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Carbon Cycle

As part of the global carbon cycle, underwater volcanoes emit between 66 to 97 million tonnes of CO2 per year. However, this is balanced by the carbon sink provided by newly formed ocean floor lava. (NOAA)

What is the carbon cycle?

Carbon makes up the backbone of all life on Earth. It’s found in the cells of all living things, is abundant in rocks and sediments, and is also found in the atmosphere and ocean. But carbon doesn’t remain in one place. Individual atoms move from one reservoir to another: between the atmosphere and the oceans, from plants to animals to decomposers, to sediments and rock, and eventually back to the atmosphere. This movement of carbon from one place to another, which is caused by natural and human processes, is known as the carbon cycle.

The oceans play a particularly important role in the carbon cycle. Surface waters exchange gases with the atmosphere, absorbing and releasing carbon dioxide, oxygen, and other gases. Plant-like phytoplankton living in the ocean convert carbon dioxide into sugars that feed marine ecosystems. When marine animals die, their bodies may sink into the depths, where the carbon they contain either gets dissolved back in to deep waters, or a smaller fraction settles to the seafloor where it is covered in sediment and locked away for centuries. Such processes may eventually create coal or oil deposits deep underground.

Cold water absorbs more carbon dioxide than warm water, so ocean water near the poles tends to soak up more of the gas. This cold, dense water then sinks to the seafloor where it flows slowly through ocean basins in deep ocean currents. Centuries later—and far from its original location—the water warms and rises again to the surface, releasing carbon dioxide back into the atmosphere.


Why is it important?

Human activity has driven up atmospheric carbon dioxide to levels not seen in more than 800,000 years. A greenhouse gas, carbon dioxide acts as a blanket, trapping heat from the sun and holding it close Earth’s surface. As we increase levels of this gas in the atmosphere, we have been causing Earth to warm, which has wide-ranging implications.

Oceans have absorbed much of the carbon dioxide people have released into the atmosphere, slowing the rate of warming. But oceans cannot continue to sop it up indefinitely, and there are signs that the rate of absorption might be slowing. Additionally, when carbon dioxide reacts with surface water, it creates carbonic acid, which is causing ocean waters to become more acidic. Such ocean acidification is harmful to ocean life, especially corals and other organisms whose shells are damaged by the acidic waters.

When carbon sinks to the deep ocean, with some reaching the ocean floor, it can be locked away for millennia, so oceans can play a critical role in ultimately helping to fight climate change.

Round and round goes carbon around our planet. At the same time, figuratively, carbon makes the world go 'round. The element is the building block of life on Earth and, in the form of carbon dioxide gas in the atmosphere, it has a powerful impact on the planet's climate. In the process, carbon also goes through rivers, oceans, and the ocean's twilight zone (see interactive). Humans have intervened and interfered with the carbon cycle and quickly transferred carbon from slow to active pools by extracting large amounts of hydrocarbons from the Earth and burning them for fuel, putting an excess of heat-trapping carbon dioxide in the atmosphere.( Illustration by Jack Cook © Woods Hole Oceanographic Institution)

What are ocean scientists doing to understand the carbon cycle?

The processes involved in the carbon cycle are large and complex. Scientists from around the world are working to understand the forces behind the movement of carbon.

Some groups collect water samples from the oceans, analyzing them for carbon content. Repeated over many years, such studies provide basic information on how the oceans are absorbing and releasing carbon dioxide. But they have revealed unexpected complexities in these processes, making it more difficult to fully understand how human activities, such as burning fossil fuels, impacts the carbon cycle.

To address those complications, scientists turn to mathematical and computer models of the carbon cycle. They input the basic variables that are known to affect the cycle, then test the model using real-world data. Variables included in the models include wind speeds, temperature, currents, and more. These models have found that natural processes affect the ocean’s ability to absorb carbon in a variety of ways.

Winds stir up the surface layer of water, stimulating absorption of carbon dioxide, but changes in wind patterns alter this absorption. In some areas, reduced winds cause the ocean to release carbon dioxide into the atmosphere, rather than absorbing it. As ocean temperatures rise, waters become saturated with carbon dioxide more quickly, and they become unable to remove additional carbon dioxide from the atmosphere. Warmer surface waters present another problem: when the temperature difference between the surface water and deeper layers is more extreme, the water stratifies into layers. These layers do not easily mix, preventing carbon dioxide from moving deeper into the water column and nutrients in the depths from rising to the surface.

Scientists are also working to understand the role of phytoplankton in the carbon cycle and how they might boost phytoplankton production. During a large “algal bloom” phytoplankton remove carbon dioxide from the atmosphere and convert it into carbohydrates, proteins, and fats that feed marine organisms. A large bloom sustains vast numbers of zooplankton and larger animals, and when enough of these organisms die and sink to the ocean’s depths, they can help lock away excess carbon. Researchers are developing ways to manage marine systems to encourage more carbon to sink deep. This, surprisingly, includes protecting the ocean’s largest occupants. For example, whales bring nutrients up from the depths, which fertilize phytoplankton. This “whale pump” is one example of ways that scientists are using natural systems to promote long-term carbon removal and storage.

Bauer, J.E., W.-J. Cai, P.A. Raymond, T.S. Bianchi, C.S. Hopkinson, & P.A. Regnier. The changing carbon cycle of the coastal ocean. Nature, vol. 504.2013. doi: 10.1038/nature12857.

Chami, R. et al. On valuing nature-based solution to climate change: A framework with application to elephants and whales. Economic Research Initiatives at Duke, working paper number 297. August 2020.

Mariani, G., et al. Let more big fish sink: Fisheries prevent blue carbon sequestration—half in unprofitable areas. Science Advances, vol. 6. 2020. doi: 10.1126/sciadv.abb4848.

NASA. Carbon cycle. Accessed on February 18. 2021.

NOAA. Carbon cycle. Accessed on February 18, 2021.

NOAA. What is the carbon cycle? Accessed on February 18, 2021.

Riebeek, Holli. The Ocean’s Carbon Balance. NASA: Earth Observatory. July 1, 2008.

Roberts, C.M. et al. Marine reserves can mitigate and promote adaptation to climate change. Proceedings of the National Academy of Sciences, vol. 114. 2017. doi: 10.1073/pnas.1701262114.

Steinberg, D.K. & M.R. Landry. Zooplankton and the Ocean Carbon Cycle. Annual Review of Marine Science, vol. 9. 2017. doi: 10.1146/annurev-marine-010814-015924.

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