Life on the edge
Mangrove forests occupy a unique boundary between the land and sea of many tropical and subtropical coastlines, where they thrive on the intertidal belt of waterlogged and saline sediments. Mangroves are the ultimate ecotone, with both terrestrial and marine elements.
They are one of the most productive ecosystems on earth, with remarkable social, ecological and economic value; the economic value of mangroves (based on the services and benefits they provide) is estimated between $200,000 and $900,000/ha/year¹.
There are around 70 species of mangrove tree, each adapted to life in salt water, that provide the backdrop for a multitude of other marine species (sea turtles, migratory birds and fish) to thrive. They also assist in moderating the impacts of natural hazards, acting as a natural barrier or ‘bio-shield’. And, they provide an abundance of resources for coastal communities, from recreation and tourism to firewood and fishing poles. Consequently, over 200 million people rely on mangroves to support their livelihood and protect their community².
The total area of mangroves worldwide is estimated around 135,000km² ³. Global mangrove distribution is dependent on many factors, including air temperature, rainfall regimes and river flow. The most extensive region of mangroves is in the Indo-West Pacific, which hosts about 27,000km² of the world’s mangroves⁴. However, mangroves have been heavily impacted by deforestation, degradation and natural hazards. And, in the past 50 years approximately 25%-35% of global mangrove cover has been lost⁵… and the areas that remain, are in a degraded condition.
The economic value of mangrove ecosystem services is well-understood, for example in the protection they offer against coastal erosion. Yet, pressure from urban development and a lack of market-consideration to mangrove co-benefits (ecosystem services and natural capital) have contributed to unsustainable rates of mangrove deforestation — a costly case of natural capital devaluation.
By 2030, around 60% of the global population will inhabit urban areas, with a high concentration in coastal zones. Coastal developments will increasingly erode the natural environment, building infrastructure that is highly vulnerable to damage (as it replaces the very thing providing shoreline protection).
Against the backdrop of climate change, mangroves undergo high levels of direct anthropogenic disturbance (although natural and human disturbance shouldn’t necessarily be distinguished as the two are increasingly intertwined). Mangroves, already stressed by natural hazards, are more susceptible to human impacts such as pollution and the introduction of invasive species, which can be a tipping point towards total loss.
Major anthropogenic threats include urban expansion, mining, local exploitation for timber and land conversion for shrimp farming (Figure 1). These activities are often considered to be the most accessible and profitable uses of the land, but a longer term outlook could see the mass restoration and conservation of these immensely valuable forests. Such an outlook needs to focus on mangrove’s role in shoreline protection and carbon sequestration and find a way to place an economic valuation on these services.
There is clear evidence to show that mangroves effectively reduce the impacts of natural hazards by dampening wave energy⁸. Mangrove belts that are only a few hundred metres wide have been shown to reduce tsunami height by anywhere between 5% and 30%; and thicker mangrove forests are even more effective at reducing wave height and water speed. Mangroves are therefore an important asset in community-level disaster risk reduction (DRR) and climate change adaptation. Even a modest reduction in the area of inundation can reduce loss of lives and economic damage⁹.
With the frequency and intensity of natural climate hazards increasing, the call to ‘build back better’ is an opportunity to utilise natural capital and connect frameworks for DRR under the structure provided by the UN’s SDGs. Mangrove forests offer an important solution, yet their value as ‘green infrastructure’ is not well realised at present.
In 2016, The World Bank published a report¹⁰ on the value of mangroves, the key points being:
- In the US, mangroves avert over $1billion in damages to residential housing, and without mangroves, the cost of damages to residential and industrial property would increase by 28%
- In the Philippines, the extent of mangrove loss between 1950 and 2010 has caused increases in flooding risk to almost 300,000 people each year. Restoring mangroves would provide the equivalent of $450 million/ year in flood protection benefits
- If mangrove deforestation continues at the current global rate, harm to people, damage to infrastructure and property will increase annually by about 25%; mangroves currently protect over 200 million people from flooding, over 20% of whom live below the poverty line as defined by the World Bank (2016)
Sea level rise
Mean sea-level levels are expected to rise significantly by 2100 in all credible climate scenarios, and global sea level rise (SLR) projections indicate devastating implications for many populations, ecosystem services and biodiversity.
The relationship between mangroves and SLR is not completely understood, however research suggests that mangroves can keep pace with some rates of SLR due to their root systems which have the ability to accrue sediment. This means they slowly migrate landward during times of SLR and seaward in times of sea level regression. This temporal response to changing sea levels has been well documented and offers hope that mangroves can adapt to future SLR, although the speed of change will be critical¹¹.
Mangroves also increase soil volume by trapping their own organic matter as well as coastal sediments as they pass through. Over time, this volume builds up, and the soil level is pushed upward by the mangrove roots creating a higher soil level, this process further allows mangroves to keep pace with SLR.
Mangroves are a crucial part of blue carbon ecosystems, as explained in our previous blue carbon blog. But one of the issues often flagged with blue carbon is vulnerability to SLR and therefore unpredictability regarding the longevity or permanence of blue carbon projects. However, if mangroves can be shown to successfully overcome climate change impacts such as SLR, they could offer a sustainable supply of blue carbon credits. This is a particularly relevant opportunity for many small island states, where the reality of climate change poses looming threats which include intensified natural disasters.
Excluding peatlands, mangroves store more carbon per unit area than any other natural ecosystem¹². Some estimates suggest that as much as 50% of total carbon stored in ecosystems is stored in blue carbon habitats; these ‘climate-combating coastal ecosystems cover less than 0.5% of the sea bed. But they are disappearing faster than anything on land and much may be lost in a couple of decades. These areas, covering features such as mangroves, salt marshes and seagrasses, are responsible for capturing and storing up to some 70% of the carbon permanently stored in the marine realm¹³.
Researchers have found that mangroves store up to 4 times more carbon per hectare than most other tropical forests — for example, 1,023 MgC/ha in Indo-Pacific mangroves is compared to approximately 250 MgC/ha from tropical forests. Long-term accumulation in the ocean ranges from 18 and 1713 g C m-² compared with 0.7 and 13.1 g C m-² in terrestrial forests¹⁴. Forest leaf litter is quickly recycled and the carbon released via oxidation, however, with mangroves the waterlogged sediment creates anoxic conditions which prevents leaf litter from oxidising their carbon, creating longer-term carbon stores.
Most of the carbon is stored in mangrove soil, derived from organic matter and debris carried in by tides. Kauffman et al (2020)¹⁵ report that global aboveground carbon stocks are 1.6 billion tonnes, whilst belowground carbon stock amounts to 10.2 billion tonnes. Figure 2 compares carbon storage above and belowground for different ecosystems.
Despite these estimates, marine habitats are frequently excluded in favour of terrestrial ones in relation to carbon sequestration objectives. The discipline is relatively novel, this is perhaps partly because the science is fairly new, and also because of the challenges in monitoring and measuring impacts in coastal ecosystems. But we hope that as new data continues to demonstrate their efficacy in carbon sequestration, there will be a wider recognition of the crucial role that they play in mitigating the impacts of climate change.
In the last few years, a handful of mangrove projects have been launched to provide offsets. These projects explore the use of blue carbon as a long-term financial mechanism for maintaining sustainable livelihoods, whereby the restoration and conservation of the mangroves is community-led. By capitalising on carbon credits from the mangroves, these projects incorporate several of the SDGs: Climate Action, No Poverty, Life on Land, and Life Below Water to name a few.
Projects like these which i) focus on long-term community development and ii) deliver success for multiple stakeholders, can be expected to drive investment and growth in mangrove-led carbon projects.
A project in practice
A mangrove project, soundly managed, provides a fascinating story of how a range of ecosystem services can be brought together, to provide a sustainable source of revenue for stakeholders and communities.
A well managed plan would balance the needs of coastal communities with the promotion of biodiversity, and a flourishing and sustainable mangrove ecosystem which maximises the opportunities for a blue carbon monetisation project. BeZero is ideally positioned to drive such initiatives and centrally placed to advise on how best to monetise nascent blue carbon opportunities, with deep data access, well-developed case studies of other blue carbon projects, and global market intelligence.
Written by Thea Kay, Blue Carbon Intern