To mark Scotland’s Climate Week, we asked NatureScot’s marine sustainability advisor, Corallie Hunt to tell us why blue carbon is so important in the fight against climate change.

Scotland was the first nation to declare a global climate emergency. Our global climate is changing due to unprecedented levels of greenhouse gases, such as carbon dioxide (CO₂), in the atmosphere. Climate change results in more extreme weather events such as floods and heatwaves, rising sea-levels, and has serious consequences for nature, already in decline due to habitat destruction, over-exploitation, and pollution. Nature, however, is critical in helping us tackle climate change, so it is essential that we address both crises together.

We often hear about the importance of forests in absorbing CO₂, but we’re quickly learning that the ocean also has an integral role in regulating the atmosphere. In fact, over a quarter of the world’s carbon dioxide is captured by marine organisms, and every second breath of oxygen we take has originated from the ocean*.

Blue carbon is the carbon that is sequestered (taken in) and stored within the marine and coastal environment. Marine plants and algae capture CO₂ from the atmosphere using sunlight via a process called photosynthesis. This provides the energy for growth. Over time, carbon is transferred down to plant roots or surrounding coastal and ocean sediments where it can be locked away from the atmosphere for thousands of years. The plants that form three key blue carbon habitats – mangroves, seagrasses, and saltmarshes – are well-adapted to surviving in the salty and low oxygen conditions found in sediments waterlogged by seawater, and they are highly effective at absorbing CO₂ from the atmosphere. For the same amount of area, these blue carbon habitats can absorb CO₂ two to four times faster than forests on land.

In Scotland, the definition of blue carbon has been expanded from the three traditional habitats mentioned above, to include kelp beds, calcifying habitats and species such as native oysters and a coralline red algae called maerl, and seabed sediments. Each of these habitats play a role in the carbon cycle and research is underway to help us understand how these additional habitats accumulate carbon and contribute to longer-term blue carbon storage. Many of these habitats are protected within Marine Protected Areas for their high biodiversity value and they provide multiple other positive benefits for people. These essential ‘ecosystem services’ include providing coastal protection from storms and essential habitat for many fish species.

Key blue carbon habitats in Scotland are seagrasses, saltmarshes, and seabed sediments. Seagrasses are flowering plants that live within sheltered areas of the intertidal (the area of the coast between high tide and low tide) and subtidal (under water) zones of the coast. Scotland has about 20% of north-west Europe’s seagrass beds, with the UK’s largest meadow found within the Cromarty Firth. They provide important nursery grounds for fish, store carbon within sediments, and help to protect the coast from storms by absorbing wave energy.

Saltmarshes are found within sheltered coastal bays and are made up of creeks and channels, forming unique landscape patterns. In addition to storing carbon, saltmarshes provide habitat for a range of birds and natural flood defence, and can help to filter water.

Of all Scotland’s blue carbon habitats, seabed sediments are the biggest store of carbon due to their sheer extent. Sediments receive organic material that settles through the water column, which can build up in layers over time. Muddy sediments, such as those found within Scottish fjords, are the most carbon rich and are known as ‘hot spots’. Fjords receive a significant amount of carbon that originated from the land and has since been washed downstream where it settles onto the glacially, over-deepened seabed of the sea lochs.

Sadly, blue carbon habitats are threatened by human activity and climate change. Seagrasses have suffered huge declines in UK waters; over 40% of their area has been lost since 1936, due to disease, climate change, invasive species, and pollution. Rising sea levels at increasing rates threaten to erode and eventually drown low lying marshes, resulting in more lost habitat. When these habitats are damaged, carbon may be emitted back into the atmosphere, where it can further contribute to climate change. Activities that physically contact the seabed, such as trawling or dredging, can seriously damage seabed biodiversity in addition to disturbing carbon buried within sediments. What happens to the disturbed carbon is still unknown and a focus of current research.

Recognising the critical importance of blue carbon habitats, the Scottish Government has formed the Scottish Blue Carbon Forum, which aims to increase the knowledge and protection of these habitats through coordinated research. NatureScot has recently published a comprehensive review of all the evidence relating to Blue Carbon habitats within Scotland as well as identifying threats and critical evidence gaps. Work is in progress at a UK level to develop a saltmarsh carbon code with ambitions to incorporate saltmarsh into the UK Greenhouse Gas Inventory. Many community-based restoration projects are also underway to create or enhance blue carbon habitats and their ecosystem services. NatureScot has recently published the Scottish Seagrass Restoration Handbook, and the Marine and Coastal Enhancement Framework, which provides essential information about the design and good principles of restoration projects to enhance project success. To find out about some of the current projects, visit Project Seagrass Scotland or Seawilding, for further information.

By protecting and enhancing blue carbon habitats we can halt the loss of biodiversity and maximise the ability of the natural environment to absorb CO₂ from the atmosphere. We are also increasing the resilience of the marine environment and helping coastal communities to respond to climate change impacts. Blue carbon is one tool in the toolbox to help us solve and adapt to the joint climate and nature crises, but much is still to be discovered.

*Source:https://www.globalcarbonproject.org/carbonbudget/22/files/GCP_CarbonBudget_2022.pdf and https://www.un.org/en/climatechange/science/climate-issues/ocean