Rita Adele Steyn

The global ocean removes carbon from the atmosphere and stores it - in the water, in the sediments of the seafloor, and in the living organisms that inhabit both open ocean and coastlines. The ocean holds billions of tons of carbon, and global leaders must consider it in their plans to tackle climate change.

The Global Carbon Cycle

As one of the driving forces of global warming, carbon dioxide is the primary gas emitted through human activities. Before the industrial revolution (1750) the level of carbon dioxide in the atmosphere was 280 ppm (parts per million), and today it stands at around 420 ppm (and increasing). All this carbon is part of the larger global carbon cycle, a process that moves carbon around the globe between sources and sinks.

Sources produce carbon, and sinks hold carbon in reserve, locking it away (sometimes for decades or even centuries) either in living, organic matter, or in non-living materials. This means that both sources and sinks can operate on long and short-term time scales, but ultimately the goal is for there to be a balance - for carbon sequestration to equal production. By creating this balance, the global climate remains relatively stable.

The carbon cycle has both terrestrial, or land based, and aquatic, portions. The ocean is the Earth’s second largest carbon sink, storing vast amounts of both organic and inorganic carbon. For some perspective, the ocean also absorbs about 30% of current human CO2 emissions.

What is Blue Carbon?

From tiny plants and algae, to large whales, carbon can be found in all living things. Blue carbon is the carbon stored in coastal and marine ecosystems, specifically salt marshes, mangroves, and seagrass.

Even though these ecosystems cover a small proportion of the planet, they are capable of storing carbon at a much faster rate than terrestrial systems, and can continue to do so for millions of years. Interestingly, much of the carbon stored in these coastal ecosystems is found in their sediments (up to 99% in some cases!).

Blue Carbon Sinks

A carbon sink is something that absorbs more carbon than it releases, and thus acts as a pool or reservoir of ‘trapped’ carbon. By accumulating carbon, the concentration of carbon dioxide in the atmosphere decreases.

In the temperate zones, saltmarshes (also known as tidal marshes) are thought to accumulate carbon 55 times faster than tropical rainforests, storing it in their soils for millennia.

Seagrass meadows are capable of storing carbon up to 35 times faster than tropical rainforests.  Despite covering less than 0.2% of the seafloor, they account for an estimated 10-18% of the ocean’s carbon storage capacity. These underwater grasslands can be found globally (even in the Arctic!).

Mangroves store carbon not only in their intricate webs of roots and branches, but also in the soil beneath them. They sequester up to 5 times more carbon dioxide than terrestrial forests, but are found only in the tropical and subtropical zones.

Other Ocean Carbon Sinks

Seawater itself acts as a massive carbon sink. Much of this exists as dissolved inorganic carbon, often in the form of bicarbonate and carbonate (formed when carbon dioxide dissolves into seawater).

A large proportion of carbon is stored in the deep ocean. Cold, carbon-rich water sinks at the poles, driving the slow-moving ocean conveyor belt. Organic carbon, like dead organisms and faeces, also rains down into the depths of the ocean as marine snow. At the very bottom of the ocean, sediments hold huge amounts of carbon - comparable in size to the organic carbon from land, terrestrial soils, the atmosphere and seawater combined!

Shelled marine organisms remove carbon from seawater to make their hard shells, acting as a small, biological carbon sink. Other calcifying organisms, like corals and sea urchins also store carbon in their skeletons. Even some phytoplankton (such as foraminifera and coccolithophores) create tiny homes for themselves from calcium carbonate. But these species are at great risk of ocean acidification, which could jeopardize their ability to act as carbon sinks.

Blue Carbon Sources

Unfortunately, carbon sinks can also become sources.

Many coastal ecosystems are disappearing at a drastic rate. When disturbed or removed, saltmarshes, mangroves, and seagrasses release some of their stored carbon back into the atmosphere.

Since 1980, the world has lost 25% of its mangrove forests. And it’s been estimated that mangrove deforestation accounts for around 10% of global deforestation emissions. Based on current rates of loss, around 30-40% of saltmarshes and seagrasses (and an even higher percentage of mangroves) could be lost in the next 100 years.

It was also recently discovered that bottom trawling (a type of fisheries practise) releases carbon dioxide from seafloor sediments. This amounts to 1 gigaton of carbon yearly - about the same as the entire aviation industry!

Protecting Blue Carbon Sinks

If we are to keep 1.5 degrees warming within reach, it is vital to conserve, and restore, these blue carbon ecosystems.

“For far too long, the ocean has been mostly absent from serious policy discussions about reducing carbon emissions.”

Jane Lubchenco, Oregon State University and Co-chair of the HLP Expert Group

Blue carbon can play a role at national and international levels to fight the climate crisis. We need dedicated conservation efforts to ensure that coastal ecosystems continue to act as carbon sinks. We must halt their degradation and destruction, to avoid contributing further to global warming. And we need to restore previously carbon-rich habitats to their former glory.

Not only should we protect and enhance carbon sinks, but we must also drastically reduce global emissions. For the world to stay within 1.5C of warming, we must reach net-zero and go beyond. We can only do this by considering ocean-based solutions to climate change.

Protecting priority blue carbon ecosystems - mangroves, salt marshes, and seagrass meadows - is the only way to turn the black smoke of our emissions into the precious blue diamonds of our future.

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