Driving Engine of The Global Carbon Cycle

Plants everywhere work hard to take carbon dioxide out of the atmosphere and turn it into food and oxygen. Plants live everywhere on land and in the ocean. Ocean plants can be large, like seagrass and kelp. They can also be tiny, called phytoplankton. You might know phytoplankton as algae and cyanobacteria. Although small, the whole community of tiny plant-like creatures is important for understanding the ocean’s response to increasing carbon dioxide in the atmosphere, which causes global warming. We are interested in using computer models to explore how the amount of carbon in phytoplankton will change due to global warming, and how that could affect the amount of carbon dioxide in the atmosphere.

Carbon in the bodies of phytoplankton can sink deep into the ocean, taking carbon from the surface ocean and the atmosphere down into the ocean’s depths and trapping it there for hundreds to thousands of years. This sinking and trapping of carbon is known as the biological carbon pump (Figure 1). The biological carbon pump is important for the future climate because its strength affects how much carbon dioxide is in the atmosphere. A strong pump means more carbon is moving from the surface ocean to the deep ocean. A weaker pump means less carbon gets into the deep ocean. Carbon dioxide in the atmosphere can change by as much as 400 parts per million (ppm) between a very strong and a very weak biological carbon pump. To put this in perspective, since industrialization, humans have increased carbon dioxide by about 150 ppm as of 2026 by burning fossil fuels. A weaker pump is predicted due to global warming [1]. This Frontiers for Young Minds article provides more detail on the biological carbon pump.

Not all phytoplankton are made of the same amount of carbon. They also do not all respond to a warming climate in the same way. To improve our understanding of the future, we needed to know where the different types of phytoplankton live and how many might live in a warming climate.

Carbon in Each Phytoplankton Cell

Phytoplankton are single-celled organisms and there are many different species. They are made up of many elements, including carbon, phosphorus, and nitrogen. On average, for every one part phosphorus present in phytoplankton, there are 16 parts nitrogen and 106 parts carbon. This ratio is known as the Redfield ratio [2]. However, individual phytoplankton cells often do not follow these rules—some phytoplankton have different amounts of carbon. To better illustrate how individual phytoplankton can be different from the Redfield ratio rules, we simplified the ratios in Figure 2. In that example, the carbon-poor phytoplankton has just two carbons for every phosphorus, while the carbon-rich phytoplankton has six carbons for every phosphorus.

Just like a cactus is in exactly the right environment in the desert and a palm tree is meant to live in the tropics, phytoplankton are no different. Many environmental factors determine where and how many phytoplankton grow. Their growth depends on the temperature, amount of light, and amount of nutrients. If phytoplankton do not have enough light, there is not enough energy for photosynthesis. If there are not enough nutrients, they do not have the necessary building materials for their bodies.

Nutrients are brought from the ocean’s depths to the surface water when the water mixes up and down. In the future, ocean water is predicted to mix less because the surface water is getting warmer and become lighter. Less mixing means less nutrients in the surface waters. You can learn more about the ocean’s ability to mix by reading this Frontiers for Young Minds article.

When the environmental factors are not just right, phytoplankton get stressed out. In a warming climate, stressed phytoplankton tend to become more carbon-rich. Other local changes, like the amount of light and nutrients, can also make phytoplankton more carbon-rich or carbon-poor. Today, the subtropics are nutrient poor, and the phytoplankton there are more carbon-rich than in other regions of the ocean. If these phytoplankton become stressed out by a warming environment, they become even more carbon-rich. This means that the carbon found in individual phytoplankton cells can change. The amount of carbon in a phytoplankton cell is important because the more carbon there is, the stronger the biological carbon pump and the lower the levels of carbon dioxide in the atmosphere.

Carbon In Phytoplankton Communities

A community of phytoplankton is made of a variety of phytoplankton cells living in the same region of the ocean. The temperate and subtropic regions, shown in dark green in Figure 3, show mostly carbon-rich phytoplankton communities. Scientists found that carbon-rich phytoplankton do well in the nutrient-poor subtropics [3]. The light green areas are the polar and tropical ocean areas. In these nutrient-rich regions, communities with mostly carbon-poor phytoplankton thrive. As the climate warms, phytoplankton communities can respond. How communities change is important for the future, because a community-wide shift to more carbon-rich phytoplankton will make the biological carbon pump stronger and lower the levels of atmospheric carbon dioxide.

What We Wanted To Find Out

There are many unanswered questions about what is going to happen in a warming climate. Using our computer models of the global ocean, we tried to answer these three questions [4]:

• Will phytoplankton grow more or less?
• Will individual phytoplankton cells become richer or poorer in carbon?
• What will phytoplankton communities be like?

The combined answers to these questions determine whether the biological carbon pump will be stronger or weaker in the future. For example, even if there are fewer total phytoplankton, if they have more carbon in their bodies and the phytoplankton community shifts to having more 6-handed than 2-handed members, the biological carbon pump could get stronger overall. We tried to answer these questions in two regions: the subtropics and the waters around Antarctica (Figure 3).

What Will Happen In The Subtropics?

Our computer model tells us that, in the future, the warmer subtropics will have fewer total phytoplankton (six cells today, Figure 3B vs. four cells in the future, Figure 3D). This is because there will be less up and down mixing in a warmer ocean, and so, there will be less nutrients in the surface ocean. However, our model indicates that the individual phytoplankton will contain more carbon. The 2-handed phytoplankton become 3-handed, and the 6-handed become 8-handed. This is because warming and fewer nutrients stress out all phytoplankton. Also, there will be three times as many carbon-rich phytoplankton (three 8-handed) as carbon-poor phytoplankton (one 3-handed) in the future (Figure 3D). Compare this to only two times as many 6-handed phytoplankton as 2-handed today (Figure 3B). This is because the carbon-rich phytoplankton are smaller and grow better than the larger, carbon-poor phytoplankton under low nutrient conditions. We need to count the carbon atoms to find our combined answer to the three questions. Under today’s conditions (Figure 3B) there are 28 carbon atoms total. In the future (Figure 3D), there are 27 total. Thus, the combined answer to the three questions is that the biological carbon pump becomes weaker (27 carbon atoms vs. 28 carbon atoms). It will not pull down carbon dioxide from the atmosphere as much as before, but it will not be as bad as people once thought, thanks to the changes in the carbon content of the phytoplankton cells and communities.

What Will Happen Around Antarctica?

In the Antarctic area of the ocean, sea ice is expected to decrease under global warming, providing new ice-free ocean areas. Our computer model predicts that phytoplankton growth will increase in these waters. Like in the subtropics, these individual phytoplankton also hold more carbon. Let us count the carbon atoms again. Figures 3C, E show that phytoplankton in Antarctic waters have 20 total carbon atoms today and 42 in the future. Let’s break these numbers into the three questions again. First, there will be more phytoplankton growth (nine cells in the future, Figure 3E vs. six cells today, Figure 3C). Second, all phytoplankton will have more carbon (2-handed become 3-handed, and 6-handed become 8-handed). Third, the mixture of carbon-rich and carbon-poor phytoplankton in the community remains the same. In the end, more carbon will likely be taken up by phytoplankton in the Antarctic waters in the future. As a result, the biological carbon pump in this region will become stronger.

What Does It All Mean For Global Warming?

In this article, we have shown how the biological carbon pump may change in the future in two regions of the world’s oceans. To understand the global response, we must consider all the changes occurring worldwide. Our computer model tells us there will be fewer phytoplankton globally, but most phytoplankton cells will become more carbon-rich. These two effects tend to balance each other out. We found that the global ocean carbon pump will become weaker overall in the future, but thanks to the carbon-rich phytoplankton cells, the weakening will probably be about one-third smaller than if the phytoplankton did not become more carbon-rich. This means that the biological carbon pump will continue to operate and keep global warming in check.

Another conclusion is that life always tries to survive by responding to a changing climate. Our prediction that marine phytoplankton will become more carbon-rich is one such response. This is what life does and, in this case, humans are being helped by this response even as we continue to burn fossil fuels. In short, increased carbon dioxide and global warming are predicted to have major effects on the ocean’s phytoplankton. But our results show that it might not be as bad as people once thought.

Glossary

Phytoplankton: ↑ A group of one-celled organisms that use energy from the sun to make sugar through photosynthesis.

Community: ↑ All the different species living in the same area.

Global Warming: ↑ The rise in Earth’s average temperature, mainly caused by human activities such as burning coal, oil, and gas, which add carbon dioxide to the atmosphere.

Computer Models: ↑ Mathematical representation of the ways things works in nature. Researchers often use computer models because there are too many calculations to do them by hand.

Biological Carbon Pump: ↑ The ocean process that moves carbon dioxide from the atmosphere into tiny plants, then carries carbon into deeper waters as organisms and waste sink.

Redfield Ratios: ↑ The ratio of carbon to nitrogen to phosphorus building blocks in phytoplankton. The typical ratio is 106:16:1.

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Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.