement. This means that the ocean takes in a massive amount of CO₂, helping to reduce the greenhouse gases in the air that contribute to global warming. By soaking up this extra CO₂, the ocean helps to keep our planet’s climate more stable. Without the ocean’s ability to absorb CO₂, climate change would be much more severe, making the ocean’s role in managing carbon dioxide essential for a healthy Earth.

The amount of CO₂ that moves back and forth between the air and the ocean changes depending on factors such as changing seawater temperatures. If an ocean region takes in CO₂, it is called a carbon sink. If it releases CO₂ into the air, it is called a carbon source. Scientists measure this CO₂ exchange with electronic sensors, but these measurements are usually taken in small areas. Studies that cover much larger areas of Earth’s oceans often have less detail and can miss out on important processes. This means we might not fully understand how much small changes in the ocean help with CO₂ absorption. By learning more about these small changes, we can better understand how the ocean helps keep our planet healthy. Ocean eddies are one cause of these small changes.

What are Ocean Eddies?

Imagine the ocean is like a large bowl of soup. When you blow on the soup, it creates swirling patterns on the surface. Similarly, the wind blows across the ocean, creating small, spinning circles of water called eddies (Figure 1). Eddies in the ocean are often created by the twisting and turning of powerful currents, such as the Gulf Stream, which flows along the U.S. East Coast. When these strong currents meander or waver, they can create a sort of swirling disturbance in the water. Over time, this swirling disturbance can become more pronounced and eventually “break off” from the main current, forming a circular eddy. Although they can be 200 km wide, eddies are considered small compared to the vastness of the global oceans—the Pacific Ocean, for example, is about 20,000 km wide! Eddies move warm water to cold places and cold water to warm places, mixing things up in the ocean. This helps tiny sea creatures and nutrients travel around, making the ocean a healthier place for all the fish and animals that live there. It is like the ocean’s way of stirring its big soup pot, to keep everything balanced and lively!

Eddies Move CO₂ Between the Air and Ocean

Scientists are still learning how ocean eddies affect how much CO₂ moves between the ocean and the air. It is tricky to study these effects because there are not many measurements available, and studying eddies is complex and computer intensive. Recent research has shown that in some parts of the ocean, eddies are good at pulling CO₂ into the ocean [1]. However, scientists are still trying to figure out exactly how eddies influence ocean CO₂ absorption as a whole, across all the world’s oceans. There are still lots of questions to answer, like whether eddies make certain areas of the ocean better at capturing CO₂ from the air or whether our current studies might be missing important details.

To investigate how eddies affect the exchange of CO₂ between the ocean and air, we use a method called coarse-graining [2, 3]. This technique helps us separate eddy features smaller than about 200 km across from larger-scale flows in the ocean, using a super-detailed ocean simulated with a powerful computer [4]. Using this simulated ocean, we can estimate how eddies contribute to air-sea CO₂ exchange, as shown in Figure 2. We found some interesting results.

Eddies make the biggest changes to CO₂ exchange in specific ocean regions, like areas near the poles and in the middle latitudes (30–45° latitude north and south) of the ocean where powerful currents meet. In these areas with lots of eddies, eddies are responsible for more than 30% of the variations in how much CO₂ is exchanged between the ocean and the atmosphere, influencing whether more CO₂ is absorbed or released. On the other hand, in quiet parts of the ocean, eddies hardly affect CO₂ exchange at all. This shows us that understanding and studying eddies is really important to better understand how the ocean as a whole absorbs and releases CO₂.

How do Eddies Impact Local Carbon Absorption?

Now we know that ocean eddies cause some of the CO₂ movement between the ocean and the air. But the big question is whether these swirls help the ocean absorb more CO₂ (making the ocean a carbon sink) or release it into the air (making the ocean a carbon source). To figure this out, we analyzed all the oceans and found that, in some places, ocean eddies are helping the ocean absorb CO₂.

While in other places the eddies are causing the ocean to release CO₂ into the air (Figure 3). Taken altogether, eddies can affect the absorption/release of over 300,000 tons of CO₂ every year. That is equivalent to the amount of CO₂ that would be released by 30,000 car trips around the earth! This shows that the influence of these small ocean swirls on air-sea CO₂ exchange is large and can be very different from place to place in the ocean.

The ocean becomes a carbon source or a carbon sink because of how ocean eddies twist and turn in different ways. We found that if an eddy is spinning in a way that brings water with more CO₂ to a part of the ocean with less CO₂, the receiving part will get too much CO₂ and release it into the air. But if the eddy spins the other way, bringing water with less CO₂ to an area that does not have much CO₂, the ocean absorbs more CO₂ from the air. In short, the spinning of ocean eddies sets whether the ocean is a carbon source or sink, and these changes in different parts of the ocean.

As the ocean gets warmer due to global warming, scientists think that eddies might become even more active and important in influencing how CO₂ moves between the ocean and the air [5]. For example, with more heatwaves in the ocean, higher ocean temperatures might make the swirling currents work differently, which could affect how much CO₂ gets moved around. To understand this better, scientists need to monitor the ocean more closely and improve their computer models to predict how eddies will influence our climate. Knowing more about ocean eddies will help us plan better ways to protect our planet from climate change.

Glossary

Carbon Sink: ↑ An ocean area that absorbs and stores carbon dioxide from the air.

Carbon Source: ↑ An ocean area that releases carbon dioxide (CO₂) into the air.

Eddies: ↑ Small, spinning currents in the ocean, like mini whirlpools, that moves water and nutrients around.

Gulf Stream: ↑ A strong, warm ocean current in the Atlantic Ocean that flows from the Gulf of Mexico along the eastern coast of the United States and across the Atlantic to Europe.

Coarse-graining: ↑ A method to look at the big picture by combining smaller details into larger groups.

Air-Sea CO₂ Exchange: ↑ The process by which CO₂ moves between the ocean and the air, helping to control the amount of CO₂ in the atmosphere, which influences climate change.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

YG and M-LT were supported by the Yale Center for Natural Carbon Capture. The authors thank Elizabeth Grant for her valuable feedback, which improved this manuscript. Support for this research was provided by the Yale Center for Natural Carbon Capture. We would like to acknowledge high performance computing support from Cheyenne and Casper supercomputers provided by National Center for Atmospheric Research’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation.