What is the ocean carbon cycle?

Deep Dives VI

Welcome to another edition of Deep Dives by Beached - an in-depth exploration of everything oceanic, from scientific discoveries to marine mysteries. 

Expect a bit of history, science and adventure, as we dive deep into the big blue 🤿

The carbon cycle is one of the most important functions that keeps our planet habitable.

Without it, Earth would be a vastly different world – one we likely wouldn't recognise and certainly couldn't survive in.

This invisible process is constantly at work, moving carbon between Earth's major reservoirs – the atmosphere, the ocean, plants, rocks¹, sediment, soil and animals – in a continuous cycle of exchange. Some of these exchanges are instantaneous, while others unfold over millions of years, but each plays a crucial part in regulating our planet's temperature and supporting life as we know it.

The slowest movement of carbon occurs in rocks and sediments. When organisms die, some become buried in layers of earth or ocean sediment. Eventually, their remains transform into sedimentary rocks and, under the right conditions, into fossil fuels – effectively locking away that carbon for millions of years. Meanwhile, the natural weathering of rocks slowly releases carbon back into the cycle creating the perfect balance so we can all live here.

Moving at a slightly faster pace is the exchange of carbon through living things on land. Plants pull carbon dioxide from the air during photosynthesis and it then moves through the food chain as animals eat plants (and other animals), before being released back into the atmosphere through respiration or decomposition.

But the fastest and most dynamic movement of carbon happens between air and sea. The ocean surface is constantly exchanging gases with the atmosphere and though its ability to absorb carbon dioxide depends largely on temperature and wind, the ocean holds around 50 times more carbon than the atmosphere does. Wind stirs up the surface layer of water, stimulating absorption.

Cold waters, particularly near the poles², soak up more carbon dioxide than warmer tropical waters³ and as this cold, dense water sinks it carries the carbon with it, storing it in seabed sediment. However, this isn’t the only way carbon reaches the deep ocean..

Through photosynthesis, phytoplankton (microscopic marine plants) convert carbon dioxide into energy that fuels the marine food web. When phytoplankton bloom in large numbers, they remove vast amounts of carbon dioxide from the water, which in turn draws more down from the atmosphere. And when they die their bodies sink through the water column with other organic matter in what is known as marine snow, dissolving back into deep water or falling to the seabed and storing even more carbon away in deep-sea sediment.

Even whales play a part in this process. As they dive to feed and return to the surface to breathe, they release nutrient-rich waste that fertilises phytoplankton – a process referred to as the "whale pump."

But this finely-tuned system is under great threat. Burning fossil fuels puts excess amounts of carbon into the atmosphere, throwing off the balance between the air and the ocean. Much of this excess has already been absorbed by the ocean – around 30% or so – which has slowed the rate of warming, but it has also taken a significant toll on marine life. When carbon dioxide dissolves in seawater, it creates carbonic acid, making the ocean more acidic. Since 1750, the pH of ocean surface waters has dropped by 0.1 – a 30% increase in acidity.

One of the greatest threats of a warming ocean is acidification. Carbonic acid reacts with carbonate ions in the water - the very same ions that organisms like corals, crabs, and snails need to build their shells and skeletons. With less carbonate available, these animals must expend more energy to build their protective structures, which end up thinner and more fragile. The more acidic water can even dissolve existing shells.

Warmer ocean temperatures also make things difficult for phytoplankton, who grow best in cool, nutrient-rich waters. As our oceans warm, their abundance could decrease, limiting the ocean's ability to absorb carbon.

Over time, scientists have predicted that the ocean will absorb up to 85% of the extra carbon we've added to the atmosphere through burning fossil fuels. But this process is slow, tied to the movement of water from the surface to the depths, and there are worrying signs that the ocean's capacity to continue this absorption is already diminishing.

This is particularly concerning given that carbon dioxide levels in our atmosphere are already higher than they've been in the last 3.6 million years. So far, the ocean has been our greatest ally in moderating these unprecedented levels, but it cannot continue indefinitely. Any change to the ocean's ability to store and transport carbon could have far-reaching consequences for our planet's climate – and all the life that depends on it.

All of that to say, we owe a lot to the ocean.

Bleached coral. Photo by Veronique Mocellin/AIMS.

Cover image by NASA Earth Observatory/Joshua Stevens.

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Amie 🐋

1. Most of Earth’s carbon – about 65,500 billion metric tons – is stored away in rocks.

2. This is why the polar regions play such an outsized role in capturing carbon from our atmosphere.

3. In warmer waters, the temperature difference between the surface and deeper water can mean the water stratifies into layers. This lack of mixing prevents carbon dioxide from reaching deeper in the water column while also preventing nutrients from reaching higher.