Our oceans are the largest carbon sink on the planet! Absorbing around 25% of CO₂ emissions and capturing around 90% of the excess heat generated from them.

This amazing feat of climate control is powered by a complex marine ecosystem, where some of the smallest organisms play the biggest part. However, this ecosystem is currently under threat like never before, and without it, we may be looking at a very uncertain future for our planet.

So what exactly makes the ocean a ‘Carbon Sink’? And, how can we ensure that our seas keep performing this vital role?

Why the Ocean is a Carbon Sink

A ‘Carbon Sink’ is a natural or artificial reservoir that absorbs and stores carbon from the atmosphere, locking it away through physical and biological processes.

In the ocean, CO₂ is able to dissolve in seawater through a series of complex chemical reactions. Slowly, this process acidifies the surrounding seawater, as the chemical reaction to dissolve CO₂ increases the number of hydrogen ions in the water. This acidification is then mitigated by the ocean currents, which mix the surface waters with water from the deep ocean.

However, the ocean is not just a passive absorber of CO₂. Photosynthetic marine organisms also take up CO₂ via photosynthesis, helping to buffer this ocean acidity. These organisms then incorporate carbon into their organic matter, which then becomes food for other creatures. Whilst seagrasses, kelps and many other marine plants play their part in this process, it is microscopic phytoplankton that are the true heroes of this story.

 

Small but Mighty!

Phytoplankton are the main reason that our oceans are the biggest carbon sink on the planet, as their enormous biomass absorbs as much carbon as all the plants and trees on land combined!

Not only that, but these little single-cell plants are the basis for the majority of the marine food-chain. In fact, it is estimated that roughly half of the energy that flows through the ocean food web is derived from these tiny organisms.

Zooplankton, krill, small fish, and filter-feeding invertebrates such as clams and oysters, are among those that depend heavily on these mini-plants as a food source. The carbon taken up by the phytoplankton then gets passed up the food-chain, for instance, into the zooplankton and krill, which then become food for much larger animals.

A good amount of this carbon also finds its way onto the sea floor, via particles known as ‘marine snow’. This includes everything from dead plankton, to the digestive waste of other animals. As these particles fall through the water column, they stick together and form aggregates. When these aggregates eventually reach the seafloor, they form a compost-like layer of organic-rich sediment. Overtime further layers of this sediment are deposited, eventually locking this carbon away.

It’s this very process, over the course of millennia, that created many of the fossil fuels we now burn today for energy, re-releasing this carbon back into the atmosphere.

 

A Plastic Problem?

We are now releasing stores of carbon from former carbon sinks into the atmosphere, much faster than the ocean can re-absorb it. However, it’s not just the burning of fossil fuels which is the issue; plastic pollution and other physical waste which ends up in our seas is also disrupting the ocean’s ability to absorb carbon.

Floating “garbage patches”, where plastics accumulate in large masses on the surface, are especially damaging. Some of these are miles wide, and effect the ability of marine organisms like phytoplankton to photosynthesize just below the surface of the water.

These patches also interfere with ocean circulation, reducing the mixing of surface and deep waters. This can lead to the build up of CO₂ in the upper waters, reducing the oceans effectiveness in absorbing CO₂ from the atmosphere.

These problems could also affect the numbers of phytoplankton in our ocean, depriving them of food from photosynthesis, and also creating water too acidic for some of them to survive. Lower numbers of phytoplankton will not only reduce their overall impact on dealing with atmospheric carbon, but it would also fundamentally effect the entire marine food-chain which also helps to lock it away.

 

What can we do about it?

As individuals we can make changes to our lives that could help our oceans absorb carbon.

Firstly, we can make decisions that reduce our carbon impact altogether, whether that’s reducing our energy consumption at home or using sustainable transport. However, it is vital that we also reduce our reliance on plastics, especially single use plastic. There are many ways to do this and it can start as simply as using re-usable non-plastic food containers, choosing products with sustainable packaging, or switching household utensils to plastic free alternatives. When we do encounter plastic which cannot be re-used, we can also make sure we dispose of it appropriately by recycling it whenever possible.

When made by enough people, small changes like these could make an enormous difference for our oceans, enabling them to sustain the phytoplankton and other organisms that keep our planet’s climate habitable!