Thirty years ago, no-one had heard of the term ‘Blue Carbon’, but it is now in common usage, even a new buzzword! But what does it actually mean and why is it important? We spoke to our Chief Scientific Adviser, Martin Attrill, about Blue Carbon, and the role of it in the fight against climate change.
Blue Carbon refers to the carbon captured by the world’s Ocean ecosystems, particularly coastal and marine habitats like mangroves, salt marshes, and seagrasses. These ecosystems absorb and store large amounts of carbon dioxide (CO2) from the atmosphere, potentially helping to mitigate climate change. In recent years, as we have become more aware of the capabilities of marine systems, Blue Carbon has gained significant attention as an essential tool in the fight against global warming – a natural form of Carbon Capture and Storage (CCS).
The term is therefore derived from the ability of coastal and marine ecosystems to sequester carbon. Like more familiar terrestrial forests, which are well-known for their carbon-absorbing properties, blue carbon ecosystems may be at least as efficient in locking away carbon. Mangrove forests, seagrass meadows and salt marshes act as ‘carbon sinks’, meaning they absorb more carbon than they release, trapping it in both plant matter and the sediments below them. Other marine habitats, such as kelp beds, take up lots of carbon, but as they do not grow on soft sediment (equivalent to terrestrial soil), their storage capacity is less obvious.
The world is increasingly looking at nature-based solutions to combat climate change, and blue carbon ecosystems are now at the forefront of these efforts, although we have so much to find out and learn compared with well-studied systems on land. Some studies that have been done suggest that coastal ecosystems can sequester carbon at rates of up to 10 times greater than terrestrial forests. For example, it has been estimated that the world’s mangroves can store up around 12 billion tonnes of carbon, but we are finding that there is huge variation in the amount of carbon stored, even within the same seagrass bed or mangrove forest. Global coastal blue carbon ecosystems have been estimated to capture at least 0.2 gigatons of CO2 annually, so they could certainly provide a valuable contribution as the world seeks to limit global temperature rise to 1.5°C above pre-industrial levels.
As with many cases when we move from land to sea, marine ecosystems function quite differently from those on land – and so this makes it harder to work out exactly where the absorbed CO2 goes. While much of the carbon produced by a forest stays in that forest, in the open sea the production can be spread widely – some of this may end up buried in sediment a long way from the source, or taken up by long-lived animals, for example. It seems clear that blue carbon ecosystems take up significant amounts of carbon, but it is also very likely that a lot of this carbon may get locked up within the wider marine environment (not just in the plants and their sediment) and not escape back to the atmosphere. We certainly have a lot still to learn!
Much of the global seabed is covered by fine, soft sediment (especially in the deep sea) and this is one of the great carbon stores on the planet. The deep-sea sediment in particular, as with some of the Blue Carbon habitats like seagrass beds, is thousands of years old and represents a vast amount of time when the system has been building up stored carbon. What is essential, therefore, is for this stock not to be disturbed or else much of this carbon will be released and add to emissions. Seafloor sediments can, for example, be disturbed by trawling; this carbon is released into the water column and can eventually escape into the atmosphere. A quite controversial study suggested that bottom trawling may release 0.6-1.5 gigatons of CO2 annually – equivalent to the emissions of the entire aviation industry, but these numbers have been strongly criticised. Nevertheless, the process, as with other physical impacts on the seabed, will disturb carbon stocks that have been stored for centuries – and so protected areas are essential not just to provide refuge for biodiversity to recover, but also to help in the battle with climate change.
The OCT is at the heart of this battle, with our Blue Meadows programme looking to protect the biodiversity and carbon within thousands of hectares of UK seagrass beds and to restore large new beds to allow more carbon capture and homes for a vast array of marine organisms.