The ocean’s role as a carbon superpower was only recently quantified in the 1990s and, as it turns out, it is responsible for diverting up to a third of all anthropogenic carbon from entering the atmosphere since the industrial revolution^[1]Khatiwala, S. et al. Global ocean storage of anthropogenic carbon. Biogeosciences 10, 2169–2191 (2013).. A study conducted by Woods Hole Oceanographic Institution (WHOI) states that the ocean holds about 50 times the amount of carbon stored in the atmosphere, and 20 times the amount in land plants and soil^[2]When it comes to sucking up carbon emissions, ‘the ocean has been forgiving.’ That might not last. PBS NewsHour … Continue reading. How does it do this? Let’s talk about “blue carbon” and how it could help us with climate change. 

The Carbon Cycle

In environmental lingo, the word “blue” tends to refer to the ocean or bodies of water – the blue economy^[3]What is the Blue Economy? World Bank https://www.worldbank.org/en/news/infographic/2017/06/06/blue-economy. (the use of ocean resources in economic growth), blue-green infrastructure^[4]5 lessons learned from blue-green infrastructure delivery. Institution of Civil Engineers (ICE) … Continue reading (the integration of natural water features in urban design), and blue carbon. Blue carbon is simply defined as carbon taken in and stored by the ocean and coastal ecosystems^[5]US Department of Commerce, N. O. and A. A. What is Blue Carbon? https://oceanservice.noaa.gov/facts/bluecarbon.html (1969).; however, blue carbon is just one part of the overall carbon cycle – the constant exchanges of carbon molecules on Earth. Here’s a quick summary:

Figure 1: The carbon cycle^[6]Carbon cycle. https://www.noaa.gov/education/resource-collections/climate/carbon-cycle..

To start, carbon enters the atmosphere as carbon dioxide, released from our breathing, human activity, and other carbon sources. The atmosphere is a “sink,” where more carbon is stored than released^[7]What is a carbon sink? | ClientEarth. https://www.clientearth.org/latest/latest-updates/stories/what-is-a-carbon-sink/ (1969).. Carbon dioxide is then drawn out of the air and taken in by plants and other autotrophs (living things that make their own food) for photosynthesis, where they trap it into sugar molecules. When another creature eats these plants, they absorb the same carbon. When they die, it returns to the soil and the atmosphere as they decompose. This basic cycle serves to show us that carbon can exist in a variety of forms, constantly migrating from one sink to another.

Usually, the carbon inputs to the atmosphere are matched by the outputs, but what happens when there is more carbon going into the atmosphere than can be absorbed? Well, that’s where blue carbon comes in! If you add a teaspoon of sugar to your coffee or tea without stirring it, the bottom of your glass would taste the sweetest. But, as you let it sit, you would start tasting the sugar throughout your drink. Gases tend to move from areas of high concentration (the dissolved sugar at the bottom) to those with a lower concentration (the rest of the drink) in a process called diffusion. So when there is a higher concentration in the atmosphere, the ocean ends up absorbing carbon dioxide to make up for the difference and restore balance to the system. Conversely, the ocean would release this stored blue carbon if there was a lower concentration in the air. It’s all about the balance. 

The Storage and Movement of Blue Carbon

After it is taken in from the air, a large amount of this blue carbon is stored in the cold, deep, twilight zone (from 200-1000 meters below the surface), deposited via several biological pumps. Biological pumps are rather complicated, but the way they work is well-illustrated by zooplankton, tiny aquatic organisms at the base of the food chain. These clever little organisms wait until the cover of night, when they are best hidden from predators, to undergo a huge vertical migration^[8]DVM: The World’s Biggest Game of Hide-and-Seek. Frontiers for Young Minds https://kids.frontiersin.org/articles/10.3389/frym.2020.00044 (1969).. They travel up to the surface to feed on carbon rich food (often other plankton), which is carried within them on their return to the deep when the sun comes out. The carbon they have consumed enters the water column through their respiration, poop, and carcasses. This whole process is a biological pump, the movement of carbon from the surface water down into the deep ocean through biological mechanisms^[9]Claustre, H., Legendre, L., Boyd, P. W. & Levy, M. The Oceans’ Biological Carbon Pumps: Framework for a Research Observational Community Approach. Front. Mar. Sci. 8, (2021)..

Figure 2: The oceanic biological pump processes^[10]Fig. 2.10 Scheme of the biological pump processes (Chisholm 2000). ResearchGate https://www.researchgate.net/figure/Scheme-of-the-biological-pump-processes-Chisholm-2000_fig19_278662655..

The activities of this plankton pump are part of a larger nutrient flow that is often referred to as “marine snow,” consisting of all kinds of organic waste and decomposing organisms that fall, like snow, into the deep^[11]Frost, E. Marine Snow: A Staple of the Deep | Smithsonian Ocean. https://ocean.si.edu/ecosystems/deep-sea/marine-snow-staple-deep.. On its way down, this carbon-rich mixture is a feast for swimming creatures, like the vampire squid (Vampyroteuthis infernalis)^[12]Pacific, A. of the. Vampire Squid. https://www.aquariumofpacific.org/onlinelearningcenter/species/vampire_squid., and benthic (bottom feeding) species like sea cucumbers (Holothuroidea)^[13]Pontin, S. Sea Cucumbers – Facts, Diet & Habitat Information. https://www.americanoceans.org/species/sea-cucumber/ (2021).. A small amount of carbon that reaches the seabed gets stored in sediment or rock, getting trapped there for long periods of time. 

Another major force in the cycling of blue carbon is sequestration, which occurs in what are called blue carbon ecosystems. To step back on land for a moment, carbon sequestration is the process in which carbon dioxide is captured from the atmosphere to be confined in plant matter or soil for long periods of time. The carbon that forests and other terrestrial ecosystems draw in is referred to as “green carbon,” which is usually stored within the trees and plants of the system. Forests are heavy hitters for carbon sequestration, but coastal ecosystems like salt marshes, mangroves, and seagrass are at a whole other level. Current studies from NOAA find that mangroves and coastal ecosystems actually sequester 10 times more carbon each year than mature tropical forests^[14]Coastal Blue Carbon. https://oceanservice.noaa.gov/ecosystems/coastal-blue-carbon/ (1969).. There are two reasons for this, the first being that they are highly productive, meaning that they tend to grow at a fast rate. Secondly, the soil in these ecosystems are mostly anaerobic, meaning without oxygen^[15]Coastal Blue Carbon. https://oceanservice.noaa.gov/ecosystems/coastal-blue-carbon/ (1969).. The bacteria responsible for decomposition needs oxygen in order to do its job. So, once a tree falls, its decomposition releases the carbon that it had sequestered within itself back into the atmosphere. When part of a seagrass meadow dies, it is slower to decompose due to the lower level of available oxygen. During this delay, dead seagrass can get buried in layers of ocean sediment, further cutting off any supply to oxygen^[16]Magazine, S. Underwater Meadows of Seagrass Could Be the Ideal Carbon Sinks. Smithsonian Magazine … Continue reading. The buried plant material retains its carbon which can remain trapped for years. 

Getting the System Back on Track

The cycling of atmospheric carbon into blue carbon is an impressive and effective process, however, this process may be slowing down. An analysis from the American Geophysical Union (AGU) suggests that the ocean will lose the ability to absorb carbon by the year 2100. The paper relates this potential decrease in carbon uptake to a change to the ocean’s currents, and the implications are dire. An additional threat we are beginning to see is that of ocean acidification, which is a direct effect of excess dissolved carbon dioxide reacting with water to create carbonic acid^[17]October 13 & Hu, 2022 Perrin Ireland Shelia. Ocean Acidification: What You Need to Know. NRDC https://www.nrdc.org/stories/what-you-need-know-about-ocean-acidification.. This natural chemical reaction is relatively harmless in smaller amounts, but as more carbon is absorbed the ocean’s waters become more acidic and less hospitable to marine life. There’s a difficult conundrum, as if the cycling of blue carbon comes to a halt, global warming would accelerate^[18]Hilmi, N. et al. The Role of Blue Carbon in Climate Change Mitigation and Carbon Stock Conservation. Front. Clim. 3, (2021)., but as it continues, the acidifying effect threatens all the sensitive species that inhabit the ocean, in particular, coral reefs (which host all other kinds of life) and shelled organisms like crabs, mussels, etc. 

Cate Hogan

Research and Writing Intern

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