Colourful Carbon – The solution to climate change?

With COP26 and new commitments from different governments to become carbon neutral in the not so distant future there has been a lot of talk about nature based solutions, restoration efforts, carbon capture and blue carbon. A few months ago, I went for a hike with a friend, I had just submitted a paper on blue carbon and, very excitedly, kept yammering on about it in great detail. He listened to me patiently and only when I finally stopped, did he turn around and ask: What actually is blue carbon?

So, I thought I’ll try and explain it here. First, we need to get back to an old acquaintance that has recently made big headlines. CO2 – carbon dioxide! For a very small molecule is has a big impact! CO2 is a gas that is being created when we burn fossil fuels or biomass (also called black and brown carbon) and upon its release creates a barrier in the atmosphere which reflects heat back onto the planet – thus the term greenhouse gas! There are others, but CO2 is by far the most prominent and why most people are so keen to reduce it. Normally abundance of CO2 is regulated by plants. They more or less feed on it. This occurs in three steps – uptake, storage and sequestration! You will have heard of the rainforest being called the green lungs of the earth as they take up CO2 and produce oxygen instead (this is part of photosynthesis). The crucial bit here is the breakdown of CO2 into carbon and oxygen, because the carbon it contains is actually a very important building block for all life. We humans, and almost every other organisms use it to build our cells. When plants grow, they essentially store carbon. Some can store carbon for many years (when growing woody bits) and some for a summer (when growing leaves). However, as soon as this organic matter is turning to mush and decomposing or burning for that matter– the stored carbon is released back into the atmosphere again. In order to make sure this doesn’t happen, dead plants need to be removed from oxygen. This means that a lot of the bacteria normally responsible for decomposing dead plant matter cannot function properly and the carbon contained in these plants becomes sequestered. This is more or less how oil and coal have been generated in the first place and often occurs through burial of dead plants.

A nice schematic showing carbon storage and potential for carbon release in trees by the MN Board of Water and Soil Resources

Back to blue carbon! If rainforests are the green lungs of the planet, oceans are the blue lungs. The term blue carbon encompasses uptake of CO2, and storage and sequestration of the carbon contained in CO2 in the oceans. And the oceans are actually pretty good at that. It is estimated that about 30% of our yearly CO2 emissions end up in the sea. How, you might be wondering? There are a lot of complex chemical cycles going on that contribute to this, but a great deal of CO2 will be taken up by marine plants (mangroves, seagrass, saltmarsh) and algae (seaweed and phytoplankton) the same way plants on land take up CO2. Marine plants, which can be found around the coasts can store carbon similarly to plants on land. This is trickier in the open ocean or for that matter around the poles. Here, the only organisms able to take up CO2 are phytoplankton.

Phytoplankton are teeny tiny single cell algae floating in the water column (I recently learned the name comes from the Greek phyto for plant and plankto for wanderer). The problem is that they do not live very long. This means, although phytoplankton is great at CO2 capture, it is not so good at storage. Hence, these teeny tiny algae need to be eaten for the carbon to be stored and sequestered effectively. And this is where all the creepy crawlies at the bottom of the sea come in and our current paper on blue carbon in Antarctica. When animals eat phytoplankton, approximately 30% of the carbon that was originally captured is then used to grow that animal and store carbon. This is done by animals that live in the water column and swim and those that live attached to the sea bottom. In Antarctica, animals living on the seafloor can live for decades if not centuries and during this whole time they grow and/or build shells and reefs, which means they store carbon. The seafloor there is also fairly well protected which means the animals and the seafloor remain undisturbed by fishing for examples.

A bit of Antarctic seafloor: A crinoid nestled in amongst bryozoans (which are tiny colonial animals feeding on plankton). Photo credit D.K.A Barnes

We found that animals living on top of the seafloor (epifauna) and in the sediment of the seafloor (infauna) store a similar amount of carbon. This is exiting because, so far we only ever knew how much carbon is stored in animals living on top of the seafloor. And looking at both, showed that we have been underestimating this blue carbon storage by 50%. But the really big whammy here was that the sediment itself on and in which the animals live stores about 10 times the amount of carbon that is already stored in animals. Carbon storage in the sediment happens when phytoplankton and small swimming critters such as Krill die and trundle through the water column down towards the seafloor (this is also called marine snow). It also happens when animals living on the bottom of the sea poop. Over time this builds up more and more sediment and the marine snow and animal poop turn into carbon storage. However, less than 1% of the original biomass of phytoplankton reaches the seafloor. So, this is a very slow process.

The path of carbon storage and potential sequestration in areas of glacier retreat. CO2 is taken up by phytoplankton, parts of which are consumed by animals on and in the seafloor and a small part of it will be buried in the sediment. Increased sedimentation from the retreating glacier might help to bury such captured carbon below the level where it is recycled back into the system.

This is now getting more and more interesting because, as climate changes progress – ice on the poles will melt, meaning glaciers will retreat. Retreating glaciers, however, will create more space on the seafloor, while also generating a lot of sediment. Because, now the seafloor is not full of ice any more, animals will be able to live there and marine snow will be able to reach the sediment. Simultaneously, there will be more sediment from the glacier to bury any stored carbon. This means that more carbon can be stored and maybe sequestered. And the more CO2 we have in the atmosphere, the more ice will melt, the more space is created and the more CO2 is taken up by animals and sediment – the greater will be the reduction of CO2 in the atmosphere. This is what is called a negative feedback cycle.

View of Sheldon Glacier on the West Antarctic Peninsula. One of the fastest retreating glaciers there.

So, fjords in Antarctica are able to store a gigantic amount of carbon (maybe up to 56909 t C/yr ~ similar to what ~21000 families produce every year) in the sediment and animals living in and on the sediment, actively fighting global warming. However, even though this seems like an incredible high number it pales in comparison with the 36 billion metric tonnes (this is a number too big to even imagine) which all of us produce together every year.

So, we can’t leave it all to the icy poles – we have to help them out a bit – especially because a world without snow and ice would be pretty sad for everybody who loves to go skiing, sledging and stuffing snow down the back their mates shirt. All of us can do small things to reduce our carbon footprint. But we can also make sure that we tell people in charge that we are actually pretty serious about this net-zero thing.

Nature is absolutely brilliant at sorting itself out – it will help us if only we are willing to help ourselves.

Top Image: Climate strike 2019, Rothera Research Station. Photo credit: Ben Mack


Zwerschke, N., Sands, C. J., Roman-Gonzalez, A., Barnes, D. K. A., Guzzi, A., Jenkins, S., Muñoz-Ramírez, C., & Scourse, J. (2022). Quantification of blue carbon pathways contributing to negative feedback on climate change following glacier retreat in West Antarctic fjords. Global Change Biology, 28(1), 8–20.

Guidi, L., Chaffron, S., Bittner, L., Eveillard, D., Larhlimi, A., Roux, S., Darzi, Y., Audic, S., Berline, L., Brum, J. R., Coelho, L. P., Espinoza, J. C. I., Malviya, S., Sunagawa, S., Dimier, C., Kandels-Lewis, S., Picheral, M., Poulain, J., Searson, S., … Gorsky, G. (2016). Plankton networks driving carbon export in the oligotrophic ocean. Nature, 532(7600), 465–470.

Barnes, D. K. A., Fleming, A., Sands, C. J., Quartino, M. L., Deregibus, D., Chester, J., & Quartino, M. L. (2018). Icebergs , sea ice , blue carbon and Antarctic climate feedbacks. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 376(2122), 20170176.

Bax, N., Sands, C., Gogarty, B., Downey, R. V., Moreau, C. V. E., Moreno, B., Held, C., Lund Paulsen, M., McGee, J., Haward, M., & Barnes, D. K. A. (2020). Perspective: Increasing Blue Carbon around Antarctica is an ecosystem service of considerable societal and economic value worth protecting. Global Change Biology, 27(1), 5– 12.

Carvalhao-Resende, T., Gibbs, D., Harris, N., & Osipova, E. (2021). World Heritage forests: Carbon sinks under pressure. UNESCO, International Union for Conservation of Nature, World Resources Institute.

Lovelock, C. E., Fourqurean, J. W., & Morris, J. T. (2017). Modeled CO2 emissions from coastal wetland transitions to other land uses: Tidal marshes, mangrove forests, and seagrass beds. Frontiers in Marine Science, 4, 143.

Deng, B. (2015). Fjords soak up a surprising amount of carbon. Nature.

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