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. https://doi.org/10.1111/gcb.15898

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. https://doi.org/10.1038/nature16942

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. https://doi.org/10.1098/rsta.2017.0176

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. https://doi.org/10.1111/gcb.15392

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. https://doi.org/10.3389/fmars.2017.00143

Deng, B. (2015). Fjords soak up a surprising amount of carbon. Nature. https://doi.org/10.1038/NATURE.2015.17464

Icebergs – What you gonna do when they come for you?

The science bit

Antarctica – the white continent, the most inhospitable place on the planet. Yet there is life here and I do not mean the bunch of scientists rambling around research stations who could not survive without the outside world. I mean colourful self-sustaining life. Where do you ask, since the most colourful life you can see is a penguin and that is black and white? Under the sea is my answer! Antarctic seas are teeming with life, they are wonderfully rich, filled with obscure little critters, some of which might seem familiar from a day’s rock pooling some of which seem to have come straight out of an alien movie but most of them supremely unique and only found in Antarctica.

Worms (Parabolasia corrugatus) that can grow over a meter long and expand or decrease their surface depending on oxygen concentration and sea urchins (Sterechinus neumayeri). These are some of the first animals to return after a disturbance to the seafloor Photo credit: C. Stronach
Worms (Parabolasia corrugatus) that can grow over a meter long and expand or decrease their surface depending on oxygen concentration and sea urchins (Sterechinus neumayeri). These are some of the first animals to return after a disturbance to the seafloor Photo credit: C. Stronach

Yet, like so many other of earths ecosystems they are under threat. In Antarctica some of this threat might come from an unexpected source – Icebergs! Icebergs can be a magnificent sight, providing a magical labyrinth of sculptures on the sea surface and range from being as large as a city to being a small sorry looking heap of ice in the water (perfect for a G&T). However, the saying the tip of the iceberg is a saying for a reason. What we can see of an iceberg on the sea surface is about 10% of the iceberg. A 10 m iceberg on the surface thus, is approximately 90 m deep. This means icebergs can be quite far reaching. Usually, icebergs are frozen into the sea-ice for large parts of the year.

Several icebergs covered in snow and frozen into the sea-ice
Icebergs frozen into sea-ice during winter at Rothera Research Station

Warming temperatures and changes in climate, however, reduce the duration of time icebergs are immobilised and mean icebergs drift around the sea surface a lot more freely moved by currents and wind. How, you might wonder, is this a threat to cheery little critters in the water? In general, there are two types of animals living in the sea, those that live in the water column (pelagic – like fish and jellyfish) and those that live on the seafloor (benthos – like starfish and anemones). Animals living on the seafloor move a lot slower, or might even be attached to the seafloor, than those living in the water column. When icebergs are moving around the sea, their submerged parts are very likely to bump into the seafloor and move along it, thereby scouring it. To the animals living on the seafloor this might be similar to a bulldozer in a forest for an Orangutan. Anything living in the wake of an iceberg will be removed and killed. This might sound harsh, but at a normal rate this is natures way of guaranteeing that there is a space to live for all animals, even for the weaker ones and it makes Antarctica’s seafloors even richer. Yet during 2007-2009 there was very little sea-ice along the West Antarctic Peninsula which meant that icebergs moved a lot more and a lot more freely than they used to. This in turn meant there was a lot more iceberg scouring on the seafloor and when we looked at it, we found that a lot of the animals that live attached to the seafloor had disappeared, the ones that we could find did not grow as old anymore and there was a lot more free space without animals on it.

(A) An iceberg grounded on the seafloor while frozen into the sea-ice and what the seafloor looks like immediately after being scoured by icebergs (B), after 10 years without disturbance (B) and when it has been protected from disturbance (C) – Source Zwerschke et al. (2021).

The seafloor was in crisis! Amount of different animals that could move around, however, stayed the same. We think this is because a lot of them are scavengers (animals that live of anything they can find such as vultures) and they are able to move into freshly scoured areas fairly quickly and munch on anything that is left over – after true vultures of the sea fashion. The outlook for anything else on the other hand, was fairly bleak. But don’t worry this is a good news story. After 2009, we started to see increases in sea-ice duration again. This meant icebergs were being kept in check more and more and the animals on the seafloor could start to recover. We kept checking up on them to see how many of the species had returned and how long they were living and we found that 10 years after the last big pulse of icebergs impact, the animals on the seafloor had recovered and the seafloor was again densely populated with vibrant, beautiful, bustling life.

Monitoring sea-ice cover and the retreat of Sheldon Glacier at a remote outcrop on Adelaide Island.

While 10 years might seem like a lifetime to some, it is actually incredibly fast if you consider how sensitive and delicate the ecosystem in Antarctica is (temperatures in the sea range from -2° to +2°C for comparison temperatures in the shallow Irish Sea are between 6.5° and 15.5°C. When the surface is covered in sea-ice the seafloor is shrouded in perpetual darkness with very little light reaching the bottom, and food in form of teeny tiny algae (phytoplankton) growing in the water column only turns up once a year). Even if you compare it to the average recovery time of a forest which is 40 years it again seems incredibly fast. Forest – 0, Antarctic seafloor – 1, I would say. Yet, before we get our party hats and streamers out, we should pause for a moment and have another good look at the data. What we see is that, actually, sea-ice duration is still not as high as it used to be and, actually, we still see a lot of iceberg impacts. At any rate more than we had seen before 2007.

We counted days of sea-ice cover for every year – dark blue (A) and on average how often the seafloor was hit by icebergs each year – turquoise (B) and how many different types of animals there are for those attached to the seafloor in dark brown and those that are more mobile in light brown (C). The bit marked with the blue bar is the time when there was very little sea-ice. Amended from Zwerschke et al. (2021)

The recovery of the seafloor under these conditions is reassuring and shows that we have some tough little bastards down there which can cope with a lot of adversity. On the other hand, what if this is just a short rebreather, the calm before the storm? What if more years without sea-ice and increased iceberg scouring are under way? How much can these little sea aliens take before they have to give up and will perish forever? There is no doubt, that a decrease in sea-ice is linked to a warming planet and while the fate of a purple starfish might not rock your boat, the prospect of 5 m sea level rise, once all the glaciers have melted should at least give cause to a raised eyebrow and a slightly uncomfortable feeling in your stomach region. Far be it from me to preach, we are all creatures of comfort but there are still little things that every one of us can do and you have heard them all before (cycle, use public transport, up-cycle, eat less meat, don’t fly so often) yet even if the problem seems humongous (sorting climate change is a biggie) and it seems impossible to tackle for the individual –  every little bit helps. You cycling to work or taking the bus instead of the car matters! Swapping to green energy has an impact! Having cauliflower instead of a steak changes everything – in so many aspects! It’s a slow burner, you will not be rewarded the next day but your children or grandchildren might and the anemones will thank you!


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Barnes, D. K. A., & Souster, T. (2011). Reduced survival of Antarctic benthos linked to climate‐induced iceberg scouring. Nature Climate Change, 1(10), 365– 368. https://doi.org/10.1038/nclimate1232

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Zwerschke N., Morley S. A., Peck L. S. , Barnes D. K. A. (2019). Can Antarctica’s shallow zoobenthos ‘bounce back’ from iceberg scouring impacts driven by climate change? Global Change Biology, https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15617