Salt marshes to fight climate change

Salt marshes to fight climate change

Climate change is upon us, and the urgency to act against it has never been so high. The ever-increasing emission of carbon dioxide over the past century must be reversed and has become a global priority. The sea level is rising, and the frequency and intensity of extreme weather events are increasing, putting our shorelines and coastal communities at increased risk of flooding. Marine coastal ecosystems are vital for human health and well-being for all the services they provide to people, from food provision to coastal protection and more. Salt marshes are coastal wetlands flooded and drained by salt water brought in by the tides that occur worldwide, particularly in middle to high latitudes. As a marine and coastal ecosystem, they are excellent habitats for climate change adaptation and mitigation, and here is why.

Salt marshes in the National Park of Banc d’Arguin, Mauritania. Credit: Ewan Trégarot

A colossal carbon sink to mitigate Greenhouse Gases emissions

The main cause of climate change is the emission of greenhouse gases such as carbon dioxide, nitrous oxide, methane. Salt marshes are highly effective to store carbon, as they absorb the carbon dioxide from the atmosphere and lock it into the ground through the capture of organic sediment rich in organic matter (in a nutshell!). Globally, for terrestrial forest systems, the average carbon burial rate range between 4 to 5 g/m2/year. For salt marshes, the rates have been measured at an average of 218 g/m2/year, which is about 50 times more than terrestrial forests! The blue carbon potential of vegetated marine ecosystems is enormous, and its valuation (in monetary terms) has the potential to create opportunities to fund the restoration, conservation, and protection of saltmarshes to contribute to climate change mitigation. This is achieved by generating carbon credits through restoration activities that follow an official methodology approved by carbon offset mechanisms, such as the Verified Carbon Standards.

 However, human activities can alter the capacity of salt marshes to sequester blue carbon, and loss of blue carbon from salt marshes has been reported due to land reclamation, chemical and physical disturbances, and eutrophication. Sea-level rise and climate change may also influence carbon sequestration within tidal marshes either positively or negatively. More research is still needed to understand those highly complex processes.

A great coastal defence against sea-level rise

Sea-level rise is a significant threat to our coastline around the world. Even if a low carbon emissions trajectory is followed, the global mean sea level is projected to rise between 0.29 and 0.59 m by 2100 relative to 1986–2005, according to the last IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. Being at the interface between land and sea, salt marshes, like other vegetated marine and coastal ecosystems, are important flood and coastal defence, and critical in reducing disaster risk in low-lying coastal zones. As a matter of fact, they support three essential functions for coastal protection: wave attenuation, storm surge reduction, and seabed elevation. Indeed, the amount of waves energy reaching the shore depends on the bathymetry or state of ocean floors, the slope, and the sediment properties of the seabed. In the presence of salt marshes, the plants’ stiffness, density, leaf length, and morphology further dissipate wave energy, increase bottom friction, and reduce current flows and turbulences. Belowground, the root systems secure sand fixation, allow high sediment accretion rates and shore stability. Through sediment accretion rate or change in elevation, the marshes can respond to the elevation in sea level as the way they build up vertically is key to keeping up with sea-level rise. To ensure this function, salt marshes also require healthy associated ecosystems, like mudflats or intertidal seagrass beds, to maintain high elevation and limit the attacks of waves on salt marshes fringe.

Protecting the remaining salt marshes is a matter of urgency, but there is also potential to restore and create new habitats. By removing coastal defences or moving them further inland, ‘Managed realignment’ is one way of creating new habitat, and is one of the UK adaptation measures to increase carbon storage, and reduce risks of flooding and coastal erosion. In fact, plans are to realign 10% of England’s coastal zone by 2030 which would create 6,200 ha of new habitat. A great example of coastal realignment is the Medmerry project, which is one of the few marine Nature-based Solutions to date according to the IUCN global standards.

Illustration of the managed realignment at Medmerry 2011. Credit: Google Earth
Illustration of the managed realignment at Medmerry 2019. Credit: Google Earth

One of the most noticeable effects of climate change is the increasing frequency and severity of storms. If the climate continues to warm and sea levels continue to rise, the effects of these storms could be devastating, putting these and other coastal communities worldwide at risk.

And much more that we know and we don’t know…

Beyond being a carbon sink and a great coastal defence against flood, these intertidal habitats are essential for healthy fisheries and communities, and they are an integral part of our economy and culture. They also provide essential food, refuge, or nursery habitat for many species of fish and shrimps of commercial interest.They mitigate flooding by slowing and absorbing rainwater and protecting water quality by filtering runoff and metabolizing excess nutrients. However, salt marshes globally have seen their ecological condition and physical properties degraded by the combined effects of climate change and anthropogenic stressors such as pollution and land-use practices. The resulting decline in the coastal protection service they provide put coastal infrastructures and populations even more at risk in the face of climate change. The cumulative impact of climate change and anthropogenic activities on the state of saltmarshes and their ability to provide ecosystem services remains largely unknown. It is one of the main challenges the MaCoBioS project will attempt to solve. Our experts will soon collect in-situ data in the salt marshes of the Southern UK and Ireland across a gradient of environmental conditions and protection measures. Stay tuned for fieldwork updates and results!

You could get involved in protecting these fabulous habitats

Want to contribute to the research on salt marshes? If you are based in the UK, The Saltmarsh App gives you all you need to know to collect essential plant and soil data. Next time you take a walk by the sea or take out your binoculars to enjoy the richness and abundance of birds on saltmarshes, why not collect a few data? Or else you can help spread the word on this amazing natural habitat, highlighting its importance for human well-being and fighting climate change.

Text by Ewan Trégarot, with the help of Mialy Andriamahefazafy and Cindy Cornet

For further reading:

IPCC Special Report on the Ocean and Cryosphere in a Changing Climate: https://www.ipcc.ch/srocc/

Managed realignment: https://www.ice.org.uk/knowledge-and-resources/case-studies/managed-realignment-at-medmerry-sussex

Salt marshes research and App: https://www.ceh.ac.uk/our-science/projects/salt-marshes

Verified Carbon Standards: https://verra.org/project/vcs-program/

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