One of the benefits arising from the creation of coastal wetlands, through managed realignment, is that these new or restored habitats sequester carbon. They, therefore, have the potential to offset other carbon releases to the atmosphere and contribute to climate change mitigation. This brings an additional value from such projects alongside their primary objectives which are typically flood protection and habitat creation. 

One of the benefits arising from the creation of coastal wetlands, through managed realignment, is that these new or restored habitats sequester carbon. They, therefore, have the potential to offset other carbon releases to the atmosphere and contribute to climate change mitigation. This brings an additional value from such projects alongside their primary objectives which are typically flood protection and habitat creation. 

Whilst this carbon sequestration effect has become increasingly well recognised in recent years, there is no consistency of approach in terms of quantifying, valuing and communicating this benefit. To address this issue ABPmer has reviewed the available evidence and created a new and auditable blue carbon calculator.

Developing a ‘Blue Carbon’ Storage Calculator

On a global basis, it has been well known for many years that freshwater wetlands provide the largest terrestrial biological stores of carbon (referred to as Green Carbon) and that such habitats are especially important for trapping carbon because of their large spatial extent.  By contrast less attention has been paid to the role of marine and estuarine habitats (saltmarshes, mangroves and seagrass beds) in part because of their relatively small extent in a global context.

Recent research is providing valuable evidence that the marine carbon sequestration (referred to as Blue Carbon) rates can be particularly high in these coastal environments and indicates that they are ‘hot spots’ for carbon burial. This research is indicating they have a significant role to play in carbon storage despite their small size. From the work undertaken so far, it is clear that sedimentation/accretion is one of the key processes driving sequestration in coastal wetlands.

Coastal habitats created through managed realignment often function differently to external/mature marshes and exhibit high levels of accretion (across both marshes and mudflats), especially during the early years/decades after their creation.  Therefore managed realignment sites can be viewed, globally, as one of the ‘hottest spots’ of all for carbon trapping. 

What is less well understood is just how much carbon these new coastal habitats are trapping.  There is no consistent approach to understanding this issue. To address this gap, ABPmer has reviewed the available scientific evidence to produce a clear, simple and auditable approach to make this calculation. The ABPmer ‘Blue Carbon Calculator’ (ABC Calculator) can be used to estimate the rate of carbon sequestration in a created or restored marine wetland and establish its monetary value.

Allfleet’s Marsh Case Study 

Trial applications of the ABC Calculator have been made on the 115ha Allfleet’s Marsh site on Wallasea Island, Essex. Results indicate that the sequestration potential is double that of a typical mature saltmarsh during the early years of its evolution. This site is trapping around 491 tonnes of carbon annually (tC/yr) at 446 gC/m2/yr (as compared to around 210 gC/m2/yr which has previously been identified for mature saltmarshes globally (Chmura et al., 2003, Global carbon sequestration in tidal, saline wetland soils. Global Biogeochemical Cycles Vol 17 No. 4 1111). This comparison with mature is illustrated in the bar chart plot which shows the carbon burial rates in a range of terrestrial and marine habitats alongside the rates identified for Allfleet’s Marsh.

High  levels of sequestration are likely to continue over the long term although they will gradually reduce as bed levels rise and accretion rates drop.  In the first 7 years since it was created it has sequestered, approximately, £700,000 of ‘Carbon Dioxide equivalents’ based on the 2013 Department of Energy and Climate Change valuations.

Looking Forward

The key proposed actions for the future are:

1. Apply and test the Blue Carbon Calculator on several different MR sites to judge its effectiveness and to add to the evidence base about carbon sequestration rates in various locations;
2. Apply the Blue Carbon Calculator not just to past changes but future projected changes in MR sites in to order to describe the full lifetime carbon sequestion potential of MRs
3. Pursue further practical research in the field to build up an evidence base with which to enhance our understanding of key issues;
4. Seek to apply the lessons from this work to future national and international policy making and in particular to carbon offsetting programmes and UK Government’s commitment to put ‘put natural capital (including carbon storage functioning) at the centre of economic thinking’.