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Illustration of a blue whale doing its thing.
Cutting carbon

The ocean's role in lowering CO2

Avoiding CO2 emissions is a key part of tackling the climate crisis. But capturing and storing CO2 is another tool in our CO-toolbox. While the energy industry works out new ways to do this on land, ocean habitats are already busy absorbing CO2 and keeping it locked away from our atmosphere.

Trees are an easy analogy

Whenever we tell Bulb members that they’ve lowered their carbon impact by 3.2 tonnes a year on average, we compare it to the number of trees it would take to remove that much CO2 from the air (it’s 1,590, as it happens). That’s because trees are something we’re all familiar with. We don’t mean to be presumptuous, but you might be less familiar with phytoplankton. And frankly, the comparison would make for a weirder email.

The ocean’s potential for locking up CO2 is often overlooked. A tree holds onto carbon for as long as it remains standing, which we hope is a long time. It could be many decades before that carbon is released back into the air. But when carbon makes its way to the deep ocean, it’s locked away from our atmosphere for hundreds (if not thousands) of years in the form of animal bones, sediment or plant materials. 

There are lots of ways that ocean habitats capture and store CO2


The carbon captured by coastal ecosystems is called ‘coastal blue carbon.’ Wetlands like seagrass meadows, mangroves and salt marshes are excellent at sequestering carbon (taking it from the air) because the plants that live in these rich environments grow very quickly. Some studies suggest that wetlands actually sequester carbon 10 times quicker than mature rainforests. The soil itself is important, too. It’s usually anaerobic, which means that it lacks oxygen. Plants decompose much more slowly in anaerobic soil, so the carbon stored in those plants takes longer to find its way back into our atmosphere. 

Sea creatures 

All marine vertebrates store carbon in their bodies. Whales are a great example, because they weigh a lot and they live for ages. Over their long lifetimes, whales accumulate a huge amount of carbon in their fat, and their storage potential doesn’t stop there. When a great whale dies, it sinks to the sea floor, sequestering an impressive 33 tons of CO2. From here, carbon passes onto any hungry deep-sea critters, and eventually into sediment, where it can stay locked up for thousands of years. Whales also play an important role in spreading nutrients through the ocean - partly thanks to their giant movements, and partly thanks to their giant poo. These nutrients help to grow populations of phytoplankton. Which brings us neatly onto...

Marine plants 

Phytoplankton are single-celled, free-floating marine plants, and they’re tiny carbon-capturing experts, too. Like plants on land, they use CO2 and produce oxygen during photosynthesis. It’s estimated that at least 50% of the oxygen in our atmosphere comes from phytoplankton, and that these microscopic superstars absorb about 4 times as much CO2 as the Amazon rainforest

Larger marine plants (macroalgae) like kelp, might also play a significant role in fighting the climate crisis. Kelp can grow at speeds of 60cm a day, and it tends to enjoy rocky conditions close to shore. When these rocks erode, or when bits of kelp break away, the plant matter floats out to sea and sinks, storing CO2 on the deep seafloor (a bit like our friends, the whales). Scientists have suggested that around 200 million tonnes of CO2 are sequestered by macroalgae every year. That’s the same as the annual emissions of New York state. 

Surface absorption 

As well as plants and animals, seawater itself plays a role in carbon absorption. That’s because carbon dioxide dissolves on the ocean's surface. And the sea is massive, don’t forget. It’s estimated that the sea absorbs 30% of man-made CO2 emissions. But as humans emit more greenhouse gases into the atmosphere, we’re altering the chemistry of seawater by making it more acidic. This has knock-on effects on the marine life that’s busy locking away carbon in other ways. 

Protecting the ocean is an important way to prevent climate catastrophe

It’s clear that the sea plays a vital role in keeping CO2 out of the atmosphere. From restoring salt marshes and protecting whale populations to defending kelp forests with sea otter patrols, there are plenty of opportunities for us to harness marine biology and slow climate change.

There are lots of organisations doing fantastic work to protect our oceans. If you’d like to lend your support, the Marine Conservation Society or the WWF are great places to start. You could also consider an online course in the science of algae, or find out more about regenerative ocean farming techniques with Greenwave. The world is your oyster.

Don't forget, you can work out your personal carbon footprint and offset it with Bulb. Just head on over to our Carbon Calculator to get started.